Best Performance Practices Hibernate 5/6 & Spring Boot 2

Hibernate & Spring Boot Samples

1. How To Store UTC Timezone In MySQL

Description: This application is a sample of how to store date, time, and timestamps in UTC time zone. The second setting, useLegacyDatetimeCode is needed only for MySQL. Otherwise, set only hibernate.jdbc.time_zone.

Key points:

• spring.jpa.properties.hibernate.jdbc.time_zone=UTC
• spring.datasource.url=jdbc:mysql://localhost:3306/screenshotdb?useLegacyDatetimeCode=false

2. View Binding/Extracted Params Via Log4J 2

Description: View the prepared statement binding/extracted parameters via Log4J 2 logger setting.

Key points:

• for Maven, in pom.xml, exclude Spring Boot's Default Logging
• for Maven, in pom.xml, Add Log4j 2 Dependency
• in log4j2.xml add, <Logger name="org.hibernate.type.descriptor.sql" level="trace"/>

Output example:

3. How To View Query Details Via DataSource-Proxy Library

Description: View the query details (query type, binding parameters, batch size, execution time, etc) via DataSource-Proxy

Key points:

• for Maven, add in pom.xml the datasource-proxy dependency
• create an bean post processor to intercept the DataSource bean
• wrap the DataSource bean via ProxyFactory and an implementation of MethodInterceptor

Output example:

4. Batch Inserts via saveAll(Iterable<S> entities) in MySQL

Description: Batch inserts via SimpleJpaRepository#saveAll(Iterable<S> entities) method in MySQL

Key points:

• in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
• in application.properties set spring.jpa.properties.hibernate.generate_statistics (just to check that batching is working)
• in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL)
• in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
• in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
• in case of using a parent-child relationship with cascade persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_inserts=true to optimize the batching by ordering inserts
• in entity, use the assigned generator since MySQL IDENTITY will cause insert batching to be disabled
• in entity, add @Version property to avoid extra-SELECT statements fired before batching (also prevent lost updates in multi-request transactions). Extra-SELECT statements are the effect of using merge() instead of persist(); behind the scene, saveAll() uses save(), which in case of non-new entities (entities that have IDs) will call merge(), which instruct Hibernate to fire a SELECT statement to make sure that there is no record in the database having the same identifier
• pay attention on the amount of inserts passed to saveAll() to not "overwhelm" the Persistence Context; normally the EntityManager should be flushed and cleared from time to time, but during the saveAll() execution you simply cannot do that, so if in saveAll() there is a list with a high amount of data, all that data will hit the Persistence Context (1st Level Cache) and will remain in memory until the flush time; using relatively small amount of data should be ok (in this example, each batch of 30 entities run in a separate transaction and Persistent Context)
• the saveAll() method return a List<S> containing the persisted entities; each persisted entity is added into this list; if you just don't need this List then it is created for nothing
• if is not needed, then ensure that Second Level Cache is disabled via spring.jpa.properties.hibernate.cache.use_second_level_cache=false

5. Batch Inserts Via EntityManager (MySQL)

Description: This application is a sample of batching inserts via EntityManager in MySQL. This way you can easily control the flush() and clear() cycles of the Persistence Context (1st Level Cache) inside the current transaction. This is not possible via Spring Boot, saveAll(Iterable<S> entities), since this method executes a single flush per transaction. Another advantage is that you can call persist() instead of merge() - this is used behind the scene by the SpringBoot saveAll(Iterable<S> entities) and save(S entity).

If you want to execute a batch per transaction (recommended) then check this example.

Key points:

• in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
• in application.properties set spring.jpa.properties.hibernate.generate_statistics (just to check that batching is working)
• in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL)
• in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
• in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
• in case of using a parent-child relationship with cascade persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_inserts=true to optimize the batching by ordering inserts
• in entity, use the assigned generator since MySQL IDENTITY will cause insert batching to be disabled
• in your DAO layer, flush and clear the Persistence Context from time to time (e.g. for each batch); this way you avoid to "overwhelm" the Persistence Context
• if is not needed, then ensure that Second Level Cache is disabled via spring.jpa.properties.hibernate.cache.use_second_level_cache=false

Output example:

6. How To Batch Inserts Via JpaContext/EntityManager In MySQL

Description: Batch inserts via JpaContext/EntityManager in MySQL.

Key points:

• in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
• in application.properties set spring.jpa.properties.hibernate.generate_statistics (just to check that batching is working)
• in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL)
• in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
• in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
• in case of using a parent-child relationship with cascade persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_inserts=true to optimize the batching by ordering inserts
• in entity, use the assigned generator since MySQL IDENTITY will cause insert batching to be disabled
• the EntityManager is obtain per entity type via, JpaContext#getEntityManagerByManagedType(Class<?> entity)
• in DAO, flush and clear the Persistence Context from time to time; this way you avoid to "overwhelm" the Persistence Context
• if is not needed, then ensure that Second Level Cache is disabled via spring.jpa.properties.hibernate.cache.use_second_level_cache=false

Output example:

7. How To Exploit Session-Level Batching (Hibernate 5.2 Or Higher) In MySQL

Description: Batch inserts via Hibernate session-level batching (Hibernate 5.2 or higher) in MySQL.

Key points:

• in application.properties set spring.jpa.properties.hibernate.generate_statistics (just to check that batching is working)
• in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL)
• in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
• in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
• in case of using a parent-child relationship with cascade persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_inserts=true to optimize the batching by ordering inserts
• in entity, use the assigned generator since MySQL IDENTITY will cause insert batching to be disabled
• the Hibernate Session is obtained by un-wrapping it via EntityManager#unwrap(Session.class)
• the batching size is set via Session#setJdbcBatchSize(Integer size) and get via Session#getJdbcBatchSize()
• in DAO, flush and clear the Persistence Context from time to time; this way you avoid to "overwhelm" the Persistence Context
• if is not needed, then ensure that Second Level Cache is disabled via spring.jpa.properties.hibernate.cache.use_second_level_cache=false

Output example:

8. Direct Fetching Via Spring Data findById(), JPA EntityManager And Hibernate Session

Description: Direct fetching via Spring Data, EntityManager and Hibernate Session examples.

Key points:

• direct fetching via Spring Data uses findById()
• direct fetching via JPA EntityManager uses find()
• direct fetching via Hibernate Session uses get()

9. DTO Via Spring Data Projections

Note: You may also like to read the recipe, "How To Enrich DTOs With Virtual Properties Via Spring Projections"

Description: Fetch only the needed data from the database via Spring Data Projections (DTO).

Key points:

• write an interface (projection) containing getters only for the columns that should be fetched from the database
• write the proper query returning a List<projection>
• if it is applicable, limit the number of returned rows (e.g., via LIMIT)
• in this example, we can use query builder mechanism built into Spring Data repository infrastructure

Note: Using projections is not limited to use query builder mechanism built into Spring Data repository infrastructure. We can fetch projections via JPQL or native queries as well. For example, in this application we use a JPQL.

Output example (select first 2 rows; select only "name" and "age"):

10. How To Use Hibernate Attribute Lazy Loading

Description: By default, the attributes of an entity are loaded eagerly (all at once). But, we can load them lazy as well. This is useful for column types that store large amounts of data: CLOB, BLOB, VARBINARY, etc or details that should be loaded on demand. In this application, we have an entity named Author. Its properties are: id, name, genre, avatar and age. And, we want to load the avatar lazy. So, the avatar should be loaded on demand.

Key points:

• in pom.xml, activate Hibernate bytecode enhancement (e.g. use Maven bytecode enhancement plugin)
• in entity, annotate the attributes that should be loaded lazy with @Basic(fetch = FetchType.LAZY)
• in application.properties, disable Open Session in View

Check as well:
- Default Values For Lazy Loaded Attributes
- Attribute Lazy Loading And Jackson Serialization

11. How To Populate a Child-Side Parent Association via Proxy

Description: A Hibernate proxy can be useful when a child entity can be persisted with a reference to its parent (@ManyToOne or @OneToOne association). In such cases, fetching the parent entity from the database (execute the SELECT statement) is a performance penalty and a pointless action, because Hibernate can set the underlying foreign key value for an uninitialized proxy.

Key points:

• rely on EntityManager#getReference()
• in Spring, use JpaRepository#getOne() -> used in this example
• in Hibernate, use load()
• assume two entities, Author and Book, involved in a unidirectional @ManyToOne association (Author is the parent-side)
• we fetch the author via a proxy (this will not trigger a SELECT), we create a new book, we set the proxy as the author for this book and we save the book (this will trigger an INSERT in the book table)

Output example:

• the console output will reveal that only an INSERT is triggered, and no SELECT

12. How To Quickly Reproduce The N+1 Performance Issue

Description: The N+1 is an issue of lazy fetching (but, eager is not exempt). This application reproduce the N+1 behavior.

Key points:

• define two entities, Author and Book in a lazy bidirectional @OneToMany association
• fetch all Book lazy, so without Author (results in 1 query)
• loop the fetched Book collection and for each entry fetch the corresponding Author (results N queries)
• or, fetch all Author lazy, so without Book (results in 1 query)
• loop the fetched Author collection and for each entry fetch the corresponding Book (results N queries)

Output example:

13. Optimize SELECT DISTINCT Via Hibernate HINT_PASS_DISTINCT_THROUGH Hint

Description: Starting with Hibernate 5.2.2, we can optimize JPQL (HQL) query entites of type SELECT DISTINCT via HINT_PASS_DISTINCT_THROUGH hint. Keep in mind that this hint is useful only for JPQL (HQL) JOIN FETCH-ing queries. Is not useful for scalar queries (e.g., List<Integer>), DTO or HHH-13280. In such cases, the DISTINCT JPQL keyword is needed to be passed to the underlying SQL query. This will instruct the database to remove duplicates from the result set.

Key points:

• use @QueryHints(value = @QueryHint(name = HINT_PASS_DISTINCT_THROUGH, value = "false"))

Output example:

14. How To Enable Dirty Tracking In A Spring Boot Application

Note: The Hibernate Dirty Checking mechanism is responsible to identify the entitites modifications at flush-time and to trigger the corresponding UPDATE statements in our behalf.

Description: Prior to Hibernate version 5, the Dirty Checking mechanism relies on Java Reflection API for checking every property of every managed entity. Starting with Hibernate version 5, the Dirty Checking mechanism can rely on the Dirty Tracking mechanism (which is the capability of an entity to track its own attributes changes) which requires Hibernate Bytecode Enhancement to be present in the application. The Dirty Tracking mechanism sustain a better performance, especially when you have a relatively large number of entitites.

For Dirty Tracking, during Bytecode Enhancement process, the entity classes bytecode is instrumented by Hibernate by adding a tracker, $$_hibernate_tracker. At flush time, Hibernate will use this tracker to discover the entities changes (each entity tracker will report the changes). This is better than checking every property of every managed entity.

Commonly (by default), the instrumentation takes place at build-time, but it can be configured to take place at runtime or deploy-time as well. It is preferable to take place at build-time for avoiding an overhead in the runtime.

Adding Bytecode Enhancement and enabling Dirty Tracking can be done via a plugin added via Maven or Gradle (Ant can be used as well). We use Maven, therefore we add it in pom.xml.

Key points:

• Hibernate come with Bytecode Enhancement plugins for Maven, Gradle (Ant can be used as well)
• for Maven, add the Bytecode Enhancement plugin in the pom.xml file

Output example:

The Bytecode Enhancement effect can be seen on Author.class here. Notice how the bytecode was instrumented with $$_hibernate_tracker.

15. Use Java 8 Optional In Entities And Queries

Description: This application is an example of how is correct to use the Java 8 Optional in entities and queries.

Key points:

• use the Spring Data built-in query-methods that return Optional (e.g., findById())
• write your own queries that return Optional
• use Optional in entities getters
• in order to run different scenarios check the file, data-mysql.sql

16. The Best Way To Map The @OneToMany Bidirectional Association

Description: This application is a proof of concept of how is correct to implement the bidirectional @OneToMany association from the performance perspective.

Key points:

• always cascade from parent to child
• use mappedBy on the parent
• use orphanRemoval on parent in order to remove children without references
• use helper methods on parent to keep both sides of the association in sync
• use lazy fetching on both side of the association
• as entities identifiers, use assigned identifiers (business key, natural key (@NaturalId)) and/or database-generated identifiers and override (on child-side) properly the equals() and hashCode() methods as here
• if toString() need to be overridden, then pay attention to involve only the basic attributes fetched when the entity is loaded from the database

Note: Pay attention to remove operations, especially to removing child entities. The CascadeType.REMOVE and orphanRemoval=true may produce too many queries. In such scenarios, relying on bulk operations is most of the time the best way to go for deletions.

17. Query Fetching

Description: This application is an example of how to write a query via JpaRepository, EntityManager and Session.

Key points:

• for JpaRepository use @Query or Spring Data Query Creation
• for EntityManager and Session use the createQuery() method

18. Why And How To Avoid The AUTO Generator Type In Hibernate 5 And MySQL

Description: In MySQL & Hibernate 5, the GenerationType.AUTO generator type will result in using the TABLE generator. This adds a significant performance penalty. Turning this behavior to IDENTITY generator can be obtained by using GenerationType.IDENTITY or the native generator.

Key points:

• use GenerationType.IDENTITY instead of GenerationType.AUTO
• use the native generator - exemplified in this application

Output example:

19. How To Avoid The Redundant save() Call

Description: This application is an example when calling save() for an entity is redundant (not necessary).

Key points:

• at flush time, Hibernate relies on dirty checking mechanism to determine the potential modifications in entities
• for each modification, Hibernate automatically triggers the corresponding UPDATE statement without the need to explicitly call the save() method
• behind the scene, this redundancy (calling save() when is not necessarily) doesn't affect the number of triggered queries, but it implies a performance penalty in the underlying Hibernate processes

20. Why To Avoid PostgreSQL (BIG)SERIAL In Batching Inserts Via Hibernate

Description: In PostgreSQL, using GenerationType.IDENTITY will disable insert batching. The (BIG)SERIAL is acting "almost" like MySQL, AUTO_INCREMENT. In this application, we use the GenerationType.SEQUENCE which permits insert batching, and we optimize it via the hi/lo optimization algorithm.

Key points:

• use GenerationType.SEQUENCE instead of GenerationType.IDENTITY
• rely on the hi/lo algorithm to fetch a hi value in a database roundtrip (the hi value is useful for generating a certain/given number of identifiers in-memory; until you haven't exhausted all in-memory identifiers there is no need to fetch another hi)
• you can go even further and use the Hibernate pooled and pooled-lo identifier generators (these are optimizations of hi/lo that allows external services to use the database without causing duplication keys errors)
• optimize batching via spring.datasource.hikari.data-source-properties.reWriteBatchedInserts=true

Output example:

21. JPA Inheritance - SINGLE_TABLE

Description: This application is a sample of using JPA Single Table inheritance strategy (SINGLE_TABLE).

Key points:

• this is the default inheritance strategy (@Inheritance(strategy=InheritanceType.SINGLE_TABLE))
• all the classes in an inheritance hierarchy are represented via a single table in the database
• subclasses attributes non-nullability is ensured via @NotNull and MySQL triggers
• the default discriminator column memory footprint was optimized by declaring it of type TINYINT

Output example (below is a single table obtained from 3 entities):

22. Count and Assert SQL Statements

Description: This application is a sample of counting and asserting SQL statements triggered "behind the scene". Is very useful to count the SQL statements in order to ensure that your code is not generating more SQL statements that you may think (e.g., N+1 can be easily detected by asserting the number of expected statements).

Key points:

• for Maven, in pom.xml, add dependencies for DataSource-Proxy library and Vlad Mihalcea's db-util library
• create the ProxyDataSourceBuilder with countQuery()
• reset the counter via SQLStatementCountValidator.reset()
• assert INSERT, UPDATE, DELETE and SELECT via assertInsert/Update/Delete/Select/Count(long expectedNumberOfSql)

Output example (when the number of expected SQLs is not equal with the reality an exception is thrown):

23. How To Setup JPA Callbacks

Description: This application is a sample of setting the JPA callbacks (Pre/PostPersist, Pre/PostUpdate, Pre/PostRemove and PostLoad).

Key points:

• in entity, write callback methods and use the proper annotations
• callback methods annotated on the bean class must return void and take no arguments

Output example:

24. How To Use @MapsId For Sharing Identifier In @OneToOne Relationship

Description: Instead of regular unidirectional/bidirectional @OneToOne better rely on an unidirectional @OneToOne and @MapsId. This application is a proof of concept.

Key points:

• use @MapsId on child side
• use @JoinColumn to customize the name of the primary key column
• mainly, for @OneToOne associations, @MapsId will share the primary key with the parent table (id property acts as both primary key and foreign key)

Note:

• @MapsId can be used for @ManyToOne as well

25. How To Fetch DTO Via SqlResultSetMapping And EntityManager

Description: Fetching more data than needed is prone to performance penalities. Using DTO allows us to extract only the needed data. In this application we rely on SqlResultSetMapping and EntityManager.

Key points:

• use SqlResultSetMapping and EntityManager
• for using Spring Data Projections check this item

26. How To Fetch DTO Via SqlResultSetMapping And NamedNativeQuery

Note: If you want to rely on the {EntityName}.{RepositoryMethodName} naming convention for simply creating in the repository interface methods with the same name as of native named query then skip this application and check this one.

Description: Fetching more data than needed is prone to performance penalities. Using DTO allows us to extract only the needed data. In this application we rely on SqlResultSetMapping, NamedNativeQuery.

Key points:

• use SqlResultSetMapping, NamedNativeQuery
• for using Spring Data Projections check this item

27. How To Fetch DTO Via javax.persistence.Tuple And Native SQL

Description: Fetching more data than needed is prone to performance penalities. Using DTO allows us to extract only the needed data. In this application we rely on javax.persistence.Tuple and native SQL.

Key points:

• use java.persistence.Tuple in a Spring repository and mark the query as nativeQuery = true
• for using Spring Data Projections check this item

28. How To Fetch DTO via javax.persistence.Tuple and JPQL

Description: Fetching more data than needed is prone to performance penalities. Using DTO allows us to extract only the needed data. In this application we rely on javax.persistence.Tuple and JPQL.

Key points:

• use java.persistence.Tuple in a Spring repository
• for using Spring Data Projections check this item

29. How To Fetch DTO Via Constructor Expression and JPQL

Description: Fetching more data than needed is prone to performance penalities. Using DTO allows us to extract only the needed data. In this application we rely on Constructor Expression and JPQL.

Key points:

• write a proper constructor in the DTO class
• use a query as SELECT new com.bookstore.dto.AuthorDto(a.name, a.age) FROM Author a
• for using Spring Data Projections check this item

See also:
How To Fetch DTO Via Constructor And Spring Data Query Builder Mechanism

30. How To Fetch DTO Via ResultTransformer And Native SQL

Description: Fetching more data than needed is prone to performance penalties. Using DTO allows us to extract only the needed data. In this application we rely on Hibernate, ResultTransformer and native SQL.

Key points:

• use AliasToBeanConstructorResultTransformer for DTO without setters, but with constructor
• use Transformers.aliasToBean() for DTO with setters
• use EntityManager.createNativeQuery() and unwrap(org.hibernate.query.NativeQuery.class)
• starting with Hibernate 5.2, ResultTransformer is deprecated, but until a replacement will be available (probably in Hibernate 6.0) it can be used (read further)
• for using Spring Data Projections check this recipe

31. How To Fetch DTO Via ResultTransformer and JPQL

Description: Fetching more data than needed is prone to performance penalties. Using DTO allows us to extract only the needed data. In this application we rely on Hibernate, ResultTransformer and JPQL.

Key points:

• use AliasToBeanConstructorResultTransformer for DTO without setters, with constructor
• use Transformers.aliasToBean() for DTO with setters
• use EntityManager.createQuery() and unwrap(org.hibernate.query.Query.class)
• starting with Hibernate 5.2, ResultTransformer is deprecated, but until a replacement will be available (in Hibernate 6.0) it can be used (read further)
• for using Spring Data Projections check this item

32. How To Fetch DTO Via Blaze-Persistence Entity Views

Description: Fetching more data than needed is prone to performance penalities. Using DTO allows us to extract only the needed data. In this application we rely on Blaze-Persistence entity views.

Key points:

• for Maven, add in pom.xml the dependencies specific to Blaze-Persistence
• configure Blaze-Persistence via CriteriaBuilderFactory and EntityViewManager
• write an entity view via an interface in Blaze-Persistence fashion
• write a Spring-centric repository by extending EntityViewRepository
• call method of this repository such as, findAll(), findOne(), etc
• for using Spring Data Projections check this item

33. How Regular @ElementCollection (Without @OrderColumn) Works

Description: This application reveals the possible performance penalties of using @ElementCollection. In this case, without @OrderColumn. As you can see in the next item (34) adding @OrderColumn can mitigate some performance penalties.

Key points:

• an @ElementCollection doesn't have a primary key
• an @ElementCollection is mapped in a separate table
• avoid @ElementCollection when you have a lot of inserts/deletes on this collection; inserts/deletes will cause Hibernate to delete all the existing table rows, process the collection in-memory, and re-insert the remaining table rows to mirror the collection from memory
• the more entries we have in this collection the greater the performance penalty will be

Output example:

34. How @ElementCollection With @OrderColumn Works

Description: This application reveals the performance penalties of using @ElementCollection. In this case, with @OrderColumn. But, as you can see in this application (in comparison with item 33), by adding @OrderColumn can mitigate some performance penalties when operations takes place near the collection tail (e.g., add/remove at/from the end of the collection). Mainly, all elements situated before the adding/removing entry are left untouched, so the performance penalty can be ignored if we affect rows close to the collection tail.

Key points:

• an @ElementCollection doesn't have a primary key
• an @ElementCollection is mapped in a separate table
• prefer @ElementCollection with @OrderColumn when you have a lot of inserts and deletes near the collection tail
• the more elements are inserted/removed from the beginning of the collection the greater the performance penalty will be

Output example:

35. How To Avoid Lazy Initialization Issues Caused By Disabling Open Session In View Via Explicit (Default) Values

Note: Before reading this item try to see if Hibernate5Module is not what you are looking for.

Description: The Open-Session in View anti-pattern is activated by default in SpringBoot. Now, imagine a lazy association (e.g., @OneToMany) between two entities, Author and Book (an author has associated more books). Next, a REST controller endpoint fetches an Author without the associated Book. But, the View (more precisely, Jackson), forces the lazy loading of the associated Book as well. Since OSIV will supply the already opened Session, the proxies initializations take place successfully. The solution to avoid this performance penalty starts by disabling the OSIV. Further, explicitly initialize the un-fetched lazy associations. This way, the View will not force lazy loading.

Key points:

• disable OSIV by adding in application.properties this setting: spring.jpa.open-in-view=false
• fetch an Author entity and initialize its associated Book explicitly with (default) values (e.g., null)
• set @JsonInclude(Include.NON_EMPTY) on this entity-level to avoid rendering null or what is considered empty in the resulted JSON

NOTE: If OSIV is enabled, the developer can still initialize the un-fetched lazy associations manually as long as he does this outside of a transaction to avoid flushing. But, why is this working? Since the Session is open, why the manually initialization of the associations of a managed entity doesn't trigger the flush? The answer can be found in the documentation of OpenSessionInViewFilter which specifies that: This filter will by default not flush the Hibernate Session, with the flush mode set to FlushMode.NEVER. It assumes to be used in combination with service layer transactions that care for the flushing: The active transaction manager will temporarily change the flush mode to FlushMode.AUTO during a read-write transaction, with the flush mode reset to FlushMode.NEVER at the end of each transaction. If you intend to use this filter without transactions, consider changing the default flush mode (through the "flushMode" property).

36. How To Use Spring Projections(DTO) And Inner Joins

Description: This application is a proof of concept for using Spring Projections(DTO) and inner joins written via JPQL and native SQL (for MySQL).

Key points:

• define two entities (e.g., Author and Book in a (lazy) bidirectional @OneToMany association)
• populate the database with some test data (e.g., check the file resources/data-mysql.sql)
• write interfaces (Spring projections) that contains getters for the columns that should be fetched from the database (e.g., check AuthorNameBookTitle.java)
• write inner joins queries using JPQL/SQL

37. How To Use Spring Projections(DTO) And Left Joins

Description: This application is a proof of concept for using Spring Projections(DTO) and left joins written via JPQL and native SQL (for MySQL).

Key points:

• define two entities (e.g., Author and Book in a (lazy) bidirectional @OneToMany association)
• populate the database with some test data (e.g., check the file resources/data-mysql.sql)
• write interfaces (Spring projections) that contains getters for the columns that should be fetched from the database (e.g., check AuthorNameBookTitle.java)
• write left joins queries using JPQL/SQL

38. How To Use Spring Projections(DTO) And Right Joins

Description: This application is a proof of concept for using Spring Projections(DTO) and right joins written via JPQL and native SQL (for MySQL).

Key points:

• define two entities (e.g., Author and Book in a (lazy) bidirectional @OneToMany association)
• populate the database with some test data (e.g., check the file resources/data-mysql.sql)
• write interfaces (Spring projections) that contains getters for the columns that should be fetched from the database (e.g., check AuthorNameBookTitle.java)
• write right joins queries using JPQL/SQL

39. How To Use Spring Projections(DTO) And Inclusive Full Joins (PostgreSQL)

Description: This application is a proof of concept for using Spring Projections(DTO) and inclusive full joins written via JPQL and native SQL (for PostgreSQL).

Key points:

• define two entities (e.g., Author and Book in a (lazy) bidirectional @OneToMany association)
• populate the database with some test data (e.g., check the file resources/data-mysql.sql)
• write interfaces (Spring projections) that contains getters for the columns that should be fetched from the database (e.g., check AuthorNameBookTitle.java)
• write inclusive full joins queries using JPQL/SQL

40. How To Use Spring Projections(DTO) And Exclusive Left Joins

Description: This application is a proof of concept for using Spring Projections(DTO) and exclusive left joins written via JPQL and native SQL (for MySQL).

Key points:

• define two entities (e.g., Author and Book in a (lazy) bidirectional @OneToMany association)
• populate the database with some test data (e.g., check the file resources/data-mysql.sql)
• write interfaces (projections) that contains getters for the columns that should be fetched from the database (e.g., check AuthorNameBookTitle.java)
• write exclusive left joins queries using JPQL/SQL

41. How To Use Spring Projections(DTO) And Exclusive Right Joins

Description: This application is a proof of concept for using Spring Projections(DTO) and exclusive right joins written via JPQL and native SQL (for MySQL).

Key points:

• define two entities (e.g., Author and Book in a (lazy) bidirectional @OneToMany association)
• populate the database with some test data (e.g., check the file resources/data-mysql.sql)
• write interfaces (Spring projections) that contains getters for the columns that should be fetched from the database (e.g., check AuthorNameBookTitle.java)
• write exclusive right joins queries using JPQL/SQL

42. How To Use Spring Projections(DTO) And Exclusive Full Joins (PostgreSQL)

Description: This application is a proof of concept for using Spring Projections(DTO) and exclusive full joins written via JPQL and native SQL (for PostgreSQL).

Key points:

• define two entities (e.g., Author and Book in a (lazy) bidirectional @OneToMany association)
• populate the database with some test data (e.g., check the file resources/data-mysql.sql)
• write interfaces (Spring projections) that contains getters for the columns that should be fetched from the database (e.g., check AuthorNameBookTitle.java)
• write exclusive full joins queries using JPQL/SQL

43. Why You Should Avoid Time-Consuming Tasks In Spring Boot Post-Commit Hooks

Description: This application is a proof of concept for using Spring post-commit hooks and how they may affect the persistence layer performance.

Key points:

• avoid time-consuming tasks in Spring post-commit hooks since the database connection will remain open until this code finshes

44. How To Exploit Spring Projections(DTO) And Join Unrelated Entities In Hibernate 5.1+

Description: This application is a proof of concept for using Spring Projections (DTO) and join unrelated entities. Hibernate 5.1 introduced explicit joins on unrelated entities and the syntax and behaviour are similar to SQL JOIN statements.

Key points:

• define serveral entities (e.g., Author and Book unrelated entities)
• populate the database with some test data (e.g., check the file resources/data-mysql.sql)
• write interfaces (Spring projections) that contains getters for the columns that should be fetched from the database (e.g., BookstoreDto)
• write joins queries using JPQL/SQL (e.g., queries all authors names and book titles of the given price)

45. Why To Avoid Lombok @EqualsAndHashCode And @Data In Entities And How To Override equals() And hashCode()

Description: Entities should implement equals() and hashCode() as here. The main idea is that Hibernate requires that an entity is equal to itself across all its state transitions (transient, attached, detached and removed). Using Lombok @EqualsAndHashCode (or @Data) will not respect this requirment.

Key points:
AVOID THESE APPROACHES

• Using Lombok default behavior of @EqualsAndHashCode (entity: LombokDefaultBook, test: LombokDefaultEqualsAndHashCodeTest)
• Using Lombok @EqualsAndHashCode with primary key only (entity: LombokIdBook, test: LombokEqualsAndHashCodeWithIdOnlyTest)
• Rely on default equals() and hashCode() (entity: DefaultBook, test: DefaultEqualsAndHashCodeTest)
• Rely on default equals() and hashCode() containing only the database-generated identifier (entity: IdBook, test: IdEqualsAndHashCodeTest)

PREFER THESE APPROACHES

• Rely on business key (entity: BusinessKeyBook, test: BusinessKeyEqualsAndHashCodeTest)
• Rely on @NaturalId (entity: NaturalIdBook, test: NaturalIdEqualsAndHashCodeTest)
• Rely on manually assigned identifiers (entity: IdManBook, test: IdManEqualsAndHashCodeTest)
• Rely on database-generated identifiers (entity: IdGenBook, test: IdGenEqualsAndHashCodeTest)

46. How To Avoid LazyInitializationException Via JOIN FETCH

See also:

• LEFT JOIN FETCH
• JOIN VS. JOIN FETCH

Description: Typically, when we get a LazyInitializationException we tend to modify the association fetching type from LAZY to EAGER. That is very bad! This is a code smell. Best way to avoid this exception is to rely on JOIN FETCH (if you plan to modify the fetched entities) or JOIN + DTO (if the fetched data is only read). JOIN FETCH allows associations to be initialized along with their parent objects using a single SELECT. This is particularly useful for fetching associated collections.

This application is a JOIN FETCH example for avoiding LazyInitializationException.

Key points:

• define two related entities (e.g., Author and Book in a @OneToMany lazy-bidirectional association)
• write a JPQL JOIN FETCH to fetch an author including his books
• write a JPQL JOIN FETCH (or JOIN) to fetch a book including its author

Output example:

47. How To Merge Entity Collections

Description: This is a Spring Boot example based on the following article. Is a functional implementation of the Vlad's example. It is highly recommended to read that article.

Key points:

• remove the existing database rows that are no longer found in the incoming collection
• update the existing database rows which can be found in the incoming collection
• add the rows found in the incoming collection, which cannot be found in the current database snapshot

48. How To Delay Connection Acquisition As Needed (Hibernate 5.2.10)

Description: This is a Spring Boot example that exploits Hibernate 5.2.10 capability of delaying the connection acquisition as needed. By default, in resource-local mode, a database connection is aquried immediately after calling a method annotated with @Transactional. If this method contains some time-consuming tasks before the first SQL statement then the connection is hold open for nothing. But, Hibernate 5.2.10 allows us to delay the connection acquisition as needed. This example rely on HikariCP as the default connection pool for Spring Boot.

Key points:

• set spring.datasource.hikari.auto-commit=false in application.properties
• set spring.jpa.properties.hibernate.connection.provider_disables_autocommit=true in application.properties

Output example:

49. How To Generate Sequences Of Identifiers Via Hibernate hi/lo Algorithm

Note: If systems external to your application need to insert rows in your tables then don't rely on hi/lo algorithm since, in such cases, it may cause errors resulted from generating duplicated identifiers. Rely on pooled or pooled-lo algorithms (optimizations of hi/lo).

Description: This is a Spring Boot example of using the hi/lo algorithm for generating 1000 identifiers in 10 database roundtrips for batching 1000 inserts in batches of 30.

Key points:

• use the SEQUENCE generator type (e.g., in PostgreSQL)
• configure the hi/lo algorithm as in Author.java entity

Output example:

50. The Best Way To Implement A Bidirectional @ManyToMany Association

Description: This application is a proof of concept of how it is correct to implement the bidirectional @ManyToMany association from the performance perspective.

Key points:

• choose an owning and a mappedBy side
• materialize the relationships collections via Set not List
• use helper methods on the owner of the relationship to keep both sides of the association in sync
• on the owner of the relationship use CascadeType.PERSIST and CascadeType.MERGE, but avoid CascadeType.REMOVE/ALL
• on the owner of the relationship set up join table
• @ManyToMany is lazy by default; keep it this way!
• as entities identifiers, use assigned identifiers (business key, natural key (@NaturalId)) and/or database-generated identifiers and override (on both sides) properly the equals() and hashCode() methods as here
• if toString() need to be overridden, then pay attention to involve only for the basic attributes fetched when the entity is loaded from the database

51. Prefer Set Instead of List in @ManyToMany Associations

Description: This is a Spring Boot example of removing rows in case of a bidirectional @ManyToMany using List, respectively Set. The conclusion is that Set is much better! This applies to unidirectional as well!

Key points:

• using Set is much more efficent than List

Output example:

52. How To View Query Details Via log4jdbc

Description: View the query details via log4jdbc.

Key points:

• for Maven, in pom.xml, add log4jdbc dependency

Output sample:

53. How To View Binding Params Via TRACE

Description: View the prepared statement binding/extracted parameters via TRACE.

Key points:

• in application.properties add: logging.level.org.hibernate.type.descriptor.sql=TRACE
• or, even better (for filtering SQLs capabilities), in a Logback specific configuration file add the proper logger

Output sample:

54. How To Store java.time.YearMonth As Integer Or Date Via Hibernate Types Library

Description: Hibernate Types is a set of extra types not supported by default in Hibernate Core. One of these types is java.time.YearMonth. This is a Spring Boot application that uses Hibernate Type to store this YearMonth in a MySQL database as integer or date.

Key points:

• for Maven, add Hibernate Types as a dependency in pom.xml
• in entity use @TypeDef to map typeClass to defaultForType

Output example:

55. How To Execute SQL Functions In JPQL Query

Note: Using SQL functions in the WHERE part (not in the SELECT part) of query in JPA 2.1 can be done via function() as here.

Description: Trying to use SQL functions (standard or defined) in JPQL queries may result in exceptions if Hibernate will not recognize them and cannot parse the JPQL query. For example, the MySQL, concat_ws function is not recognized by Hibernate. This application is a Spring Boot application based on Hibernate 5.3, that registers the concat_ws function via MetadataBuilderContributor and inform Hibernate about it via, metadata_builder_contributor property. This example uses @Query and EntityManager as well, so you can see two use cases.

Key points:

• use Hibernate 5.3 (or, to be precisely, 5.2.18) (e.g., use Spring Boot 2.1.0.RELEASE)
• implement MetadataBuilderContributor and register the concat_ws MySQL function
• in application.properties, set spring.jpa.properties.hibernate.metadata_builder_contributor to point out Hibernate to MetadataBuilderContributor implementation

Output example:

56. Log Slow Queries Via DataSource-Proxy

Description: This application is a sample of logging only slow queries via DataSource-Proxy. A slow query is a query that has an execution time bigger than a specificed threshold in milliseconds.

Key points:

• for Maven, add in pom.xml the DataSource-Proxy dependency
• create an bean post processor to intercept the DataSource bean
• wrap the DataSource bean via ProxyFactory and an implementation of MethodInterceptor
• choose a threshold in milliseconds
• define a listener and override afterQuery()

Output example:

57. Offset Pagination - Trigger SELECT COUNT Subquery And Return Page<dto>

Description: This application fetches data as Page<dto> via Spring Boot offset pagination. Most of the time, the data that should be paginated is read-only data. Fetching the data into entities should be done only if we plan to modify that data, therefore, fetching read only data as Page<entity> is not preferable since it may end up in a significant performance penalty. The SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, there is one query needed for fetching the data and one for counting the total number of records).

Key points:

• create a Spring projection (DTO) to contains getters only for the columns that should be fetched
• write a repository that extends PagingAndSortingRepository
• fetch data via a JPQL or native query (that includes counting) into a List<dto>
• use the fetched List<dto> and the proper Pageable to create a Page<dto>

58. Offset Pagination - Trigger SELECT COUNT Subquery And Return List<dto>

Description: This application fetches data as List<dto> via Spring Boot offset pagination. Most of the time, the data that should be paginated is read-only data. Fetching the data into entities should be done only if we plan to modify that data, therefore, fetching read only data as List<entity> is not preferable since it may end up in a significant performance penalty. The SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, there is one query needed for fetching the data and one for counting the total number of records).

Key points:

• create a Spring projection (DTO) to contains getters only for the columns that should be fetched
• write a repository that extends PagingAndSortingRepository
• fetch data via a JPQL or native query (that includes counting) into a List<dto>

59. How To Customize HikariCP Settings Via Properties

If you use the spring-boot-starter-jdbc or spring-boot-starter-data-jpa "starters", you automatically get a dependency to HikariCP

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that set up HikariCP via application.properties only. The jdbcUrl is set up for a MySQL database. For testing purposes, the application uses an ExecutorServicefor simulating concurrent users. Check the HickariCP report revealing the connection pool status.

Key points:

• in application.properties, rely on spring.datasource.hikari.* to configure HikariCP

Output sample:

60. How To Customize HikariCP Settings Via Properties And DataSourceBuilder

If you use the spring-boot-starter-jdbc or spring-boot-starter-data-jpa "starters", you automatically get a dependency to HikariCP

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that set up HikariCP via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purposes, the application uses an ExecutorService for simulating concurrent users. Check the HickariCP report revealing the connection pool status.

Key points:

• in application.properties, configure HikariCP via a custom prefix, e.g., app.datasource.*
• write a @Bean that returns the DataSource

Output sample:

61. Running a SpringBoot Application Under Payara Server Using a Payara Data Source (JDBC Resource and Connection Pool)

This application is detailed in this DZone article.

62. How To Customize BoneCP Settings Via Properties And DataSourceBuilder

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that set up BoneCP via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purposes, the application uses an ExecutorService for simulating concurrent users.

Key points:

• in pom.xml add the BoneCP dependency
• in application.properties, configure BoneCP via a custom prefix, e.g., app.datasource.*
• write a @Bean that returns the DataSource

Output sample:

63. How To Customize ViburDBCP Settings Via Properties And DataSourceBuilder

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that set up ViburDBCP via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purposes, the application uses an ExecutorService for simulating concurrent users.

Key points:

• in pom.xml add the ViburDBCP dependency
• in application.properties, configure ViburDBCP via a custom prefix, e.g., app.datasource.*
• write a @Bean that returns the DataSource

Output sample:

64. How To Customize C3P0 Settings Via Properties And DataSourceBuilder

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that set up C3P0 via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purposes, the application uses an ExecutorService for simulating concurrent users.

Key points:

• in pom.xml add the C3P0 dependency
• in application.properties, configure C3P0 via a custom prefix, e.g., app.datasource.*
• write a @Bean that returns the DataSource

Output sample:

65. How To Customize DBCP2 Settings Via Properties And DataSourceBuilder

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that set up DBCP2 via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purposes, the application uses an ExecutorService for simulating concurrent users.

Key points:

• in pom.xml add the DBCP2 dependency
• in application.properties, configure DBCP2 via a custom prefix, e.g., app.datasource.*
• write a @Bean that returns the DataSource

66. How To Customize Tomcat Settings Via Properties And DataSourceBuilder

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that set up Tomcat via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purposes, the application uses an ExecutorService for simulating concurrent users.

Key points:

• in pom.xml add the Tomcat dependency
• in application.properties, configure Tomcat via a custom prefix, e.g., app.datasource.*
• write a @Bean that returns the DataSource

Output sample:

67. How To Configure Two Data Sources With Two Connection Pools

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that uses two data sources (two MySQL databases, one named authorsdb and one named booksdb) with two connection pools (each database uses its own HikariCP connection pool with different settings). Based on the above items is pretty easy to configure two connection pools from two different providers as well.

Key points:

• in application.properties, configure two HikariCP connection pools via a two custom prefixes, e.g., app.datasource.ds1 and app.datasource.ds2
• write a @Bean that returns the first DataSource and mark it as @Primary
• write another @Bean that returns the second DataSource
• configure two EntityManagerFactory and point out the packages to scan for each of them
• put the domains and repositories for each EntityManager in the right packages

Output sample:

68. How To Provide a Fluent API Via Setters For Building Entities

Note: If you want yo provide a Fluent API without altering setters then consider this item.

Description: This is a sample application that alter the entities setters methods in order to empower a Fluent API.

Key points:

• in entitites, return this instead of void in setters

Fluent API example:

69. How To Provide a Fluent API Via Additional Methods For Building Entities

Note: If you want yo provide a Fluent API by altering setters then consider this item.

Description: This is a sample application that add in entities additional methods (e.g., for setName, we add name) methods in order to empower a Fluent API.

Key points:

• in entities, add for each setter an additional method that return this instead of void

Fluent API example:

70. How To Implement Slice<T> findAll()

Most probably this is all you want: How To Fetch Slice<entity>/Slice<dto> Via fetchAll/fetchAllDto

Some implementations of Slice<T> findAll():

• This is a thin implementation based on a hard-coded SQL: "SELECT e FROM " + entityClass.getSimpleName() + " e;"
• This is just another minimalist implementation based on CriteriaBuilder instead of hard-coded SQL
• This is an implementation that allows us to provide a Sort, so sorting results is possible
• This is an implementation that allows us to provide a Sort and a Spring Data Specification
• This is an implementation that allows us to provide a Sort, a LockModeType, a QueryHints and a Spring Data Specification
• This is an implementation that allows us to provide a Spring Data Pageable and/or Specification by extending the SimpleJpaRepository from Spring Data. Bascially, this implementation is the only one that returns Page<T> instead of Slice<T>, but it doesn't trigger the extra SELECT COUNT since it was eliminated by overriding the Page<T> readPage(...) method from SimpleJpaRepository. The main drawback is that by returing a Page<T> you don't know if there is a next page or the current one is the last. Nevertheless, there are workarounds to have this as well. In this implementation you cannot set LockModeType or query hints.

Story: Spring Boot provides an offset based built-in paging mechanism that returns a Page or Slice. Each of these APIs represents a page of data and some metadata. The main difference is that Page contains the total number of records, while Slice can only tell if there is another page available. For Page, Spring Boot provides a findAll() method capable to take as arguments a Pageable and/or a Specification or Example. In order to create a Page that contains the total number of records, this method triggers an SELECT COUNT extra-query next to the query used to fetch the data of the current page. This can be a performance penalty since the SELECT COUNT query is triggered every time we request a page. In order to avoid this extra-query, Spring Boot provides a more relaxed API, the Slice API. Using Slice instead of Page removes the need of this extra SELECT COUNT query and returns the page (records) and some metadata without the total number of records. So, while Slice doesn't know the total number of records, it still can tell if there is another page available after the current one or this is the last page. The problem is that Slice work fine for queries containing the SQL, WHERE clause (including those that uses the query builder mechanism built into Spring Data), but it doesn't work for findAll(). This method will still return a Page instead of Slice therefore the SELECT COUNT query is triggered for Slice<T> findAll(...);.

Description: This is a suite of samples applications that provides different versions of a Slice<T> findAll(...) method. We have from a minimalist implementation that relies on a hardcoded query as: "SELECT e FROM " + entityClass.getSimpleName() + " e"; (this recipe), to a custom implementation that supports sorting, specification, lock mode and query hints to an implementation that relies on extending SimpleJpaRepository.

Key points:

• write an abstract class that expose the Slice<T> findAll(...) methods (SlicePagingRepositoryImplementation)
• implement the findAll() methods to return Slice<T> (or Page<T>, but without the total number of elements)
• return a SliceImpl (Slice<T>) or a PageImpl (Page<T>) without the total number of elements
• implement a new readSlice() method or override the SimpleJpaRepository#readPage() page to avoid SELECT COUNT
• pass the entity class (e.g., Author.class) to this abstract class via a class repository (AuthorRepository)

71. Offset Pagination - Trigger COUNT(*) OVER And Return List<dto>

Description: Typically, in offset pagination, there is one query needed for fetching the data and one for counting the total number of records. But, we can fetch this information in a single database rountrip via a SELECT COUNT subquery nested in the main SELECT. Even better, for databases vendors that support Window Functions there is a solution relying on COUNT(*) OVER() as in this application that uses this window function in a native query against MySQL 8. So, prefer this one instead of SELECT COUNT subquery.

Key points:

• create a DTO projection that contains getters for the columns that should be fetched and an extra-column for mapping the return of the COUNT(*) OVER() window function
• write a native query relying on this window function

Example:

72. How To Implement Keyset Pagination in Spring Boot

Description: When we rely on an offset paging we have the performance penalty induced by throwing away n records before reached the desired offset. Larger n leads to a significant performance penalty. When we have a large n is better to rely on keyset pagination which maintain a "constant" time for large datasets. In order to understand how bad offset can perform please check this article:

Screenshot from that article (offset pagination):

Need to know if there are more records?
By its nature, keyset doesn't use a SELECT COUNT to fetch the number of total records. But, with a little tweak, we can easily say if there are more records, therefore to show a button of type Next Page. Mainly, if you need such a thing then consider this application whose climax is listed below:

public AuthorView fetchNextPage(long id, int limit) {
     List<Author> authors = authorRepository.fetchAll(id, limit + 1);

     if (authors.size() == (limit + 1)) {
          authors.remove(authors.size() - 1);
          return new AuthorView(authors, true);
     }

     return new AuthorView(authors, false);
}

Or, like this (rely on Author.toString() method):

public Map<List<Author>, Boolean> fetchNextPage(long id, int limit) {
     List<Author> authors = authorRepository.fetchAll(id, limit + 1);

     if(authors.size() == (limit + 1)) {
          authors.remove(authors.size() -1);
          return Collections.singletonMap(authors, true);
     }

     return Collections.singletonMap(authors, false);
}

A Previous Page button can be implemented easily based on the first record.

Key points:

• choose the column(s) to act as the latest visited record (e.g., id)
• use the column(s) in the WHERE and ORDER BY clauses of your SQL

73. How To Implement Offset Pagination in Spring Boot

Description: This is a classical Spring Boot offset pagination example. However, is not advisable to use this approach in production because of its performance penalties explained further.

When we rely on an offset pagination, we have the performance penalty induced by throwing away n records before reaching the desired offset. Larger n leads to a significant performance penalty. Another penalty is the extra-SELECT needed to count the total number of records. In order to understand how bad offset pagination can perform please check this article. A screenshot from that article is below: Nevertheless, maybe this example is a little bit extreme. For relatively small datasets, offset pagination is not so bad (it is close in performance to keyset pagination), and, since Spring Boot provides built-in support for offset pagination via the Page API, it is very easy to use it. However, depending on the case, we can optimize a little bit the offset pagination as in the following examples:

Fetch a page as a Page:

• Trigger COUNT(*) OVER And Return Page<dto>
• Trigger COUNT(*) OVER And Return Page<entity> Via Extra Column
• Trigger SELECT COUNT Subquery And Return Page<dto>
• Trigger SELECT COUNT Subquery And Return Page<entity> Via Extra Column
• Trigger SELECT COUNT Subquery And Return Page<projection> That Maps Entities And The Total Number Of Records Via Projection

Fetch a page as a List:

• Trigger COUNT(*) OVER And Return List<dto>
• Trigger COUNT(*) OVER And Return List<entity> Via Extra Column
• Trigger SELECT COUNT Subquery And Return List<dto>
• Trigger SELECT COUNT Subquery And Return List<entity> Via Extra Column
• Trigger SELECT COUNT Subquery And Return List<projection> That Maps Entities And The Total Number Of Records Via Projection

But: If offset pagination is causing you performance issues and you decide to go with keyset pagination then please check here (keyset pagination).

Key points of classical offset pagination:

• write a repository that extends PagingAndSortingRepository
• call or write methods that returns Page<entity>

Examples of classical offset pagination:

• call the built-in findAll(Pageable) without sorting:
  repository.findAll(PageRequest.of(page, size));
• call the built-in findAll(Pageable) with sorting:
  repository.findAll(PageRequest.of(page, size, new Sort(Sort.Direction.ASC, "name")));
• use Spring Data query creation to define new methods in your repository:
  Page<Author> findByName(String name, Pageable pageable);
Page<Author> queryFirst10ByName(String name, Pageable pageable);

74. How To Optimize Batch Inserts of Parent-Child Relationships In MySQL

Description: Let's suppose that we have a one-to-many relationship between Author and Book entities. When we save an author, we save his books as well thanks to cascading all/persist. We want to create a bunch of authors with books and save them in the database (e.g., a MySQL database) using the batch technique. By default, this will result in batching each author and the books per author (one batch for the author and one batch for the books, another batch for the author and another batch for the books, and so on). In order to batch authors and books, we need to order inserts as in this application.

Key points: Beside all setting specific to batching inserts in MySQL, we need to set up in application.properties the following property: spring.jpa.properties.hibernate.order_inserts=true

Example without ordered inserts:

Example with ordered inserts:

75. How To Batch Updates In MySQL

Implementations:

• Single entity update
• Parent-child relationship update

Description: Batch updates in MySQL.

Key points:

• in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
• in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL, statements get rewritten into a single string buffer and sent in a single request)
• in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
• in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
• in case of using a parent-child relationship with cascade all/persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_updates=true to optimize the batching by ordering updates
• before Hibernate 5, we need to set in application.properties a setting for enabling batching for versioned entities during update and delete operations (entities that contains @Version for implicit optimistic locking); this setting is: spring.jpa.properties.hibernate.jdbc.batch_versioned_data=true; starting with Hibernate 5, this setting should be true by default

Output example for single entity:

Output example for parent-child relationship:

76. How To Batch Deletes That Don't Involve Associations In MySQL

Description: Batch deletes that don't involve associations in MySQL.

Note: Spring deleteAllInBatch() and deleteInBatch() don't use delete batching and don't take advantage of automatic optimstic locking mechanism to prevent lost updates (e.g., @Version is ignored). They rely on Query.executeUpdate() to trigger bulk operations. These operations are fast, but Hibernate doesn’t know which entities are removed, therefore, the Persistence Context is not updated accordingly (it's up to you to flush (before delete) and close/clear (after delete) the Persistence Context accordingly to avoid issues created by unflushed (if any) or outdated (if any) entities). The first one (deleteAllInBatch()) simply triggers a delete from entity_name statement and is very useful for deleting all records. The second one (deleteInBatch()) triggers a delete from entity_name where id=? or id=? or id=? ... statement, therefore, is prone to cause issues if the generated DELETE statement exceedes the maximum accepted size. This issue can be controlled by deleting the data in chunks, relying on IN operator, and so on. Bulk operations are faster than batching which can be achieved via the deleteAll(), deleteAll(Iterable<? extends T> entities) or delete() method. Behind the scene, the two flavors of deleteAll() relies on delete(). The delete()/deleteAll() methods rely on EntityManager.remove() therefore the Persistence Context is synchronized accordingly. Moreover, if automatic optimstic locking mechanism (to prevent lost updates) is enabled then it will be used.

Key points for regular delete batching:

• for deleting in batches rely on deleteAll(), deleteAll(Iterable<? extends T> entities) or delete() method
• in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
• in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL, statements get rewritten into a single string buffer and sent in a single request)
• in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
• in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
• before Hibernate 5, we need to set in application.properties a setting for enabling batching for versioned entities during update and delete operations (entities that contains @Version for implicit optimistic locking); this setting is: spring.jpa.properties.hibernate.jdbc.batch_versioned_data=true; starting with Hibernate 5, this setting should be true by default

Output example:

77. How To Batch Deletes In MySQL Via orphanRemoval=true

Description: Batch deletes in MySQL via orphanRemoval=true.

Note: Spring deleteAllInBatch() and deleteInBatch() don't use delete batching and don't take advantage of cascading removal, orphanRemoval and automatic optimstic locking mechanism to prevent lost updates (e.g., @Version is ignored). They rely on Query.executeUpdate() to trigger bulk operations. These operations are fast, but Hibernate doesn’t know which entities are removed, therefore, the Persistence Context is not updated accordingly (it's up to you to flush (before delete) and close/clear (after delete) the Persistence Context accordingly to avoid issues created by unflushed (if any) or outdated (if any) entities). The first one (deleteAllInBatch()) simply triggers a delete from entity_name statement and is very useful for deleting all records. The second one (deleteInBatch()) triggers a delete from entity_name where id=? or id=? or id=? ... statement, therefore, is prone to cause issues if the generated DELETE statement exceedes the maximum accepted size. This issue can be controlled by deleting the data in chunks, relying on IN operator, and so on. Bulk operations are faster than batching which can be achieved via the deleteAll(), deleteAll(Iterable<? extends T> entities) or delete() method. Behind the scene, the two flavors of deleteAll() relies on delete(). The delete()/deleteAll() methods rely on EntityManager.remove() therefore the Persistence Context is synchronized accordingly. If automatic optimstic locking mechanism (to prevent lost updates) is enabled then it will be used. Moreover, cascading removals and orphanRemoval works as well.

Key points for using deleteAll()/delete():

• in this example, we have a Author entity and each author can have several Book (one-to-many)
• first, we use orphanRemoval=true and CascadeType.ALL
• second, we dissociate all Book from the corresponding Author
• third, we explicitly (manually) flush the Persistent Context; is time for orphanRemoval=true to enter into the scene; thanks to this setting, all disassociated books will be deleted; the generated DELETE statements are batched (if orphanRemoval is set to false, a bunch of updates will be executed instead of deletes)
• forth, we delete all Author via the deleteAll() or delete() method (since we have dissaciated all Book, the Author deletion will take advantage of batching as well)

78. How To Batch Deletes In MySQL Via SQL ON DELETE CASCADE

Description: Batch deletes in MySQL via ON DELETE CASCADE. Auto-generated database schema will contain the ON DELETE CASCADE directive.

Note: Spring deleteAllInBatch() and deleteInBatch() don't use delete batching and don't take advantage of cascading removal, orphanRemoval and automatic optimistic locking mechanism to prevent lost updates (e.g., @Version is ignored), but both of them take advantage on ON DELETE CASCADE and are very efficient. They trigger bulk operations via Query.executeUpdate(), therefore, the Persistence Context is not synchronized accordingly (it's up to you to flush (before delete) and close/clear (after delete) the Persistence Context accordingly to avoid issues created by unflushed (if any) or outdated (if any) entities). The first one simply triggers a delete from entity_name statement, while the second one triggers a delete from entity_name where id=? or id=? or id=? ... statement. For delete in batches rely on deleteAll(), deleteAll(Iterable<? extends T> entities) or delete() method. Behind the scene, the two flavors of deleteAll() relies on delete(). Mixing batching with database automatic actions (ON DELETE CASCADE) will result in a partially synchronized Persistent Context.

Key points:

• in this application, we have a Author entity and each author can have several Book (one-to-many)
• first, we remove orphanRemoval or set it to false
• second, we use only CascadeType.PERSIST and CascadeType.MERGE
• third, we set @OnDelete(action = OnDeleteAction.CASCADE) next to @OneToMany
• fourth, we set spring.jpa.properties.hibernate.dialect to org.hibernate.dialect.MySQL5InnoDBDialect (or, MySQL8Dialect)
• fifth, we run through a set of deleteFoo() methods that uses bulk and batching deletes as well

Output example:

79. How To Use Hibernate @NaturalId In Spring Boot Style

Alternative implementation: In case that you want to avoid extending SimpleJpaRepository check this implementation.

Description: This is a SpringBoot application that maps a natural business key using Hibernate @NaturalId. This implementation allows us to use @NaturalId as it was provided by Spring.

Key points:

• in the entity (e.g., Book), mark the properties (business keys) that should act as natural IDs with @NaturalId; commonly, there is a single such property, but multiple are suppored as well as here
• for non-mutable ids, mark the columns as @NaturalId(mutable = false) and @Column(nullable = false, updatable = false, unique = true, ...)
• for mutable ids, mark the columns as @NaturalId(mutable = true) and @Column(nullable = false, updatable = true, unique = true, ...)
• override the equals() and hashCode() using the natural id(s)
• define a @NoRepositoryBean interface (NaturalRepository) to define two methods, named findBySimpleNaturalId() and findByNaturalId()
• provide an implementation for this interface (NaturalRepositoryImpl) relying on Hibernate, Session, bySimpleNaturalId() and byNaturalId() methods
• use @EnableJpaRepositories(repositoryBaseClass = NaturalRepositoryImpl.class) to register this implementation as the base class
• for the entity, write a classic repository
• inject this class in your services and call findBySimpleNaturalId() or findByNaturalId()

80. How To Set Up P6Spy in Spring Boot

Description: This is a Spring Boot application that uses P6Spy. P6Spy is a framework that enables database data to be seamlessly intercepted and logged with no code changes to the application.

Key points:

• in pom.xml, add the P6Spy Maven dependency
• in application.properties, set up JDBC URL as, jdbc:p6spy:mysql://localhost:3306/db_users
• in application.properties, set up driver class name as, com.p6spy.engine.spy.P6SpyDriver
• in the application root folder add the file spy.properties (this file contains P6Spy configurations); in this application, the logs will be outputed to console, but you can easy switch to a file; more details about P6Spy configurations can be found in documentation

Output sample:

81. How To Retry Transactions After OptimisticLockException Exception (@Version)

Note: Optimistic locking mechanism via @Version works for detached entities as well.

Description: This is a Spring Boot application that simulates a scenario that leads to an optimistic locking exception. When such exception occur, the application retry the corresponding transaction via db-util library developed by Vlad Mihalcea.

Key points:

• for Maven, in pom.xml, add the db-util dependency
• configure the OptimisticConcurrencyControlAspect bean
• mark the method (not annotated with @Transactional) that is prone to throw (or that calls a method that is prone to throw (this method can be annotated with @Transactional)) an optimistic locking exception with @Retry(times = 10, on = OptimisticLockingFailureException.class)

Output sample:

82. How To Retry Transaction After OptimisticLockException Exception (Hibernate Version-less Optimistic Locking Mechanism)

Note: Optimistic locking mechanism via Hibernate version-less doesn't work for detached entities (don't close the Persistent Context).

Description: This is a Spring Boot application that simulates a scenario that leads to an optimistic locking exception (e.g., in Spring Boot, OptimisticLockingFailureException) via Hibernate version-less optimistic locking. When such exception occur, the application retry the corresponding transaction via db-util library developed by Vlad Mihalcea.

Key points:

• for Maven, in pom.xml, add the db-util library dependency
• configure the OptimisticConcurrencyControlAspect bean
• annotate the corresponding entity (e.g., Inventory) with @DynamicUpdate and @OptimisticLocking(type = OptimisticLockType.DIRTY)
• mark the method (not annotated with @Transactional) that is prone to throw (or that calls a method that is prone to throw (this method can be annotated with @Transactional)) an optimistic locking exception with @Retry(times = 10, on = OptimisticLockingFailureException.class)

83. How To Enrich DTO With Virtual Properties Via Spring Projections

Note: You may also like to read the recipe, "How To Create DTO Via Spring Data Projections"

Description: This is an application sample that fetches only the needed columns from the database via Spring Data Projections (DTO) and enrich the result via virtual properties.

Key points:

• we fetch from the database only the author name and age
• in the projection interface, AuthorNameAge, use the @Value and Spring SpEL to point to a backing property from the domain model (in this case, the domain model property age is exposed via the virtual property years)
• in the projection interface, AuthorNameAge, use the @Value and Spring SpEL to enrich the result with two virtual properties that don't have a match in the domain model (in this case, rank and books)

Output example:

84. How To Use Query Creation Mechanism For JPA To Limit Result Size

Description: Spring Data comes with the query creation mechanism for JPA that is capable to interpret a query method name and convert it into a SQL query in the proper dialect. This is possible as long as we respect the naming conventions of this mechanism. This is an application that exploit this mechanism to write queries that limit the result size. Basically, the name of the query method instructs Spring Data how to add the LIMIT (or similar clauses depending on the RDBMS) clause to the generated SQL queries.

Key points:

• define a Spring Data regular repository (e.g., AuthorRepository)
• write query methods respecting the query creation mechanism for JPA naming conventions

Examples:
- List<Author> findFirst5ByAge(int age);
- List<Author> findFirst5ByAgeGreaterThanEqual(int age);
- List<Author> findFirst5ByAgeLessThan(int age);
- List<Author> findFirst5ByAgeOrderByNameDesc(int age);
- List<Author> findFirst5ByGenreOrderByAgeAsc(String genre);
- List<Author> findFirst5ByAgeGreaterThanEqualOrderByNameAsc(int age);
- List<Author> findFirst5ByGenreAndAgeLessThanOrderByNameDesc(String genre, int age);
- List<AuthorDto> findFirst5ByOrderByAgeAsc();
- Page<Author> queryFirst10ByName(String name, Pageable p);
- Slice<Author> findFirst10ByName(String name, Pageable p);

The list of supported keywords is listed below:

85. How To Generate A Schema Via schema-*.sql In MySQL

Note: As a rule, in real applications avoid generating schema via hibernate.ddl-auto or set it to validate. Use schema-*.sql file or better Flyway or Liquibase migration tools.

Description: This application is an example of using schema-*.sql to generate a schema(database) in MySQL.

Key points:

• in application.properties, set the JDBC URL (e.g., spring.datasource.url=jdbc:mysql://localhost:3306/bookstoredb?createDatabaseIfNotExist=true)
• in application.properties, disable DDL auto (just don't add explicitly the hibernate.ddl-auto setting)
• in application.properties, instruct Spring Boot to initialize the schema from schema-mysql.sql file

86. How To Generate Two Databases Via schema-*.sql And Match Entities To Them Via @Table In MySQL

Note: As a rule, in real applications avoid generating schema via hibernate.ddl-auto or set it to validate. Use schema-*.sql file or better Flyway or Liquibase.

Description: This application is an example of using schema-*.sql to generate two databases in MySQL. The databases are matched at entity mapping via @Table.

Key points:

• in application.properties, set the JDBC URL without the database, e.g., spring.datasource.url=jdbc:mysql://localhost:3306
• in application.properties, disable DDL auto (just don't specify hibernate.ddl-auto)
• in aaplication.properties, instruct Spring Boot to initialize the schema from schema-mysql.sql file
• in Author entity, specify that the corresponding table (author) is in the database authorsdb via @Table(schema="authorsdb")
• in Book entity, specify that the corresponding table (book) is in the database booksdb via @Table(schema="booksdb")

Output example:

• Persisting a Author results in the following SQL: insert into authorsdb.author (age, genre, name) values (?, ?, ?)
• Persisting a Book results the following SQL: insert into booksdb.book (isbn, title) values (?, ?)

87. How To Stream Result Set Via Spring Data In MySQL

Note: For web-applications, pagination should be the way to go, not streaming. But, if you choose streaming then keep in mind the golden rule: keep th result set as small as posible. Also, keep in mind that the Execution Plan might not be as efficient as when using SQL-level pagination.

Description: This application is an example of streaming the result set via Spring Data and MySQL. This example can be adopted for databases that fetches the entire result set in a single roundtrip causing performance penalties.

Key points:

• rely on forward-only result set (default in Spring Data)
• rely on read-only statement (add @Transactional(readOnly=true))
• set the fetch-size set (e.g. 30, or row-by-row; Integer.MIN_VALUE (recommended in MySQL))
• for MySQL, set Statement fetch-size to Integer.MIN_VALUE, or add useCursorFetch=true to the JDBC URL and set Statement fetch-size to a positive integer (e.g., 30)

88. How To Migrate MySQL Database Using Flyway - MySQL Database Created Via createDatabaseIfNotExist

Note: For production, don't rely on hibernate.ddl-auto (or counterparts) to export schema DDL to the database. Simply remove (disable) hibernate.ddl-auto or set it to validate. Rely on Flyway or Liquibase.

Description: This application is an example of migrating a MySQL database via Flyway when the database exists (it is created before migration via MySQL specific parameter, createDatabaseIfNotExist=true).

Key points:

• for Maven, in pom.xml, add the Flyway dependency
• remove (disable) spring.jpa.hibernate.ddl-auto
• in application.properties, set the JDBC URL as follows: jdbc:mysql://localhost:3306/bookstoredb?createDatabaseIfNotExist=true
• each SQL file containing the schema update add it in classpath:db/migration
• each SQL file name it as V1.1__Description.sql, V1.2__Description.sql, ...

89. How To Migrate MySQL Database Using Flyway - Database Created Via spring.flyway.schemas

Note: For production, don't rely on hibernate.ddl-auto (or counterparts) to export schema DDL to the database. Simply remove (disable) hibernate.ddl-auto or set it to validate. Rely on Flyway or Liquibase.

Description: This application is an example of migrating a MySQL database when the database is created by Flyway via spring.flyway.schemas. In this case, the entities should be annotated with @Table(schema = "bookstoredb") or @Table(catalog = "bookstoredb"). Here, the database name is bookstoredb.

Key points:

• for Maven, in pom.xml, add the Flyway dependency
• remove (disable) spring.jpa.hibernate.ddl-auto
• in application.properties, set the JDBC URL as follows: jdbc:mysql://localhost:3306/
• in application.properties, add spring.flyway.schemas=bookstoredb, where bookstoredb is the database that should be created by Flyway (feel free to add your own database name)
• each entity that should be stored in this database should be annotated with, @Table(schema/catalog = "bookstoredb")
• each SQL file containing the schema update add it in classpath:db/migration
• each SQL file name it as V1.1__Description.sql, V1.2__Description.sql, ...

Output of migration history example:

90. How To Auto-Create And Migrate Schemas For Two Data Sources (MySQL and PostgreSQL) Using Flyway

Note: For production don't rely on hibernate.ddl-auto to create your schema. Remove (disable) hibernate.ddl-auto or set it to validate. Rely on Flyway or Liquibase.

Description: This application is an example of auto-creating and migrating schemas for MySQL and PostgreSQL. In addition, each data source uses its own HikariCP connection pool. In case of MySQL, where schema=database, we auto-create the schema (authorsdb) based on createDatabaseIfNotExist=true. In case of PostgreSQL, where a database can have multiple schemas, we use the default postgres database and auto-create in it the schema, booksdb. For this we rely on Flyway, which is capable to create a missing schema.

Key points:

• for Maven, in pom.xml, add the Flyway dependency
• remove (disable) spring.jpa.hibernate.ddl-auto or set it to validate
• in application.properties, configure the JDBC URL for MySQL as, jdbc:mysql://localhost:3306/authorsdb?createDatabaseIfNotExist=true and for PostgreSQL as, jdbc:postgresql://localhost:5432/postgres?currentSchema=booksdb
• in application.properties, set spring.flyway.enabled=false to disable default behavior
• programmatically create a DataSource for MySQL and one for PostgreSQL
• programmatically create a FlywayDataSource for MySQL and one for PostgreSQL
• programmatically create an EntityManagerFactory for MySQL and one for PostgreSQL
• for MySQL, place the migration SQLs files in db\migration\mysql
• for PostgreSQL, place the migration SQLs files in db\migration\postgresql

91. How To Auto-Create And Migrate Two Schemas In PostgreSQL Using Flyway

Note: For production, don't rely on hibernate.ddl-auto (or counterparts) to export schema DDL to the database. Simply remove (disable) hibernate.ddl-auto or set it to validate. Rely on Flyway or Liquibase.

Description: This application is an example of auto-creating and migrating two schemas in PostgreSQL using Flyway. In addition, each data source uses its own HikariCP connection pool. In case of PostgreSQL, where a database can have multiple schemas, we use the default postgres database and auto-create two schemas, authors and books. For this we rely on Flyway, which is capable to create the missing schemas.

Key points:

• for Maven, in pom.xml, add the Flyway dependency
• remove (disable) spring.jpa.hibernate.ddl-auto or set it to validate
• in application.properties, configure the JDBC URL for books as jdbc:postgresql://localhost:5432/postgres?currentSchema=books and for authors as jdbc:postgresql://localhost:5432/postgres?currentSchema=authors
• in application.properties, set spring.flyway.enabled=false to disable default behavior
• programmatically create two DataSource, one for books and one for authors
• programmatically create two FlywayDataSource, one for books and one for authors
• programmatically create two EntityManagerFactory, one for books and one for authors
• for books, place the migration SQLs files in db\migration\books
• for authors, place the migration SQLs files in db\migration\authors

92. How To JOIN FETCH an @ElementCollection

Description: This application is an example applying JOIN FETCH to fetch an @ElementCollection.

Key points:

• by default, @ElementCollection is loaded lazy, keep it lazy
• use JOIN FETCH in the repository

93. How To Map An Entity To a Query (@Subselect) in a Spring Boot Application

Note: Consider using @Subselect only if using DTO, DTO and extra queries, or map a database view to an entity is not a solution.

Description: This application is an example of mapping an entity to a query via Hibernate, @Subselect. Mainly, we have two entities in a bidirectional one-to-many association. An Author has wrote several Book. The idea is to write a read-only query to fetch from Author only some fields (e.g., DTO), but to have the posibility to call getBooks() and fetch the Book in a lazy manner as well. As you know, a classic DTO cannot be used, since such DTO is not managed and we cannot navigate the associations (don’t support any managed associations to other entities). Via Hibernate @Subselect we can map a read-only and immutable entity to a query. This time, we can lazy navigate the associations.

Key points:

• define a new entity that contains only the needed fields from the Author (including association to Book)
• for these fields, define only getters
• mark the entity as @Immutable since no write operations are allowed
• flush pending state transitions for the used entities by @Synchronize
• use @Subselect to write the needed query, map an entity to an SQL query

94. How To Use Hibernate Soft Deletes In A Spring Boot Application

Description: This application is an example of using Hibernate soft deletes in a Spring Boot application.

Key points:

• define an abstract class BaseEntity with a field named deleted
• the entities (e.g., Author and Book entities) that should take advantage of soft deletes should extend BaseEntity
• these entities should be marked with Hibernate, @Where annotation like this: @Where(clause = "deleted = false")
• these entities should be marked with Hibernate, @SQLDelete annotation to trigger UPDATE SQLs in place of DELETE SQLs, as follows: @SQLDelete(sql = "UPDATE author SET deleted = true WHERE id = ?")
• for fetching all entities including those marked as deleted or for fetching only the entities marked as deleted we need to rely on SQL native queries

Output example:

95. How To Programmatically Customize HikariCP Settings Via DataSourceBuilder

If you use the spring-boot-starter-jdbc or spring-boot-starter-data-jpa "starters", you automatically get a dependency to HikariCP

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that set up HikariCP via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purposes, the application uses an ExecutorService for simulating concurrent users. Check the HickariCP report revealing the connection pool status.

Key points:

• write a @Bean that returns the DataSource programmatically

96. How To Setup Spring Data JPA Auditing

Description: Auditing is useful for maintaining history records. This can later help us in tracking user activities.

Key points:

• create an abstract base entity (e.g., BaseEntity) and annotate it with @MappedSuperclass and @EntityListeners({AuditingEntityListener.class})
• in this base entity, add the following fields that will be automatically persisted:
        - @CreatedDate protected LocalDateTime created;
        - @LastModifiedDate protected LocalDateTime lastModified;
        - @CreatedBy protected U createdBy;
        - @LastModifiedBy protected U lastModifiedBy;
• enable auditing via @EnableJpaAuditing(auditorAwareRef = "auditorAware")
• provide an implementation for AuditorAware (this is needed for persisting the user that performed the modification; use Spring Security to return the currently logged-in user)
• expose this implementation via @Bean
• entites that should be audited should extend the base entity
• store the date-time in database in UTC

97. Hibernate Envers Auditing (spring.jpa.hibernate.ddl-auto=create)

Description: Auditing is useful for maintaining history records. This can later help us in tracking user activities.

Key points:

• each entity that should be audited should be annotated with @Audited
• optionally, annotate entities with @AuditTable to rename the table used for auditing
• rely on ValidityAuditStrategy for fast database reads, but slower writes (slower than the default DefaultAuditStrategy)

98. Attributes Lazy Loading Via Subentities

Description: By default, the attributes of an entity are loaded eager (all at once). This application is an alternative to How To Use Hibernate Attribute Lazy Loading from here. This application uses a base class to isolate the attributes that should be loaded eagerly and subentities (entities that extends the base class) for isolating the attributes that should be loaded on demand.

Key points:

• create the base class (this is not an entity), BaseAuthor, and annotate it with @MappedSuperclass
• create AuthorShallow subentity of BaseAuthor and don't add any attribute in it (this will inherit the attributes from the superclass)
• create AuthorDeep subentity of BaseAuthor and add to it the attributes that should be loaded on demand (e.g., avatar)
• map both subentities to the same table via @Table(name = "author")
• provide the typical repositories, AuthorShallowRepository and AuthorDeepRepository

Run the following requests (via BookstoreController):

• fetch all authors shallow (without avatars): localhost:8080/authors/shallow
• fetch all authors deep (with avatars): localhost:8080/authors/deep

Check as well:

• Attribute Lazy Loading (basic)
• Default Values For Lazy Loaded Attributes
• Attribute Lazy Loading And Jackson Serialization

99. DTO Via Constructor And Spring Data Query Builder Mechanism

Description: Fetching more data than needed is prone to performance penalities. Using DTO allows us to extract only the needed data. In this application we rely on constructor and Spring Data Query Builder Mechanism.

Key points:

• write a proper constructor in the DTO class
• rely on Spring Data Query Builder Mechanism to write the SQL
• for using Spring Data Projections check this item

See also:
Dto Via Constructor Expression and JPQL

100. How To Page The Result Set of a JOIN

Description: Using JOIN is very useful for fetching DTOs (data that is never modified, not in the current or subsequent requests). For example, consider two entities, Author and Book in a lazy-bidirectional @OneToMany association. And, we want to fetch a subset of columns from the parent table (author) and a subset of columns from the child table (book). This job is a perfect fit for JOIN which can pick up columns from different tables and build a raw result set. This way we fetch only the needed data. Moreover, we may want to serve the result set in pages (e.g., via LIMIT). This application contains several approaches for accomplishing this task with offset pagination.

Key points:

• pagination via Page (with SELECT COUNT and COUNT(*) OVER() window function)
• pagination via Slice and List
• pagination via DENSE_RANK() for avoiding the truncation of the result set (an author can be fetched with only a subset of his books)

101. LEFT JOIN FETCH

See also:

• How To Avoid LazyInitializationException Via JOIN FETCH
• JOIN VS. JOIN FETCH

Description: Let's assume that we have two entities engaged in a one-to-many (or many-to-many) lazy bidirectional (or unidirectional) relationship (e.g., Author has more Book). And, we want to trigger a single SELECT that fetches all Author and the corresponding Book. This is a job for JOIN FETCH which is converted behind the scene into a INNER JOIN. Being an INNER JOIN, the SQL will return only Author that have Book. If we want to return all Author, including those that doesn't have Book, then we can rely on LEFT JOIN FETCH. Similar, we can fetch all Book, including those with no registered Author. This can be done via LEFT JOIN FETCH or LEFT JOIN.

Key points:

• define two related entities (e.g., Author and Book in a one-to-many lazy bidirectional relationship)
• write a JPQL LEFT JOIN FETCH to fetch all authors and books (fetch authors even if they don't have registered books)
• write a JPQL LEFT JOIN FETCH to fetch all books and authors (fetch books even if they don't have registered authors)

102. JOIN VS. JOIN FETCH

See also:

• How To Avoid LazyInitializationException Via JOIN FETCH
• LEFT JOIN FETCH

Description: This is an application meant to reveal the differences between JOIN and JOIN FETCH. The important thing to keep in mind is that, in case of LAZY fetching, JOIN will not be capable to initialize the associated collections along with their parent objects using a single SQL SELECT. On the other hand, JOIN FETCH is capable to accomplish this kind of task. But, don't underestimate JOIN, because JOIN is the proper choice when we need to combine/join the columns of two (or more) tables in the same query, but we don't need to initialize the associated collections on the returned entity (e.g., very useful for fetching DTO).

Key points:

• define two related entities (e.g., Author and Book in a one-to-many lazy-bidirectional relationship)
• write a JPQL JOIN and JOIN FETCH to fetch an author including his books
• write a JPQL JOIN to fetch a book (1)
• write a JPQL JOIN to fetch a book including its author (2)
• write a JOIN FETCH to fetch a book including its author

Notice that:

• via JOIN, fetching Book of Author requires additional SELECT statements being prone to N+1 performance penalty
• via JOIN (1), fetching Author of Book requires additional SELECT statements being prone to N+1 performance penalty
• via JOIN (2), fetching Author of Book works exactly as JOIN FETCH (requires a single SELECT)
• via JOIN FETCH, fetching each Author of a Book requires a single SELECT

103. Entity Inside Spring Projection

Description: If, for some reason, you need an entity in your Spring projection (DTO), then this application shows you how to do it via an example. In this case, there are two entities, Author and Book, involved in a lazy bidirectional one-to-many association (it can be other association as well, or even no materialized association). And, we want to fetch in a Spring projection the authors as entities, Author, and the title of the books.

Key points:

• define two related entities (e.g., Author and Book in a one-to-many lazy bidirectional relationship)
• define the proper Spring projection having public Author getAuthor() and public String getTitle()
• write a JPQL to fetch data

104. Entity Inside Spring Projection (no association)

Description: If, for some reason, you need an entity in your Spring projection (DTO), then this application shows you how to do it via an example. In this case, there are two entities, Author and Book, that have no materialized association between them, but, they share the genre attribute. We use this attribute to join authors with books via JPQL. And, we want to fetch in a Spring projection the authors as entities, Author, and the title of the books.

Key points:

• define two unrelated entities (e.g., Author and Book)
• define the proper Spring projection having public Author getAuthor() and public String getTitle()
• write a JPQL to fetch data

105. Avoid Entity In DTO Via Constructor Expression (no association)

Description: Let's assume that we have two entities, Author and Book. There is no materialized association between them, but, both entities shares an attribute named, genre. We want to use this attribute to join the tables corresponding to Author and Book, and fetch the result in a DTO. The result should contain the Author entity and only the title attribute from Book. Well, when you are in a scenario as here, it is strongly advisable to avoid fetching the DTO via constructor expression. This approach cannot fetch the data in a single SELECT, and is prone to N+1. Way better than this consists of using Spring projections, JPA Tuple or even Hibernate ResultTransformer. These approaches will fetch the data in a single SELECT. This application is a DON'T DO THIS example. Check the number of queries needed for fetching the data. In place, do it as here: Entity Inside Spring Projection (no association).

106. How To DTO an @ElementCollection

Description: This application is an example of fetching a DTO that includes attributes from an @ElementCollection.

Key points:

• by default, @ElementCollection is loaded lazy, keep it lazy
• use a Spring projection and JOIN in the repository

107. Ordering The Set Of Associated Entities In @ManyToMany Association Via @OrderBy

Description: In case of @ManyToMany association, we always should rely on Set (not on List) for mapping the collection of associated entities (entities of the other parent-side). Why? Well, please see Prefer Set Instead of List in @ManyToMany Relationships. But, is well-known that HashSet doesn't have a predefined entry order of elements. If this is an issue then this application relies on @OrderBy which adds an ORDER BY clause in the SQL statement. The database will handle the ordering. Further, Hibernate will preserve the order via a LinkedHashSet.

This application uses two entities, Author and Book, involved in a lazy bidirectional many-to-many relationship. First, we fetch a Book by title. Further, we call getAuthors() to fetch the authors of this book. The fetched authors are ordered descending by name. The ordering is done by the database as a result of adding @OrderBy("name DESC"), and is preserved by Hibernate.

Key points:

• ask the database to handle ordering and Hibernate to preserve this order via @OrderBy
• this works with HashSet, but doesn't provide consistency across all transition states (e.g., transient state)
• for consistency across the transient state as well, consider using explicitly LinkedHashSet instead of HashSet

Note: Alternatively, we can use @OrderColumn. This gets materialized in an additional column in the junction table. This is needed for maintaining a permanent ordering of the related data.

108. Versioned Optimistic Locking And Detached Entities Sample

Description: This is a sample application that shows how versioned (@Version) optimistic locking and detached entity works. Running the application will result in an optimistic locking specific exception (e.g., the Spring Boot specific, OptimisticLockingFailureException).

Key points:

• in a transaction, fetch an entity via findById(1L); commit transaction and close the Persistence Context
• in a second transaction, fetch another entity via findById(1L) and update it; commit the transaction and close the Persistence Context
• outside transactional context, update the detached entity (fetched in the first transaction)
• in a third transaction, call save() and pass to it the detached entity; trying to merge (EntityManager.merge()) the entity will end up in an optimistic locking exception since the version of the detached and just loaded entity don't match

109. How To Simulate OptimisticLockException Shaped Via @Version

Note: Optimistic locking via @Version works for detached entities as well.

Description: This is a Spring Boot application that simulates a scenario that leads to an optimistic locking exception. So, running the application should end up with a Spring specific ObjectOptimisticLockingFailureException exception.

Key points:

• set up versioned optimistic locking mechanism
• rely on two concurrent threads that call the same @Transactional method used for updating data

110. How To Retry Transaction Via TransactionTemplate After OptimisticLockException Exception (@Version)

Note: Optimistic locking via @Version works for detached entities as well.

Description: This is a Spring Boot application that simulates a scenario that leads to an optimistic locking exception. When such exception occurs, the application retry the corresponding transaction via db-util library developed by Vlad Mihalcea.

Key points:

• in pom.xml, add the db-util dependency
• configure the OptimisticConcurrencyControlAspect bean
• rely on TransactionTemplate

111. How To Simulate OptimisticLockException In Version-less Optimistic Locking

Note: Version-less optimistic locking doesn't work for detached entities (do not close the Persistence Context).

Description: This is a Spring Boot application that simulates a scenario that leads to an optimistic locking exception. So, running the application should end up with a Spring specific ObjectOptimisticLockingFailureException exception.

Key points:

• set up the version-less optimistic locking mechanism
• rely on two concurrent threads that call the same a @Transactional method used for updating data

112. How To Retry Transaction Via TransactionTemplate After OptimisticLockException Shaped Via Hibernate Version-less Optimistic Locking Mechanism

Note: Version-less optimistic locking doesn't work for detached entities (do not close the Persistence Context).

Description: This is a Spring Boot application that simulates a scenario that leads to an optimistic locking exception. When such exception occur, the application retry the corresponding transaction via db-util library developed by Vlad Mihalcea.

Key points:

• for Maven, in pom.xml, add the db-util dependency
• configure the OptimisticConcurrencyControlAspect bean
• rely on TransactionTemplate

113. HTTP Long Conversation Via Versioned Optimistic Locking And Detached Entities In The HTTP Session

Description: This is a sample application that shows how to take advantage of versioned optimistic locking and detached entities in HTTP long conversations. The climax consists of storing the detached entities across multiple HTTP requests. Commonly, this can be accomplished via HTTP session.

Key points:

• prepare the entity via @Version
• rely on @SessionAttributes for storing the detached entities

Sample output (check the message caused by optimistic locking exception):

114. Filter Association Via Hibernate @Where

Note: Rely on this approach only if you simply cannot use JOIN FETCH WHERE or @NamedEntityGraph.

Description: This application is a sample of using Hibernate @Where for filtering associations.

Key points:

• use @Where(clause = "condition to be met") in entity (check the Author entity)

115. Batch Inserts In Spring Boot Style

Description: Batch inserts (in MySQL) in Spring Boot style.

Key points:

• in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
• in application.properties set spring.jpa.properties.hibernate.generate_statistics (just to check that batching is working)
• in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL)
• in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
• in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
• in case of using a parent-child relationship with cascade persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_inserts=true to optimize the batching by ordering inserts
• in entity, use the assigned generator since the Hibernate IDENTITY will cause insert batching to be disabled
• if is not needed then ensure that Second Level Cache is disabled via spring.jpa.properties.hibernate.cache.use_second_level_cache=false

Output example:

116. Offset Pagination - Trigger COUNT(*) OVER And Return Page<entity> Via Extra Column

Description: Typically, in offset pagination, there is one query needed for fetching the data and one for counting the total number of records. But, we can fetch this information in a single database rountrip via a SELECT COUNT subquery nested in the main SELECT. Even better, for databases vendors that support Window Functions there is a solution relying on COUNT(*) OVER() as in this application that uses this window function in a native query against MySQL 8. So, prefer this one instead of SELECT COUNT subquery.This application fetches data as Page<entity> via Spring Boot offset pagination, but, if the fetched data is read-only, then rely on Page<dto> as here.

Key points:

• write a repository that extends PagingAndSortingRepository
• in the entity, add an extra column for representing the total number of records and annotate it as @Column(insertable = false, updatable = false)
• fetch data via a native query (that includes counting) into a List<entity>
• use the fetched List<entity> and Pageable to create a Page<entity>

117. Offset Pagination - Trigger SELECT COUNT Subquery And Return List<entity> Via Extra Column

Description: This application fetches data as List<entity> via Spring Boot offset pagination. The SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, one query is needed for fetching the data and one for counting the total number of records).

Key points:

• write a repository that extends PagingAndSortingRepository
• in the entity, add an extra column for representing the total number of records and annotate it as @Column(insertable = false, updatable = false)
• fetch data via a native query (that includes SELECT COUNT subquery) into a List<entity>

118. Offset Pagination - Trigger SELECT COUNT Subquery And Return List<projection> That Maps Entities And The Total Number Of Records Via Projection

Description: This application fetches data as List<projection> via Spring Boot offset pagination. The projection maps the entity and the total number of records. This information is fetched in a single database rountrip because the SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, there is one query needed for fetching the data and one for counting the total number of records). Use this approch only if the fetched data is not read-only. Otherwise, prefer List<dto> as here.

Key points:

• write a Spring projection that maps the entity and the total number of records
• write a repository that extends PagingAndSortingRepository
• fetch data via a JPQL query (that includes SELECT COUNT subquery) into a List<projection>

119. Offset Pagination - Trigger COUNT(*) OVER And Return List<entity> Via Extra Column

Description: Typically, in offset pagination, there is one query needed for fetching the data and one for counting the total number of records. But, we can fetch this information in a single database rountrip via a SELECT COUNT subquery nested in the main SELECT. Even better, for databases vendors that support Window Functions there is a solution relying on COUNT(*) OVER() as in this application that uses this window function in a native query against MySQL 8. So, prefer this one instead of SELECT COUNT subquery.This application fetches data as List<entity> via Spring Boot offset pagination, but, if the fetched data is read-only, then rely on List<dto> as here.

Key points:

• write a repository that extends PagingAndSortingRepository
• in the entity, add an extra column for representing the total number of records and annotate it as @Column(insertable = false, updatable = false)
• fetch data via a native query (that includes COUNT(*) OVER subquery) into a List<entity>

120. Offset Pagination - Trigger SELECT COUNT Subquery And Return Page<entity> Via Extra Column

Description: This application fetches data as Page<entity> via Spring Boot offset pagination. Use this only if the fetched data will be modified. Otherwise, fetch Page<dto> as here. The SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT. Therefore, there will be a single database roundtrip instead of two (typically, there is one query needed for fetching the data and one for counting the total number of records).

Key points:

• write a repository that extends PagingAndSortingRepository
• in the entity, add an extra column for representing the total number of records and annotate it as @Column(insertable = false, updatable = false)
• fetch data via a native query (that includes counting) into a List<entity>
• use the fetched List<entity> and Pageable to create a Page<entity>

121. Offset Pagination - Trigger SELECT COUNT Subquery And Return Page<projection> That Maps Entities And The Total Number Of Records Via Projection

Description: This application fetches data as Page<projection> via Spring Boot offset pagination. The projection maps the entity and the total number of records. This information is fetched in a single database rountrip because the SELECT COUNT triggered for counting the total number of records is a subquery of the main SELECT.

Key points:

• define a Spring projection that maps the entity and the total number of records
• write a repository that extends PagingAndSortingRepository
• fetch data via a JPQL query into a List<projection>
• use the fetched List<projection> and Pageable to create a Page<projection>

122. Offset Pagination - Trigger COUNT(*) OVER And Return Page<dto>

Description: Typically, in offset pagination, there is one query needed for fetching the data and one for counting the total number of records. But, we can fetch this information in a single database rountrip via a SELECT COUNT subquery nested in the main SELECT. Even better, for databases vendors that support Window Functions there is a solution relying on COUNT(*) OVER() as in this application that uses this window function in a native query against MySQL 8. So, prefer this one instead of SELECT COUNT subquery. This application return a Page<dto>.

Key points:

• create a Spring projection (DTO) to contains getters only for the columns that should be fetched
• write a repository that extends PagingAndSortingRepository
• fetch data via a native query (that includes counting) into a List<dto>
• use the fetched List<dto> and Pageable to create a Page<dto>

Example:

123. How To Fetch Slice<entity>/Slice<dto> Via fetchAll/fetchAllDto

Story: Spring Boot provides an offset based built-in paging mechanism that returns a Page or Slice. Each of these APIs represents a page of data and some metadata. The main difference is that Page contains the total number of records, while Slice can only tell if there is another page available. For Page, Spring Boot provides a findAll() method capable to take as arguments a Pageable and/or a Specification or Example. In order to create a Page that contains the total number of records, this method triggers an SELECT COUNT extra-query next to the query used to fetch the data of the current page . This can be a performance penalty since the SELECT COUNT query is triggered every time we request a page. In order to avoid this extra-query, Spring Boot provides a more relaxed API, the Slice API. Using Slice instead of Page removes the need of this extra SELECT COUNT query and returns the page (records) and some metadata without the total number of records. So, while Slice doesn't know the total number of records, it still can tell if there is another page available after the current one or this is the last page. The problem is that Slice work fine for queries containing the SQL, WHERE clause (including those that uses the query builder mechanism built into Spring Data), but it doesn't work for findAll(). This method will still return a Page instead of Slice therefore the SELECT COUNT query is triggered for Slice<T> findAll(...);.

Workaround: The trick is to simply define a method named fetchAll() that uses JPQL and Pageable to return Slice<entity>, and a method named fetchAllDto() that uses JPQL and Pageable as well to return Slice<dto>. So, avoid naming the method findAll().

Usage example:
public Slice<Author> fetchNextSlice(int page, int size) {
    return authorRepository.fetchAll(PageRequest.of(page, size, new Sort(Sort.Direction.ASC, "age")));
}

public Slice<AuthorDto> fetchNextSliceDto(int page, int size) {
    return authorRepository.fetchAllDto(PageRequest.of(page, size, new Sort(Sort.Direction.ASC, "age")));
}

124. How To Use Spring Projections(DTOs) And Inclusive Full Joins (MySQL)

Description: This application is a proof of concept for using Spring Projections(DTO) and inclusive full joins written in native SQL (for MySQL).

Key points:

• define two entities (e.g., Author and Book in a lazy bidirectional @OneToMany relationship)
• populate the database with some test data (e.g., check the file resources/data-mysql.sql)
• write interfaces (projections) that contains getters for the columns that should be fetched from the database (e.g., check AuthorNameBookTitle.java)
• write inclusive full joins queries using native SQL

125. How To Declare Immutable Entities And Store Them In Second Level Cache (e.g., EhCache)

Description: This application is a sample of declaring an immutable entity. Moreover, the immutable entity will be stored in Second Level Cache via EhCache implementation.

Key points of declaring an immutable entity:

• annotate the entity with @Immutable (org.hibernate.annotations.Immutable)
• avoid any kind of associations
• set hibernate.cache.use_reference_entries configuration to true

126. How To Programmatically Customize HikariCP Settings Via DataSourceBuilder

If you use the spring-boot-starter-jdbc or spring-boot-starter-data-jpa "starters", you automatically get a dependency to HikariCP

Note: The best way to tune the connection pool parameters consist in using Flexy Pool by Vlad Mihalcea. Via Flexy Pool you can find the optim settings that sustain high-performance of your connection pool.

Description: This is a kickoff application that set up HikariCP via DataSourceBuilder. The jdbcUrl is set up for a MySQL database. For testing purposes, the application uses an ExecutorService for simulating concurrent users. Check the HickariCP report revealing the connection pool status.

Key points:

• write a @Bean that returns the DataSource programmatically

Output sample:

127. How To Use Hibernate @NaturalIdCache For Skipping The Entity Identifier Retrieval

Description: This is a SpringBoot - MySQL application that maps a natural business key using Hibernate @NaturalId. This implementation allows us to use @NaturalId as it was provided by Spring. Moreover, this application uses Second Level Cache (EhCache) and @NaturalIdCache for skipping the entity identifier retrieval from the database.

Key points:

• enable Second Level Cache (EhCache)
• annotate entity with @NaturalIdCache for caching natural ids
• optionally, annotate entity with @Cache(usage = CacheConcurrencyStrategy.READ_WRITE, region = "Book") for caching entites as well

Output sample (for MySQL with IDENTITY generator, @NaturalIdCache and @Cache):

128. How To Calculate Non-Persistent Property via JPA @PostLoad

Description: This application is an example of calculating a non-persistent property of an entity based on the persistent entity attributes. In this case, we will use JPA, @PostLoad.

Key points:

• annotate the non-persistent field and property with @Transient
• define a method annotated with @PostLoad that calculates this non-persistent property based on the persistent entity attributes

129. How To Calculate Entity Persistent Property Via Hibernate @Generated

Description: This application is an example of calculating an entity persistent property at INSERT and/or UPDATE time via Hibernate, @Generated.

Key points:

Calculate at INSERT time:

• annotate the corresponding persistent field with @Generated(value = GenerationTime.INSERT)
• annotate the corresponding persistent field with @Column(insertable = false)

Calculate at INSERT and UPDATE time:

• annotate the corresponding persistent field with @Generated(value = GenerationTime.ALWAYS)
• annotate the corresponding persistent field with @Column(insertable = false, updatable = false)

Further, apply:

Method 1:

• if the database schema is generated via JPA annotations (not recommended) then use columnDefinition element of @Column to specify as an SQL query expression the formula for calculating the persistent property

Method 2:

• if the database schema is not generated via JPA annotations (recommended way) then add the formula as part of schema in CREATE TABLE

Note: In production, you should not rely on columnDefinition. You should disable hibernate.ddl-auto (by omitting it) or set it to validate, and add the SQL query expression in CREATE TABLE (in this application, check the discount column in CREATE TABLE, file schema-sql.sql). Nevertheless, not even schema-sql.sql is ok in production. The best way is to rely on Flyway or Liquibase.

130. How To Calculate Non-Persistent Property via Hibernate @Formula

Description: This application is an example of calculating a non-persistent property of an entity based on the persistent entity attributes. In this case, we will use Hibernate, @Formula.

Key points:

• annotate the non-persistent property with @Transient
• annotate the non-persistent field with @Formula
• as the value of @Formula add the SQL query expression that calculates this non-persistent property based on the persistent entity attributes

131. How To Add created, createdBy, lastModified And lastModifiedBy In Entities Via Hibernate

Note: The same thing can be obtained via Spring Data JPA auditing as here.

Description: This application is an example of adding in an entity the fields, created, createdBy, lastModified and lastModifiedBy via Hibernate support. These fields will be automatically generated/populated.

Key points:

• write an abstract class (e.g., BaseEntity) annotated with @MappedSuperclass
• in this abstract class, define a field named created and annotate it with the built-in @CreationTimestamp annotation
• in this abstract class, define a field named lastModified and annotate it with the built-in @UpdateTimestamp annotation
• in this abstract class, define a field named createdBy and annotate it with the @CreatedBy annotation
• in this abstract class, define a field named lastModifiedBy and annotate it with the @ModifiedBy annotation
• implement the @CreatedBy annotation via AnnotationValueGeneration
• implement the @ModifiedBy annotation via AnnotationValueGeneration
• every entity that want to take advantage of created, createdBy, lastModified and lastModifiedBy will extend the BaseEntity
• store the date-time in UTC

132. Hibernate Envers Auditing (schema-mysql.sql)

Description: Auditing is useful for maintaining history records. This can later help us in tracking user activities.

Key points:

• each entity that should be audited should be annotated with @Audited
• optionally, annotate entities with @AuditTable to rename the table used for auditing
• rely on ValidityAuditStrategy for fast database reads, but slower writes (slower than the default DefaultAuditStrategy)
• remove (disable) spring.jpa.hibernate.ddl-auto or set it to validate for avoiding schema generated from JPA annotations
• create schema-mysql.sql and provide the SQL statements needed by Hibernate Envers
• if the schema is not automatically found, then point it via spring.jpa.properties.org.hibernate.envers.default_catalog for MySQL or spring.jpa.properties.org.hibernate.envers.default_schema for the rest

133. How To Programmatically Setup Flyway And MySQL DataSource

Note: For production, don't rely on hibernate.ddl-auto (or counterparts) to export schema DDL to the database. Simply remove (disable) hibernate.ddl-auto or set it to validate. Rely on Flyway or Liquibase.

Description: This application is a kickoff for setting Flyway and MySQL DataSource programmatically.

Key points:

• for Maven, in pom.xml, add the Flyway dependency
• remove (disable) spring.jpa.hibernate.ddl-auto or set it to validate
• configure DataSource and Flyway programmatically

134. How To Migrate PostgreSQL Database Using Flyway - Use The Default Database postgres And Schema public

Note: For production, don't rely on hibernate.ddl-auto (or counterparts) to export schema DDL to the database. Simply remove (disable) hibernate.ddl-auto or set it to validate. Rely on Flyway or Liquibase.

Description: This application is an example of migrating a PostgreSQL database via Flyway for the default database postgres and schema public.

Key points:

• for Maven, in pom.xml, add the Flyway dependency
• remove (disable) spring.jpa.hibernate.ddl-auto or set it to validate
• in application.properties, set the JDBC URL as follows: jdbc:postgresql://localhost:5432/postgres
• each SQL file containing the schema update add it in classpath:db/migration
• each SQL file name it as V1.1__Description.sql, V1.2__Description.sql, ...

135. How To Migrate Schema Using Flyway In PostgreSQL - Use The Default Database postgres And Schema Created Via spring.flyway.schemas

Note: For production, don't rely on hibernate.ddl-auto (or counterparts) to export schema DDL to the database. Simply remove (disable) hibernate.ddl-auto or set it to validate. Rely on Flyway or Liquibase.

Description: This application is an example of migrating a schema (bookstore) created by Flyway via spring.flyway.schemas in the default postgres database. In this case, the entities should be annotated with @Table(schema = "bookstore").

Key points:

• for Maven, in pom.xml, add the Flyway dependency
• remove (disable) spring.jpa.hibernate.ddl-auto or set it to validate
• in application.properties, set the JDBC URL as follows: jdbc:postgresql://localhost:5432/postgres
• in application.properties, add spring.flyway.schemas=bookstore, where bookstore is the schema that should be created by Flyway in the postgres database (feel free to add your own database name)
• each entity that should be stored in this database should be annotated with, @Table(schema = "bookstore")
• each SQL file containing the schema update add it in classpath:db/migration
• each SQL file name it as V1.1__Description.sql, V1.2__Description.sql, ...

136. How To Programmatically Setup Flyway And PostgreSQL DataSource

Note: For production, don't rely on hibernate.ddl-auto (or counterparts) to export schema DDL to the database. Simply remove (disable) hibernate.ddl-auto or set it to validate. Rely on Flyway or Liquibase.

Description: This application is a kickoff for setting Flyway and PostgreSQL DataSource programmatically.

Key points:

• for Maven, in pom.xml, add the Flyway dependency
• remove (disable) spring.jpa.hibernate.ddl-auto or set it to validate
• configure DataSource and Flyway programmatically

137. How To Auto-Create And Migrate Two Databases In MySQL Using Flyway

Note: For production, don't rely on hibernate.ddl-auto (or counterparts) to export schema DDL to the database. Simply remove (disable) hibernate.ddl-auto or set it to validate. Rely on Flyway or Liquibase.

Description: This application is an example of auto-creating and migrating two databases in MySQL using Flyway. In addition, each data source uses its own HikariCP connection pool. In case of MySQL, where a database is the same thing with schema, we create two databases, authorsdb and booksdb.

Key points:

• for Maven, in pom.xml, add the Flyway dependency
• remove (disable) spring.jpa.hibernate.ddl-auto or set it to validate
• in application.properties, configure the JDBC URL for booksdb as jdbc:mysql://localhost:3306/booksdb?createDatabaseIfNotExist=true and for authorsdb as jdbc:mysql://localhost:3306/authorsdb?createDatabaseIfNotExist=true
• in application.properties, set spring.flyway.enabled=false to disable default behavior
• programmatically create two DataSource, one for booksdb and one for authorsdb
• programmatically create two FlywayDataSource, one for booksdb and one for authorsdb
• programmatically create two EntityManagerFactory, one for booksdb and one for authorsdb
• for booksdb, place the migration SQLs files in db\migration\booksdb
• for authorsdb, place the migration SQLs files in db\migration\authorsdb

138. Hibernate hi/lo Algorithm And External Systems Issue

Description: This is a Spring Boot sample that exemplifies how the hi/lo algorithm may cause issues when the database is used by external systems as well. Such systems can safely generate non-duplicated identifiers (e.g., for inserting new records) only if they know about the hi/lo presence and its internal work. So, better rely on pooled or pooled-lo algorithm which doesn't cause such issues.

Key points:

• use the SEQUENCE generator type (e.g., in PostgreSQL)
• configure the hi/lo algorithm as in Author.java entity
• insert a few records via hi/lo
• insert a few records natively (this acts as an external system that relies on NEXTVAL('hilo_sequence') and is not aware of hi/lo presence and/or behavior)

Output sample: Running this application should result in the following error:
ERROR: duplicate key value violates unique constraint "author_pkey"
Detail: Key (id)=(2) already exists.

139. How To Generate Sequences Of Identifiers Via Hibernate pooled Algorithm

Note: Rely on pooled-lo or pooled especially if, beside your application, external systems needs to insert rows in your tables. Don't rely on hi/lo since, in such cases, it may cause errors resulted from generating duplicated identifiers.

Description: This is a Spring Boot example of using the pooled algorithm. The pooled is an optimization of hi/lo. This algorithm fetched from the database the current sequence value as the top boundary identifier (the current sequence value is computed as the previous sequence value + increment_size). This way, the application will use in-memory identifiers generated between the previous top boundary exclusive (aka, lowest boundary) and the current top boundary inclusive.

Key points:

• use the SEQUENCE generator type (e.g., in PostgreSQL)
• configure the pooled algorithm as in Author.java entity
• insert a few records via pooled
• insert a few records natively (this acts as an external system that relies on NEXTVAL('hilo_sequence') and is not aware of pooled presence and/or behavior)

Conclusion: In contrast to the classical hi/lo algorithm, the Hibernate pooled algorithm doesn't cause issues to external systems that wants to interact with our tables. In other words, external systems can concurrently insert rows in the tables relying on pooled algorithm. Nevertheless, old versions of Hibernate can raise exceptions caused by INSERT statements triggered by external systems that uses the lowest boundary as identifier. This is a good reason to update to Hibernate latest versions (e.g., Hibernate 5.x), which have fixed this issue.

140. How To Generate Sequences Of Identifiers Via Hibernate pooled-lo Algorithm

Note: Rely on pooled-lo or pooled especially if, beside your application, external systems needs to insert rows in your tables. Don't rely on hi/lo since, in such cases, it may cause errors resulted from generating duplicated identifiers.

Description: This is a Spring Boot example of using the pooled-lo algorithm. The pooled-lo is an optimization of hi/lo similar with pooled. Only that, the strategy of this algorithm fetches from the database the current sequence value and use it as the in-memory lowest boundary identifier. The number of in-memory generated identifiers is equal to increment_size.

Key points:

• use the SEQUENCE generator type (e.g., in PostgreSQL)
• configure the pooled-lo algorithm as in Author.java entity
• insert a few records via pooled-lo
• insert a few records natively (this acts as an external system that relies on NEXTVAL('hilo_sequence') and is not aware of pooled-lo presence and/or behavior)

141. Fetching Associations In Batches Via @BatchSize

Description: This application uses Hibernate specific @BatchSize at class/entity-level and collection-level. Consider Author and Book entities invovled in a bidirectional-lazy @OneToMany association.

• First use case fetches all Author entities via a SELECT query. Further, calling the getBooks() method of the first Author entity will trigger another SELECT query that initializes the collections of the first three Author entities returned by the previous SELECT query. This is the effect of @BatchSize at Author's collection-level.

• Second use case fetches all Book entities via a SELECT query. Further, calling the getAuthor() method of the first Book entity will trigger another SELECT query that initializes the authors of the first three Book entities returned by the previous SELECT query. This is the effect of @BatchSize at Author class-level.

Note: Fetching associated collections in the same query with their parent can be done via JOIN FETCH or entity graphs as well. Fetching children with their parents in the same query can be done via JOIN FETCH, entity graphs and JOIN as well.

Key points:

• Author and Book are in a lazy relationship (e.g., @OneToMany bidirectional relationship)
• Author entity is annotated with @BatchSize(size = 3)
• Author's collection is annotated with @BatchSize(size = 3)

142. How To Use Entity Graphs (@NamedEntityGraph) In Spring Boot

Note: In a nutshell, entity graphs (aka, fetch plans) is a feature introduced in JPA 2.1 that help us to improve the performance of loading entities. Mainly, we specify the entity’s related associations and basic fields that should be loaded in a single SELECT statement. We can define multiple entity graphs for the same entity and chain any number of entities and even use sub-graphs to create complex fetch plans. To override the current FetchType semantics there are properties that can be set:

Fetch Graph (default), javax.persistence.fetchgraph
The attributes present in attributeNodes are treated as FetchType.EAGER. The remaining attributes are treated as FetchType.LAZY regardless of the default/explicit FetchType.

Load Graph, javax.persistence.loadgraph
The attributes present in attributeNodes are treated as FetchType.EAGER. The remaining attributes are treated according to their specified or default FetchType.

Nevertheless, the JPA specs doesn't apply in Hibernate for the basic (@Basic) attributes.. More details here.

Description: This is a sample application of using entity graphs in Spring Boot.

Key points:

• define two entities, Author and Book, involved in a lazy bidirectional @OneToMany association
• in Author entity use the @NamedEntityGraph to define the entity graph (e.g., load in a single SELECT the authors and the associatated books)
• in AuthorRepositry rely on Spring @EntityGraph annotation to indicate the entity graph defined at the previous step

143. How To Use Entity Sub-graphs In Spring Boot

Note: In a nutshell, entity graphs (aka, fetch plans) is a feature introduced in JPA 2.1 that help us to improve the performance of loading entities. Mainly, we specify the entity’s related associations and basic fields that should be loaded in a single SELECT statement. We can define multiple entity graphs for the same entity and chain any number of entities and even use sub-graphs to create complex fetch plans. To override the current FetchType semantics there are properties that can be set:

Fetch Graph (default), javax.persistence.fetchgraph
The attributes present in attributeNodes are treated as FetchType.EAGER. The remaining attributes are treated as FetchType.LAZY regardless of the default/explicit FetchType.

Load Graph, javax.persistence.loadgraph
The attributes present in attributeNodes are treated as FetchType.EAGER. The remaining attributes are treated according to their specified or default FetchType.

Nevertheless, the JPA specs doesn't apply in Hibernate for the basic (@Basic) attributes.. More details here.

Description: This is a sample application of using entity sub-graphs in Spring Boot. There is one example based on @NamedSubgraph and one based on the dot notation (.) in an ad-hoc entity graph.

Key points:

• define three entities, Author, Book and Publisher (Author and Book are involved in a lazy bidirectional @OneToMany relationship, Book and Publisher are also involved in a lazy bidirectional @OneToMany relationship; between Author and Publisher there is no relationship)

Using @NamedSubgraph

• in Author entity define an entity graph via @NamedEntityGraph; load the authors and the associatated books and use @NamedSubgraph to define a sub-graph for loading the publishers associated with these books
• in AuthorRepository rely on Spring @EntityGraph annotation to indicate the entity graph defined at the previous step

Using the dot notation (.)

• in PublisherRepository define an ad-hoc entity graph that fetches all publishers with associated books, and further, the authors associated with these books (e.g., @EntityGraph(attributePaths = {"books.author"}).

144. How To Define Ad-Hoc Entity Graphs In Spring Boot

Note: In a nutshell, entity graphs (aka, fetch plans) is a feature introduced in JPA 2.1 that help us to improve the performance of loading entities. Mainly, we specify the entity’s related associations and basic fields that should be loaded in a single SELECT statement. We can define multiple entity graphs for the same entity and chain any number of entities and even use sub-graphs to create complex fetch plans. To override the current FetchType semantics there are properties that can be set:

Fetch Graph (default), javax.persistence.fetchgraph
The attributes present in attributeNodes are treated as FetchType.EAGER. The remaining attributes are treated as FetchType.LAZY regardless of the default/explicit FetchType.

Load Graph, javax.persistence.loadgraph
The attributes present in attributeNodes are treated as FetchType.EAGER. The remaining attributes are treated according to their specified or default FetchType.

Nevertheless, the JPA specs doesn't apply in Hibernate for the basic (@Basic) attributes.. More details here.

Description: This is a sample application of defining ad-hoc entity graphs in Spring Boot.

Key points:

• define two entities, Author and Book, involved in a lazy bidirectional @OneToMany relationship
• the entity graph should load in a single SELECT the authors and the associatated books
• in AuthorRepository rely on Spring @EntityGraph(attributePaths = {"books"}) annotation to indicate the ad-hoc entity graph

145. How To Use Entity Graphs For @Basic Attributes In Hibernate And Spring Boot

Note: In a nutshell, entity graphs (aka, fetch plans) is a feature introduced in JPA 2.1 that help us to improve the performance of loading entities. Mainly, we specify the entity’s related associations and basic fields that should be loaded in a single SELECT statement. We can define multiple entity graphs for the same entity and chain any number of entities and even use sub-graphs to create complex fetch plans. To override the current FetchType semantics there are properties that can be set:

Fetch Graph (default), javax.persistence.fetchgraph
The attributes present in attributeNodes are treated as FetchType.EAGER. The remaining attributes are treated as FetchType.LAZY regardless of the default/explicit FetchType.

Load Graph, javax.persistence.loadgraph
The attributes present in attributeNodes are treated as FetchType.EAGER. The remaining attributes are treated according to their specified or default FetchType.

Nevertheless, the JPA specs doesn't apply in Hibernate for the basic (@Basic) attributes. In other words, by default, attributes are annotated with @Basic which rely on the default fetch policy. The default fetch policy is FetchType.EAGER. These attributes are also loaded in case of fetch graph even if they are not explicitly specified via @NamedAttributeNode. Annotating the basic attributes that should not be fetched with @Basic(fetch = FetchType.LAZY) it is not enough. Both, fetch graph and load graph will ignore these settings as long as we don't add bytecode enhancement as well.

The main drawback consists of the fact the these basic attributes are fetched LAZY by all other queries (e.g., findById()) not only by the queries using the entity graph, and most probably, you will not want this behavior.

Description: This is a sample application of using entity graphs with @Basic attributes in Spring Boot.

Key points:

• define two entities, Author and Book, involved in a lazy bidirectional @OneToMany association
• in Author entity use the @NamedEntityGraph to define the entity graph (e.g., load the authors names (only the name basic attribute; ignore the rest) and the associatated books)
• add bytecode enhancement
• annotate the basic attributes that should be ignored by the entity graph with @Basic(fetch = FetchType.LAZY)
• in AuthorRepository rely on Spring @EntityGraph annotation to indicate the entity graph defined at the previous step

146. How To Implement Soft Deletes Via SoftDeleteRepository In Spring Boot Application

Note: Spring Data built-in support for soft deletes is discussed in DATAJPA-307.

Description: This application is an example of implementing soft deletes in Spring Data style via a repository named, SoftDeleteRepository.

Key points:

• define an abstract class, BaseEntity, annotated with @MappedSuperclass
• in BaseEntity define a flag-field named deleted (default this field to false or in other words, not deleted)
• every entity that wants to take advantage of soft deletes should extend the BaseEntity classs
• write a @NoRepositoryBean named SoftDeleteRepository and extend JpaRepository
• override and implement the needed methods that provide the logic for soft deletes (check out the source code)
• repositories of entities should extend SoftDeleteRepository

Output example:

147. How To Implement Concurrent Table Based Queue Via SKIP_LOCKED In MySQL 8

Description: This application is an example of how to implement concurrent table based queue via SKIP_LOCKED in MySQL 8. SKIP_LOCKED can skip over locks achieved by other concurrent transactions, therefore is a great choice for implementing job queues. In this application, we run two concurrent transactions. The first transaction will lock the records with ids 1, 2 and 3. The second transaction will skip the records with ids 1, 2 and 3 and will lock the records with ids 4, 5 and 6.

Key points:

• define an entity that acts as a job queue (e.g., see the Book entity)
• in BookRepository setup @Lock(LockModeType.PESSIMISTIC_WRITE)
• in BookRepository use @QueryHint to setup javax.persistence.lock.timeout to SKIP_LOCKED
• rely on org.hibernate.dialect.MySQL8Dialect dialect
• run two concurrent transactions to see the effect of SKIP_LOCKED

148. How To Implement Concurrent Table Based Queue Via SKIP_LOCKED In PostgreSQL

Description: This application is an example of how to implement concurrent table based queue via SKIP_LOCKED in PostgreSQL. SKIP_LOCKED can skip over locks achieved by other concurrent transactions, therefore is a great choice for implementing job queues. In this application, we run two concurrent transactions. The first transaction will lock the records with ids 1, 2 and 3. The second transaction will skip the records with ids 1, 2 and 3 and will lock the records with ids 4, 5 and 6.

Key points:

• define an entity that acts as a job queue (e.g., see the Book entity)
• in BookRepository setup @Lock(LockModeType.PESSIMISTIC_WRITE)
• in BookRepository use @QueryHint to setup javax.persistence.lock.timeout to SKIP_LOCKED
• rely on org.hibernate.dialect.PostgreSQL95Dialect dialect
• run two concurrent transactions to see the effect of SKIP_LOCKED

149. JPA Inheritance - JOINED

Description: This application is a sample of JPA Join Table inheritance strategy (JOINED)

Key points:

• this inheritance strategy can be employed via @Inheritance(strategy=InheritanceType.JOINED)
• all the classes in an inheritance hierarchy (a.k.a., subclasses) are represented via individual tables
• by default, subclass-tables contains a primary key column that acts as a foreign key as well - this foreign key references the base class table primary key
• customizing this foreign key can be done by annotating the subclasses with @PrimaryKeyJoinColumn

150. JPA Inheritance - TABLE_PER_CLASS

Description: This application is a sample of JPA Table-per-class inheritance strategy (TABLE_PER_CLASS)

Key points:

• this inheritance strategy doesn't allow the usage of the IDENTITY generator
• this inheritance strategy can be employed via @Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
• all the classes in an inheritance hierarchy (a.k.a., subclasses) are represented via individual tables
• each subclass-table stores the columns inherited from the superclass-table (base class)

151. JPA Inheritance - @MappedSuperclass

Description: This application is a sample of using the JPA @MappedSuperclass.

Key points:

• the base class is not an entity, it can be abstract, and is annotated with @MappedSuperclass
• subclasses of the base class are mapped in tables that contains columns for the inherited attributes and for their own attibutes
• when the base class doens't need to be an entity, the @MappedSuperclass is the proper alternative to the JPA table-per-class inheritance strategy

152. How To Avoid Lazy Initialization Issues Caused By Disabling Open Session In View Via Hibernate5Module

Note: Hibernate5Module is an add-on module for Jackson JSON processor which handles Hibernate datatypes; and specifically aspects of lazy-loading.

Description: By default, in Spring Boot, the Open Session in View anti-pattern is enabled. Now, imagine a lazy relationship (e.g., @OneToMany) between two entities, Author and Book (an author has associated more books). Next, a REST controller endpoint fetches an Author without the associated Book. But, the View (more precisely, Jackson), forces the lazy loading of the associated Book as well. Since OSIV will supply the already opened Session, the Proxy initializations take place successfully.

Of course, the correct decision is to disable OSIV by setting it to false, but this will not stop Jackson to try to force the lazy initialization of the associated Book entities. Running the code again will result in an exception of type: Could not write JSON: failed to lazily initialize a collection of role: com.bookstore.entity.Author.books, could not initialize proxy - no Session; nested exception is com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.bookstore.entity.Author.books, could not initialize proxy - no Session.

Well, among the Hibernate5Module features we have support for dealing with this aspect of lazy loading and eliminate this exception. Even if OSIV will continue to be enabled (not recommended), Jackson will not use the Session opened via OSIV.

Key points:

• for Maven, add the Hibernate5Module dependency in pom.xml
• add a @Bean that returns an instance of Hibernate5Module
• annotate the Author bean with @JsonInclude(Include.NON_EMPTY) to exclude null or what is considered empty from the returned JSON

Note: The presence of Hibernate5Module instructs Jackson to initialize the lazy associations with default values (e.g., a lazy associated collection will be initialized with null). Hibernate5Module doesn't work for lazy loaded attributes. For such case consider this item.

153. How To View Binding Params Via profileSQL=true In MySQL

Description: View the prepared statement binding parameters via profileSQL=true in MySQL.

Key points:

• in application.properties append logger=Slf4JLogger&profileSQL=true to the JDBC URL (e.g., jdbc:mysql://localhost:3306/bookstoredb?createDatabaseIfNotExist=true&logger=Slf4JLogger&profileSQL=true)

Output sample:

154. How To Shuffle Small Result Sets

Description: This application is an example of shuffling small results sets. DO NOT USE this technique for large results sets, since is extremely expensive.

Key points:

• write a JPQL SELECT query and append to it ORDER BY RAND()
• each RDBMS support a function similar to RAND() (e.g., in PostgreSQL is random())

155. The Best Way To Remove Parent And Child Entities Via Bulk Deletions

Description: Commonly, deleting a parent and the associated children via CascadeType.REMOVE and/or orphanRemoval=true involved several SQL statements (e.g., each child is deleted in a dedicated DELETE statement). When the number of entities is significant, this is far from being efficient, therefore other approaches should be employed.

Consider Author and Book in a bidirectional-lazy @OneToMany association. This application exposes the best way to delete the parent(s) and the associated children in four scenarios listed below. These approaches relies on bulk deletions, therefore they are not useful if you want the deletions to take advantage of automatic optimistic locking mechanisms (e.g., via @Version):

Best way to delete author(s) and the associated books via bulk deletions when:

• One Author is in Persistent Context, no Book
• More Author are in the Persistent Context, no Book
• One Author and the associated Book are in Persistent Context
• No Author or Book is in Persistent Context

Note: The most efficient way to delete all entities via a bulk deletion can be done via the built-in deleteAllInBatch().

156. How To Bulk Updates

Description: Bulk operations (updates and deletes) are faster than batching, can benefit from indexing, but they have three main drawbacks:

• bulk updates/deletes may leave the Persistence Context in an outdated state (it's up to you to prevent this issue by flushing the Persistence Context before update/delete and close/clear it after the update/delete to avoid issues created by potentially unflushed or outdated entities)
• bulk updates/deletes don't benefit of automatic optimistic locking mechanisms (e.g., @Version is ignored), therefore the lost updates are not prevented (it is advisable to signal these updates by explicitly incrementing version (if any is present))
• bulk deletes cannot take advantage of cascading removals (CascadeType.REMOVE) and orphanRemoval

This application provides examples of bulk updates for Author and Book entities (between Author and Book there is a bidirectional lazy @OneToMany association). Both, Author and Book, has a version field.

157. Why You Should Avoid Unidirectional @OneToMany And Prefer Bidirectional @OneToMany Relationship

Description: As a rule of thumb, unidirectional @OneToMany association is less efficient than the bidirectional @OneToMany or the unidirectional @ManyToOne associations. This application is a sample that exposes the DML statements generated for reads, writes and removal operations when the unidirectional @OneToMany mapping is used.

Key points:

• regular unidirectional @OneToMany is less efficient than bidirectional @OneToMany association
• using @OrderColumn come with some optimizations for removal operations but is still less efficient than bidirectional @OneToMany association
• using @JoinColumn eliminates the junction table but is still less efficient than bidirectional @OneToMany association
• using Set instead of List or bidirectional @OneToMany with @JoinColumn relationship (e.g., @ManyToOne @JoinColumn(name = "author_id", updatable = false, insertable = false)) still performs worse than bidirectional @OneToMany association

158. How To Use Subqeries in JPQL WHERE/HAVING Clause

Description: This application is an example of using subqueries in JPQL WHERE clause (you can easily use it in HAVING clause as well).

Key points:
Keep in mind that subqueries and joins queries may or may not be semantically equivalent (joins may returns duplicates that can be removed via DISTINCT).

Even if the Execution Plan is specific to the database, historically speaking joins are faster than subqueries among different databases, but this is not a rule (e.g., the amount of data may significantly influence the results). Of course, do not conclude that subqueries are just a replacement for joins that doesn't deserve attention. Tuning subqueries can increases their performance as well, but this is an SQL wide topic. So, benchmark! Benchmark! Benchmark!

As a rule of thumb, prefer subqueries only if you cannot use joins, or if you can prove that they are faster than the alternative joins.

159. How To Execute SQL Functions In WHERE Part Of JPQL Query And JPA 2.1

Note: Using SQL functions in SELECT part (not in WHERE part) of the query can be done as here.

Description: Starting with JPA 2.1, a JPQL query can call SQL functions in the WHERE part via function(). This application is an example of calling the MySQL, concat_ws function, but user defined (custom) functions can be used as well.

Key points:

• use JPA 2.1, function()

160. Calling Stored Procedure That Returns A Value

Description: This application is an example of calling a MySQL stored procedure that returns a value (e.g., an Integer).

Key points:

• rely on @NamedStoredProcedureQuery to shape the stored procedure in the entity
• rely on @Procedure in repository

161. Calling Stored Procedure That Returns A Result Set (Entity And DTO)

Description: This application is an example of calling a MySQL stored procedure that returns a result set. The application fetches entities (e.g., List<Author>) and DTO (e.g., List<AuthorDto>).

Key points:

• rely on EntiyManager since Spring Data @Procedure will not work

162. Calling Stored Procedure That Returns A Result Set Via Native Query

Description: This application is an example of calling a MySQL stored procedure that returns a result set (entity or DTO) via a native query.

Key points:

• rely on a native call as @Query(value = "{CALL FETCH_AUTHOR_BY_GENRE (:p_genre)}", nativeQuery = true)

163. Calling Stored Procedure That Returns A Result Set Via JdbcTemplate

Note: Most probably you'll like to process the result set via BeanPropertyRowMapper as here. This is less verbose than the approach used here. Nevertheless, this approach is useful to understand how the result set looks like.

Description: This application is an example of calling a MySQL stored procedure that returns a result set via JdbcTemplate.

Key points:

• rely on JdbcTemplate and SimpleJdbcCall

164. How To Obtain Auto-Generated Keys

Description: This application is an example of retrieving the database auto-generated primary keys.

Key points:

• JPA style, retrieve the auto-generated keys via getId()
• JDBC style, retrieve the auto-generated keys via JdbcTemplate
• JDBC style, retrieve the auto-generated keys via SimpleJdbcInsert

165. How To Unproxy A Proxy

Description: A Hibernate proxy can be useful when a child entity can be persisted with a reference to its parent (@ManyToOne or @OneToOne association). In such cases, fetching the parent entity from the database (execute the SELECT statement) is a performance penalty and a pointless action. Hibernate can set the underlying foreign key value for an uninitialized proxy. This topic is discussed here.

A proxy can be unproxied via Hibernate.unproxy(). This method is available starting with Hibernate 5.2.10.

Key points:

• fetch a proxy via JpaRepository#getOne()
• unproxy the fetched proxy via Hibernate.unproxy()

166. How To Convert Boolean To Yes/No Via AttributeConverter

Description: This application is an example of converting a Boolean to Yes/No strings via AttributeConverter. This kind of conversions are needed when we deal with legacy databases that connot be changed. In this case, the legacy database stores the booleans as Yes/No.

Key points:

• implement a custom converter via AttributeConverter

167. How Efficient Is Just @OManyToOne

Note: The @ManyToOne association maps exactly to the one-to-many table relationship. The underlying foreign key is under child-side control in unidirectional or bidirectional relationship.

Description: This application shows that using only @ManyToOne is quite efficient. On the other hand, using only @OneToMany is far away from being efficient. Always, prefer bidirectional @OneToMany or unidirectional @ManyToOne. Consider two entities, Author and Book in a unidirectional @ManyToOne relationship.

Key points:

• Adding a new book is efficient
• Fetching all books of an author is efficient via a JPQL
• Pagination of books is efficient
• Remove a book is efficient
• Even if the fetched collection is not managed, dirty checking mechanism works as expected

168. How To Use JOIN FETCH And Pageable Pagination

Description: Trying to combine JOIN FETCH/LEFT JOIN FETCH and Pageable results in an exception of type org.hibernate.QueryException: query specified join fetching, but the owner of the fetched association was not present in the select list. This application is a sample of how to avoid this exception.

Key points:

• use countQuery
• use entity graph

Note: Fixing the above exception will lead to an warning of type HHH000104, firstResult / maxResults specified with collection fetch; applying in memory!. If this warning is a performance issue, and most probably it is, then follow by reading here.

169. How To Avoid HHH000104 And Use Pagination Of Parent-Child

Description: HHH000104 is a Hibernate warning that tell us that pagination of a result set is tacking place in memory. For example, consider the Author and Book entities in a lazy-bidirectional @OneToMany association and the following query:

@Transactional
@Query(value = "SELECT a FROM Author a LEFT JOIN FETCH a.books WHERE a.genre = ?1",
            countQuery = "SELECT COUNT(a) FROM Author a WHERE a.genre = ?1")
Page<Author> fetchWithBooksByGenre(String genre, Pageable pageable);

Calling fetchWithBooksByGenre() works fine only that the following warning is signaled: HHH000104: firstResult / maxResults specified with collection fetch; applying in memory! Obviously, having pagination in memory cannot be good from performance perspective. This application implement a solution for moving pagination at database-level.

Key points:

• use three or two JPQL queries for fetching Page of entities in read-write or read-only mode
• use two JPQL queries for fetching Slice or List of entities in read-write or read-only mode

170. What @Transactional(readOnly=true) Actually Do

Description: This application is meant to reveal what is the difference between @Transactional(readOnly = false) and @Transactional(readOnly = true). In a nuthsell, readOnly = false (default) fetches entites in read-write mode (managed). Before Spring 5.1, readOnly = true just set FlushType.MANUAL/NEVER, therefore the automatic dirty checking mechanism will not take action since there is no flush. In other words, Hibernate keep in the Persistent Context the fetched entities and the hydrated (loaded) state. By comparing the entity state with the hydrated state, the dirty checking mechanism can decide to trigger UPDATE statements in our behalf. But, the dirty checking mechanism take place at flush time, therefore, without a flush, the hydrated state is kept in Persistent Context for nothing, representing a performance penalty. Starting with Spring 5.1, the read-only mode is propagated to Hibernate, therefore the hydrated state is discarded immediately after loading the entities. Even if the read-only mode discards the hydrated state the entities are still loaded in the Persistent Context, therefore, for read-only data, relying on DTO (Spring projection) is better.

Key points:

• readOnly = false load data in read-write mode (managed)
• readOnly = true discard the hydrated state (starting with Spring 5.1)

171. Get Transaction Id In MySQL

Description: This application is an example of getting the current database transaction id in MySQL. Only read-write database transactions gets an id in MySQL. Every database has a specific query for getting the transaction id. Here it is a list of these queries.

Key points:

• rely on the following query, SELECT tx.trx_id FROM information_schema.innodb_trx tx WHERE tx.trx_mysql_thread_id = connection_id()

172. Inspect Persistent Context

Description: This application is a sample of inspecting the Persistent Context content via org.hibernate.engine.spi.PersistenceContext.

Key points:

• get the current Persistent Context via Hibernate SharedSessionContractImplementor
• rely on PersistenceContext API

173. How To Extract Tables Metadata

Description: This application is an example of using the Hibernate SPI, org.hibernate.integrator.spi.Integrator for extracting tables metadata.

Key points:

• implement org.hibernate.integrator.spi.Integrator and override integrate() method to return metadata.getDatabase()
• register this Integrator via LocalContainerEntityManagerFactoryBean

174. How To Map @ManyToOne Relationship To A SQL Query Via The Hibernate @JoinFormula

Description: This application is an example of mapping the JPA @ManyToOne relationship to a SQL query via the Hibernate @JoinFormula annotation. We start with two entities, Author and Book, involved in a unidirectional @ManyToOne relationship. Each book has a price. While we fetch a book by id (let's call it book A), we want to fetch another book B of the same author whose price is the next smaller price in comparison with book A price.

Key points:

• fetching the book B is done via @JoinFormula

175. How To Fetch Data From A MySQL Database View

Description: This application is an example of fetching a read-only MySQL database view in a JPA immutable entity.

Key points:

• the database view is available in data-mysql.sql file
• the entity used to map the database view is GenreAndTitleView.java

176. How To Update/Insert/Delete Data From/In A MySQL Database View

Description: This application is an example of updating, inserting and deleting data in a MySQL database view. Every update/insert/delete will automatically update the contents of the underlying table(s).

Key points:

• the database views are available in data-mysql.sql file
• respect MySQL requirements for updatable and insertable database views

177. How To Prevent A MySQL Database View From Updating/Inserting Rows That Are Not Visible Through It Via WITH CHECK OPTION

Description: This application is an example of preventing inserts/updates of a MySQL view that are not visible through this view via WITH CHECK OPTION. In other words, whenever you insert or update a row of the base tables through a view, MySQL ensures that the this operation is conformed with the definition of the view.

Key points:

• add WITH CHECK OPTION to the view
• this application will throw an exception of type java.sql.SQLException: CHECK OPTION failed 'bookstoredb.author_anthology_view

178. How To Efficiently Assign A Database Temporary Sequence Of Values To Rows

Description: This application is an example of assigning a database temporary sequence of values to rows via the window function, ROW_NUMBER(). This window function is available in almost all databases, and starting with version 8.x is available in MySQL as well.

Key points:

• commonly, you don't need to fetch in the result set the temporary sequence of values produced by ROW_NUMBER() (you will use it internally, in the query, usually in the WHERE clause and CTEs), but, this time, let's write a Spring projection (DTO) that contains a getter for the column generated by ROW_NUMBERas well
• write several native querys relying on ROW_NUMBER() window function

Output sample:

179. How To Efficiently Finding Top N Rows Of Every Group

Description: This application is an example of finding top N rows of every group.

Key points:

• write a native query relying on ROW_NUMBER() window function

Output sample:

180. How To Implement Pagination Via ROW_NUMBER() Window Function

Description: This application is an example of using ROW_NUMBER() (and COUNT(*) OVER() for counting all elements) window function to implement pagination.

Key points:

• use a native query relying on ROW_NUMBER()
• we don't return a page as Page or Slice, we return it as List, therefore Pageable is not used

181. Why the @Transactional annotation is being ignored

Description: This application is an example of fixing the case when @Transactional annotation is ignored. Most of the time, this annotation is ignored in the following scenarios:

1. @Transactional was added to a private, protected or package-protected method
2. @Transactional was added to a method defined in the same class where it is invoked

Key points:

• write a helper service and move the @Transactional methods there
• ensure that these methods are declared as public
• call @Transactional methods from other services

182. How To Generate Custom Sequence IDs

Description: This is a Spring Boot example of using the hi/lo algorithm and a custom implementation of SequenceStyleGenerator for generating custom sequence IDs (e.g, A-0000000001, A-0000000002, ...).

Key points:

• extend SequenceStyleGenerator and override the configure() and generate() methods
• set this generator in entities

183. How To Map Clob And Blob To byte[] And String

Description: This application is an example of mapping Clob and Blob as byte[] and String.

Key points:

• this is vey easy to use but the application doesn't take advantage of JDBC driver LOB-specific optimizations

184. How To Map To JDBC’s LOB Locators Clob And Blob

Description: This application is an example of mapping to JDBC's LOB locators Clob and Blob.

Key points:

• this takes advantage of JDBC driver LOB-specific optimizations

185. How To Fetch Certain Subclass From An SINGLE_TABLE Inheritance Hierarchy

Description: This application is a sample of fetching a certain subclass from a SINGLE_TABLE inheritance hierarchy. This is useful when the dedicated repository of the subclass doesn't automatically add in the WHERE clause a dtype based condition for fetching only the needed subclass.

Key points:

• explicitly add in the WHERE clause a TYPE check

186. How To Define An Association That Reference @NaturalId

Description: This is a SpringBoot application that defines a @ManyToOne relationship that doesn't reference a primary key column. It references a Hibernate @NaturalId column.

Key points:

• rely on @JoinColumn(referencedColumnName = "natural_id_column")

187. How To Implement Advanced Search Via Specification

Description: This application is an example of implementing an advanced search via Specification API. Mainly, you can give the search filters to a generic Specification and fetch the result set. Pagination is supported as well. You can chain expressions via logical AND and OR to create compound filters. Nevertheless, there is room for extensions to add brackets support (e.g., (x AND y) OR (x AND z)), more operations, conditions parser and so on and forth.

Key points:

• write a generic Specification

188. How To Create Specification Query Fetch Joins

Description: This application contains two examples of how to define JOIN in Specification to emulate JPQL join-fetch operations.

Key points:

• the first approach trigger two SELECT statements and the pagination is done in memory (very bad!)
• the second approach trigger three SELECT statements but the pagination is done in the database
• in both approaches the JOIN is defined in a Specification implementation

189. DTO Via Spring Data Projections (Projection Interface In Repository Interface)

Note: You may also like to read the recipe, "How To Enrich DTO With Virtual Properties Via Spring Projections"

Description: Fetch only the needed data from the database via Spring Data Projections (DTO). The projection interface is defined as a static interface (can be non-static as well) in the repository interface.

Key points:

• write an interface (projection) containing getters only for the columns that should be fetched from the database
• write the proper query returning a List<projection>
• if is applicable, limit the number of returned rows (e.g., via LIMIT) - here, we can use query builder mechanism built into Spring Data repository infrastructure

Note: Using projections is not limited to use query builder mechanism built into Spring Data repository infrastructure. We can fetch projections via JPQL or native queries as well. For example, in this application we use a JPQL.

Output example (select first 2 rows; select only "name" and "age"):

190. How To Ensure/Validate That Only One Association Is Non-Null

Description: Consider an entity named Review. This entity defines three @ManyToOne relationships to Book, Article and Magazine. A review can be associated with either a book, a magazine or an article. To validate this constraint, we can rely on Bean Validation as in this application.

Key points:

• rely on Bean Validation to validate that only one association is non-null
• expose the constraint via a custom annotation (@JustOneOfMany) added at class-level to the Review entity
• for preventing native query to break our constraint add the validation at database level as well (e.g., in MySQL add a TRIGGER)

191. Quickest Mapping Of Java Enums

Description: This application uses EnumType.ORDINAL and EnumType.STRING for mapping Java enum type to database. As a rule of thumb, strive to keep the data types as small as possible (e.g., for EnumType.ORDINAL use TINYINT/SMALLINT, while for EnumType.STRING use VARCHAR(max_needed_bytes)). Relying on EnumType.ORDINAL should be more efficient but is less expressive than EnumType.STRING.

Key points:

• strive for smallest data types (e.g., for EnumType.ORDINAL set @Column(columnDefinition = "TINYINT"))

192. How To Map Java enum To Database Via AttributeConverter

Description: This application maps a Java enum via AttributeConverter. In other words, it maps the enum values HORROR, ANTHOLOGY and HISTORY to the integers 1, 2 and 3 and viceversa. This allows us to set the column type as TINYINT/SMALLINT which is less space-consuming than VARCHAR(9) needed in this case.

Key points:

• define a custom AttributeConverter
• annotate with @Converter the corresponding entity field

193. How To Map Java enum To PostgreSQL enum Type

Description: This application maps a Java enum type to PostgreSQL enum type.

Key points:

• define a custom Hibernate EnumType
• register this custom EnumType via package-info.java
• annotate the corresponding entity field @Type

194. How To Map Java enum To PostgreSQL enum Type Via Hibernate Types Library

Description: This application maps a Java enum type to PostgreSQL enum type via Hibernate Types library.

Key points:

• install Hibernate Types library via pom.xml
• use @TypeDef to specify the needed type class
• annotate the corresponding entity field with @Type

195. How To Handle JSON in MySQL

Description: Hibernate Types is a library of extra types not supported by Hibernate Core by default. This is a Spring Boot application that uses this library to persist JSON data (JSON Java Object) in a MySQL json column and for querying JSON data from the MySQL json column to JSON Java Object. Updates are supported as well.

Key points:

• for Maven, add Hibernate Types as a dependency in pom.xml
• in entity use @TypeDef to map typeClass to JsonStringType

196. How To Handle JSON in PostgreSQL

Description: Hibernate Types is a library of extra types not supported by Hibernate Core by default. This is a Spring Boot application that uses this library to persist JSON data (JSON Java Object) in a PostgreSQL json column and for querying JSON data from the PostgreSQL json column to JSON Java Object. Updates are supported as well.

Key points:

• for Maven, add Hibernate Types as a dependency in pom.xml
• in entity use @TypeDef to map typeClass to JsonBinaryType

197. How To Increment The Version Of The Locked Entity Even If This Entity Was Not Modified OPTIMISTIC_FORCE_INCREMENT

Description: This application is a sample of how OPTIMISTIC_FORCE_INCREMENT works in MySQL. This is useful when you want to increment the version of the locked entity even if this entity was not modified. Via OPTIMISTIC_FORCE_INCREMENT the version is updated (incremented) at the end of the currently running transaction.

Key points:

• use a root entity, Chapter (which uses @Version)
• several editors load a chapter and perform modifications mapped via the Modification entity
• between Modification (child-side) and Chapter (parent-side) there is a lazy unidirectional @ManyToOne association
• for each modification, Hibernate will trigger an INSERT statement against the modification table, therefore the chapter table will not be modified by editors
• but, Chapter entity version is needed to ensure that modifications are applied sequentially (the author and editor are notified if a modificaton was added since the chapter copy was loaded)
• the version is forcibly increased at each modification (this is materialized in an UPDATE triggered against the chapter table at the end of the currently running transaction)
• set OPTIMISTIC_FORCE_INCREMENT in the corresponding repository
• rely on two concurrent transactions to shape the scenario that will lead to an exception of type ObjectOptimisticLockingFailureException

198. How To Increment The Version Of The Locked Entity Even If This Entity Was Not Modified PESSIMISTIC_FORCE_INCREMENT

Description: This application is a sample of how PESSIMISTIC_FORCE_INCREMENT works in MySQL. This is useful when you want to increment the version of the locked entity even if this entity was not modified. Via PESSIMISTIC_FORCE_INCREMENT the version is updated (incremented) immediately (the entity version update is guaranteed to succeed immediately after acquiring the row-level lock). The incrementation takes place before the entity is returned to the data access layer.

Key points:

• use a root entity, Chapter (which uses @Version)
• several editors load a chapter and perform modifications mapped via the Modification entity
• between Modification (child-side) and Chapter (parent-side) there is a lazy unidirectional @ManyToOne association
• for each modification, Hibernate will trigger an INSERT statement against the modification table, therefore the chapter table will not be modified by editors
• but, Chapter entity version is needed to ensure that modifications are applied sequentially (each editor is notified if a modificaton was added since his chapter copy was loaded and he must re-load the chapter)
• the version is forcibly increased at each modification (this is materialized in an UPDATE triggered against the chapter table immediately after aquiring the row-level lock)
• set PESSIMISTIC_FORCE_INCREMENT in the corresponding repository
• rely on two concurrent transactions to shape two scenarios: one that will lead to an exception of type OptimisticLockException and one that will lead to QueryTimeoutException

Note: Pay attention to the MySQL dialect: MySQL5Dialect (MyISAM) doesn't support row-level locking, MySQL5InnoDBDialect (InnoDB) acquires row-level lock via FOR UPDATE (timeout can be set), MySQL8Dialect (InnoDB) acquires row-level lock via FOR UPDATE NOWAIT.

199. How PESSIMISTIC_READ And PESSIMISTIC_WRITE Works In MySQL

Description: This application is an example of using PESSIMISTIC_READ and PESSIMISTIC_WRITE in MySQL. In a nutshell, each database system defines its own syntax for acquiring shared and exclusive locks and not all databases support both types of locks. Depending on Dialect, the syntax can vary for the same database as well (Hibernate relies on Dialect for chosing the proper syntax). In MySQL, MySQL5Dialect doesn't support locking, while InnoDB engine (MySQL5InnoDBDialect and MySQL8Dialect) supports shared and exclusive locks as expected.

Key points:

• rely on @Lock(LockModeType.PESSIMISTIC_READ) and @Lock(LockModeType.PESSIMISTIC_WRITE) on query-level
• for testing, use TransactionTemplate to trigger two concurrent transactions that read and write the same row

200. How PESSIMISTIC_WRITE Works With UPDATE/INSERT And DELETE Operations

Description: This application is an example of triggering UPDATE, INSERT and DELETE operations in the context of PESSIMISTIC_WRITE locking against MySQL. While UPDATE and DELETE are blocked until the exclusive lock is released, INSERT depends on the transaction isolation level. Typically, even with exclusive locks, inserts are possible (e.g., in PostgreSQL). In MySQL, for the default isolation level, REPEATABLE READ, inserts are prevented against a range of locked entries, but, if we switch to READ_COMMITTED, then MySQL acts as PostgreSQL as well.

Key points:

• start Transaction A and trigger a SELECT with PESSIMISTIC_WRITE to acquire an exclusive lock
• start a concurrent Transaction B that triggers an UPDATE, INSERT or DELETE on the rows locked by Transaction A
• in case of UPDATE, DELETE and INSERT + REPEATABLE_READ, Transaction B is blocked until it timeouts or Transaction A releases the exclusive lock
• in case of INSERT + READ_COMMITTED, Transaction B can insert in the range of rows locked by Transaction A even if Transaction A is holding an exclusive lock on this range

201. How To Check That Transaction Timeout And Rollback At Expiration Works As Expected

Note: Do not test transaction timeout via Thread.sleep()! This is not working! Rely on two transactions and exclusive locks or even better rely on SQL sleep functions (e.g., MySQL, SELECT SLEEP(n) seconds, PostgreSQL, SELECT PG_SLEEP(n) seconds). Most RDBMS supports a sleep function flavor.

Description: This application contains several approaches for setting a timeout period for a transaction or query. The timeout is signaled by a specific timeout exception (e.g., .QueryTimeoutException). After timeout, the transaction is rolled back. You can see this in the database (visually or query) and on log via a message of type: Initiating transaction rollback; Rolling back JPA transaction on EntityManager [SessionImpl(... <open>)].

Key points:

• set global transaction timeout via spring.transaction.default-timeout in seconds (see, application.properties)
• set transaction timeout at method-level or class-level via @Transactional(timeout = n) in seconds
• set query timeout via JPA javax.persistence.query.timeout hint in milliseconds
• set query timeout via Hibrenate org.hibernate.timeout hint in seconds

Note: If you are using TransactionTemplate then the timeout can be set via TransactionTemplate.setTimeout(n) in seconds.

202. How To Define A Composite Primary Key Via @Embeddable

Description: This application is a proof of concept of how to define a composite key via @Embeddable and @EmbeddedId. This application uses two entities, Author and Book involved in a lazy bidirectional @OneToMany association. The identifier of Author is composed by name and age via AuthorId class. The identifier of Book is just a regular auto-generated numeric value.

Key points:

• the composite key class (e.g., AuthorId) is public
• the composite key class must implement Serializable
• the composite key must define equals() and hashCode()
• the composite key must define a no-arguments constructor

203. How To Define A Composite Primary Key Via @IdClass

Description: This application is a proof of concept of how to define a composite key via @IdClass. This application uses two entities, Author and Book involved in a lazy bidirectional @OneToMany association. The identifier of Author is composed by name and age via AuthorId class. The identifier of Book is just a typical auto-generated numeric value.

Key points:

• the composite key class (e.g., AuthorId) is public
• the composite key class must implement Serializable
• the composite key must define equals() and hashCode()
• the composite key must define a no-arguments constructor

Note: The @IdClass can be useful when we cannot modify the compsite key class. Otherwise, rely on @Embeddable.

204. How To Define A Relationship in an @Embeddable Composite Primary Key

Description: This application is a proof of concept of how to define a relationship in an @Embeddable composite key. The composite key is AuthorId and it belongs to the Author class.

Key points:

• the composite key class (e.g., AuthorId) is public
• the composite key class must implement Serializable
• the composite key must define equals() and hashCode()
• the composite key must define a no-arguments constructor

205. How To Load Multiple Entities By Id

Description: This is a SpringBoot application that loads multiple entities by id via a @Query based on the IN operator and via the Hibernate 5 MultiIdentifierLoadAccess interface.

Key points:

• for using the IN operator in a @Query simply add the query in the proper repository
• for using Hibernate 5 MultiIdentifierLoadAccess in Spring Data style provide the proper implementation
• among its advantages, the MultiIdentifierLoadAccess implementation allows us to load entities by multiple ids in batches and by inspecting or not the current Persistent Context (by default, the Persistent Context is not inspected to see if the entities are already loaded or not)

206. Fetching All Entity Attributes As Spring Projection (DTO)

Description: This application is a sample of fetching all attributes of an entity (Author) as a Spring projection (DTO). Commonly, a DTO contains a subset of attributes, but, sometimes we need to fetch the whole entity as a DTO. In such cases, we have to pay attention to the chosen approach. Choosing wisely can spare us from performance penalties.

Key points:

• fetching the result set as a List<Object[]> or List<AuthorDto> via a JPQL of type SELECT a FROM Author a WILL fetch the result set as entities in Persistent Context as well - avoid this approach
• fetching the result set as a List<Object[]> or List<AuthorDto> via a JPQL of type SELECT a.id AS id, a.name AS name, ... FROM Author a will NOT fetch the result set in Persistent Context - this is efficient
• fetching the result set as a List<Object[]> or List<AuthorDto> via a native SQL of type SELECT id, name, age, ... FROM author will NOT fetch the result set in Persistent Context - but, this approach is pretty slow
• fetching the result set as a List<Object[]> via Spring Data query builder mechanism WILL fetch the result set in Persistent Context - avoid this approach
• fetching the result set as a List<AuthorDto> via Spring Data query builder mechanism will NOT fetch the result set in Persistent Context
• fetching the result set as read-only entitites (e.g., via the built-in findAll() method) should be considered after JPQL with explicit list of columns to be fetched and query builder mechanism

207. How To Efficiently Fetch Spring Projection Including @ManyToOne Or @OneToOne Associations

Description: This application fetches a Spring projection including the @ManyToOne association via different approaches. It can be easily adapted for @OneToOne association as well.

Key points:

• fetching raw data is the fastest approach

208. Pay Attention To Spring Projections That Include Associated Collections

Description: This application inspect the Persistent Context content during fetching Spring projections that includes collections of associations. In this case, we focus on a @OneToMany association. Mainly, we want to fetch only some attributes from the parent-side and some attributes from the child-side.

209. Reusing Spring projection

Description: This application is a sample of reusing an interface-based Spring projection. This is useful to avoid defining multiple interface-based Spring projections in order to cover a range of queries that fetches different subsets of fields.

Key points:

• define an interface-based Spring projection containing getters for the wider case
• rely on class-level @JsonInclude(JsonInclude.Include.NON_DEFAULT) annotation to avoid serialization of default fields (e.g., fields that are not available in the current projection and are null - these fields haven't been fetched in the current query)
• this is useful to Jackson that will not serialize in the resulted JSON the missing fields (e.g., null fields)

210. Dynamic Spring projection

Description: This application is a sample of using dynamic Spring projections.

Key points:

• declare query-methods in a generic manner (e.g., <T> List<T> findByGenre(String genre, Class<T> type);)

211. Batch Inserts Via EntityManager With Batch Per Transaction (MySQL)

Description: This application is a sample of batching inserts via EntityManager in MySQL. This way you can easily control the flush() and clear() cycles of the Persistence Context (1st Level Cache) inside the current transaction. This is not possible via Spring Boot, saveAll(Iterable<S> entities), since this method executes a single flush per transaction. Another advantage is that you can call persist() instead of merge() - this is used behind the scene by the SpringBoot saveAll(Iterable<S> entities) and save(S entity).

Moreover, this example commits the database transaction after each batch excecution. This way we avoid long-running transactions and, in case of a failure, we rollback only the failed batch and don't lose the previous batches. For each batch, the Persistent Context is flushed and cleared, therefore we maintain a thin Persistent Context. This way the code is not prone to memory errors and performance penalties caused by slow flushes.

Key points:

• in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
• in application.properties set spring.jpa.properties.hibernate.generate_statistics (just to check that batching is working)
• in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL)
• in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
• in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
• in case of using a parent-child relationship with cascade persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_inserts=true to optimize the batching by ordering inserts
• in entity, use the assigned generator since MySQL IDENTITY will cause insert batching to be disabled
• in your DAO layer, flush and clear the Persistence Context from time to time (e.g. for each batch); this way you avoid to "overwhelm" the Persistence Context
• in your DAO layer, commit the database transaction after each batch execution
• if is not needed, then ensure that Second Level Cache is disabled via spring.jpa.properties.hibernate.cache.use_second_level_cache=false

Output example:

212. How To JDBC Batch a Big JSON File To MySQL Via ForkJoinPool And HikariCP

Description: This is a Spring Boot application that reads a relatively big JSON file (200000+ lines) and inserts its content in MySQL via batching using ForkJoinPool, JdbcTemplate and HikariCP.

Key points:

• using MySQL, json type
• read the file content into a List
• the list is halved and subtasks are created until the list size is small than the batch size (e.g., by default smaller than 30)
• for MySQL, in application.properties, you may want to attach to the JDBC URL the following:
  • rewriteBatchedStatements=true -> this setting will force sending the batched statements in a single request;
  • cachePrepStmts=true -> enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled
  • useServerPrepStmts=true -> this way you switch to server-side prepared statements (may lead to signnificant performance boost); moreover, you avoid the PreparedStatement to be emulated at the JDBC Driver level;
  • we use the following JDBC URL settings:
    ...?cachePrepStmts=true&useServerPrepStmts=true&rewriteBatchedStatements=true&createDatabaseIfNotExist=true
  • Note: Older MySQL versions will not tolerate well to have toghether rewritting and server-side prepared statement activated. For being sure that these statements still valid please check the notes of the Connector/J that you are using
• set the HikariCP to provide a number of database connections that ensure that the database achives a minimum context switching (e.g., 2 * number of CPU cores)
• this application uses StopWatch to measure the time needed to transfer the file into the database
• in order to run the application you have to unzip the citylots.zip in the current location; this is the big JSON file collected from Internet;
• if you want to see details about the batch process simply activate the DatasourceProxyBeanPostProcessor.java component by uncomment the line, // @Component; This is needed because this application relies on DataSource-Proxy (for details, see the following item)

213. Batch Inserts In Spring Boot Style Via CompletableFuture

Description: This application is a sample of using CompletableFuture for batching inserts. This CompletableFuture uses an Executor that has the number of threads equal with the number of your computer cores. Usage is in Spring style.

214. How To Optimize Batch Inserts of Parent-Child Relationships And Batch Per Transaction (MySQL)

Description: Let's suppose that we have a one-to-many relationship between Author and Book entities. When we save an author, we save his books as well thanks to cascading all/persist. We want to create a bunch of authors with books and save them in the database (e.g., a MySQL database) using the batch technique. By default, this will result in batching each author and the books per author (one batch for the author and one batch for the books, another batch for the author and another batch for the books, and so on). In order to batch authors and books, we need to order inserts as in this application.

Moreover, this example commits the database transaction after each batch excecution. This way we avoid long-running transactions and, in case of a failure, we rollback only the failed batch and don't lose the previous batches. For each batch, the Persistent Context is flushed and cleared, therefore we maintain a thin Persistent Context. This way the code is not prone to memory errors and performance penalties caused by slow flushes.

Key points:

• beside all setting specific to batching inserts in MySQL, we need to set up in application.properties the following property: spring.jpa.properties.hibernate.order_inserts=true
• in your DAO layer, commit the database transaction after each batch execution

Example without ordered inserts:

Example with ordered inserts:

215. Batch Inserts In Spring Boot Style And Batch Per Transaction

Description: Batch inserts (in MySQL) in Spring Boot style. This example commits the database transaction after each batch excecution. This way we avoid long-running transactions and, in case of a failure, we rollback only the failed batch and don't lose the previous batches.

Key points:

• in application.properties set spring.jpa.properties.hibernate.jdbc.batch_size
• in application.properties set spring.jpa.properties.hibernate.generate_statistics (just to check that batching is working)
• in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL)
• in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
• in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
• in case of using a parent-child relationship with cascade persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_inserts=true to optimize the batching by ordering inserts
• in entity, use the assigned generator since the Hibernate IDENTITY will cause insert batching to be disabled
• in your DAO layer, commit the database transaction after each batch execution
• if is not needed then ensure that Second Level Cache is disabled via spring.jpa.properties.hibernate.cache.use_second_level_cache=false

Output example:

216. IN Clause Parameter Padding

Description: This application is an example of using Hibernate IN cluase parameter padding. This way we can reduce the number of Execution Plans. Mainly, Hibernate is padding parameters as follows:

• for 3 and 4 parameters -> it uses 4 bind parameters (2^2)
• for 5, 6, 7 and 8 parameters -> it uses 8 bind parameters (2^3)
• for 9, 10, 11, 12, 13, 14, 15 and 16 parameters -> it uses 16 parameters (2^4)
• ...

Key points:

• in application.properties set spring.jpa.properties.hibernate.query.in_clause_parameter_padding=true

217. DTO Via Spring Data Class-Based Projections

Description: Fetch only the needed data from the database via Spring Data Projections (DTO). In this case, via class-based projections.

Key points:

• write an class (projection) containing a constructor, getters, setters, equals() and hashCode() only for the columns that should be fetched from the database
• write the proper query returning a List<projection>
• if it is applicable, limit the number of returned rows (e.g., via LIMIT)
• in this example, we can use query builder mechanism built into Spring Data repository infrastructure

Note: Using projections is not limited to use query builder mechanism built into Spring Data repository infrastructure. We can fetch projections via JPQL or native queries as well. For example, in this application we use a JPQL.

Output example (select first 2 rows; select only "name" and "age"):

218. Session-Level Batching (Hibernate 5.2 or Higher) in MySQL

Description: Batch inserts via Hibernate session-level batching (Hibernate 5.2 or higher) in MySQL. This example commits the database transaction after each batch excecution. This way we avoid long-running transactions and, in case of a failure, we rollback only the failed batch and don't lose the previous batches. For each batch, the Persistent Context is flushed and cleared, therefore we maintain a thin Persistent Context. This way the code is not prone to memory errors and performance penalties caused by slow flushes.

Key points:

• in application.properties set spring.jpa.properties.hibernate.generate_statistics (just to check that batching is working)
• in application.properties set JDBC URL with rewriteBatchedStatements=true (optimization for MySQL)
• in application.properties set JDBC URL with cachePrepStmts=true (enable caching and is useful if you decide to set prepStmtCacheSize, prepStmtCacheSqlLimit, etc as well; without this setting the cache is disabled)
• in application.properties set JDBC URL with useServerPrepStmts=true (this way you switch to server-side prepared statements (may lead to signnificant performance boost))
• in case of using a parent-child relationship with cascade persist (e.g. one-to-many, many-to-many) then consider to set up spring.jpa.properties.hibernate.order_inserts=true to optimize the batching by ordering inserts
• in entity, use the assigned generator since MySQL IDENTITY will cause insert batching to be disabled
• the Hibernate Session is obtained by un-wrapping it via EntityManager#unwrap(Session.class)
• the batching size is set via Session#setJdbcBatchSize(Integer size) and get via Session#getJdbcBatchSize()
• in your DAO layer, commit the database transaction after each batch execution
• if is not needed, then ensure that Second Level Cache is disabled via spring.jpa.properties.hibernate.cache.use_second_level_cache=false

Output example:

219. Use Read-Only Entity Whenever You Plan To Propagate Entity Changes To The Database In A Future Persistent Context

Description: This application highlights the difference betweeen loading entities in read-write vs. read-only mode. If you plan to modify the entities in a future Persistent Context then fetch them as read-only in the current Persistent Context.

Key points:

• in the current Persistent Context, fetch entities in read-only mode
• modifiy the entities in the current Persistent Context or in detached state (the potential modifications done in the current Persistent Context will not be propagated to the database at flush time)
• in a subsequent Persistent Context, merge the detached entity and propagate changes to the database

Note: If you never plan to modify the fetched result set then use DTO (e.g., Spring projection), not read-only entities.

220. How To Publish Domain Events From Aggregate Root

Note: Domain events should be used with extra-caution! The best practices for using them are revealed in my book, Spring Boot Persistence Best Practices.

Description: Starting with Spring Data Ingalls release publishing domain events by aggregate roots becomes easier. Entities managed by repositories are aggregate roots. In a Domain-Driven Design application, these aggregate roots usually publish domain events. Spring Data provides an annotation @DomainEvents you can use on a method of your aggregate root to make that publication as easy as possible. A method annotated with @DomainEvents is automatically invoked by Spring Data whenever an entity is saved using the right repository. Moreover, Spring Data provides the @AfterDomainEventsPublication annotation to indicate the method that should be automatically called for clearing events after publication. Spring Data Commons comes with a convenient template base class (AbstractAggregateRoot) to help to register domain events and is using the publication mechanism implied by @DomainEvents and @AfterDomainEventsPublication. The events are registered by calling the AbstractAggregateRoot.registerEvent() method. The registered domain events are published if we call one of the save methods (e.g., save()) of the Spring Data repository and cleared after publication.

This is a sample application that relies on AbstractAggregateRoot and its registerEvent() method. We have two entities, Book and BookReview involved in a lazy-bidirectional @OneToMany association. A new book review is saved in CHECK status and a CheckReviewEvent is published. This event handler is responsible to check the review grammar, content, etc and switch the review status from CHECK to ACCEPT or REJECT and propagate the new status to the database. So, this event is registered before saving the book review in CHECK status and is published automatically after we call the BookReviewRepository.save() method. After publication, the event is cleared.

Key points:

• the entity (aggregate root) that publish events should extend AbstractAggregateRoot and provide a method for registering events
• here, we register a single event (CheckReviewEvent), but more can be registered
• event handling take place is CheckReviewEventHandler in an asynchronous manner via @Async

221. How To Use Hibernate Query Plan Cache

Description: This application is an example of testing the Hibernate Query Plan Cache (QPC). Hibernate QPC is enabled by default and, for entity queries (JPQL and Criteria API), the QPC has a size of 2048, while for native queries it has a size of 128. Pay attention to alter these values to accommodate all queries executed by your application. If the number of exectued queries is higher than the QPC size (especially for entity queries) then you will start to experiment performance penalties caused by entity compilation time added for each query execution.

In this application, you can adjust the QPC size in application.properties. Mainly, there are 2 JPQL queries and a QPC of size 2. Switching from size 2 to size 1 will cause the compilation of one JPQL query at each execution. Measuring the times for 5000 executions using a QPC of size 2, respectively 1 reveals the importance of QPC in terms of time.

Key points:

• for JPQL and Criteria API you can set the QPC via hibernate.query.plan_cache_max_size
• for native queries you can set the QPC via hibernate.query.plan_parameter_metadata_max_size

222. How To Cache Entities And Query Results In Second Level Cache (EhCache)

Description: This is a SpringBoot application that enables Hibernate Second Level Cache and EhCache provider. It contains an example of caching entities and an example of caching a query result.

Key points:

• enable Second Level Cache (EhCache)
• rely on @Cache
• rely on JPA hint HINT_CACHEABLE

223. Spring Boot Caching Kickoff

Description: This is a SpringBoot application representing a kickoff application for Spring Boot caching and EhCache.

Key points:

• setup EhCache
• rely on Spring caching annotations

224. How To Fetch Entity Via SqlResultSetMapping And NamedNativeQuery

Note: If you want to rely on the {EntityName}.{RepositoryMethodName} naming convention for simply creating in the repository interface methods with the same name as of native named query then skip this application and check this one.

Description: This is a sample application of using SqlResultSetMapping, NamedNativeQuery and EntityResult for fetching single entity and multiple entities as List<Object[]>.

Key points:

• use SqlResultSetMapping, NamedNativeQuery and EntityResult

225. How To Load Multiple Entities By Id Via Specification

Description: This is a SpringBoot application that loads multiple entities by id via a @Query based on the IN operator and via Specification.

Key points:

• for using the IN operator in a @Query simply add the query in the proper repository
• for using a Specification rely on javax.persistence.criteria.Root.in()

226. How To Fetch DTO Via A Custom ResultTransformer

Description: Fetching more read-only data than needed is prone to performance penalties. Using DTO allows us to extract only the needed data. Sometimes, we need to fetch a DTO made of a subset of properties (columns) from a parent-child association. For such cases, we can use SQL JOIN that can pick up the desired columns from the involved tables. But, JOIN returns an List<Object[]> and most probably you will need to represent it as a List<ParentDto>, where a ParentDto instance has a List<ChildDto>. For such cases, we can rely on a custom Hibernate ResultTransformer. This application is a sample of writing a custom ResultTransformer.

Key points:

• implement the ResultTransformer interface

227. How To Efficiently Chunk A Java List

Description: Is a common scenario to have a big List and to need to chunk it in multiple smaller List of given size. For example, if we want to employee a concurrent batch implementation we need to give to each thread a sublist of items. Chunking a list can be done via Google Guava, Lists.partition(List list, int size) method or Apache Commons Collections, ListUtils.partition(List list, int size) method. But, it can be implemented in plain Java as well. This application exposes 6 ways to do it. The trade-off is between the speed of implementation and speed of execution. For example, while the implementation relying on grouping collector is not performing very well, it is quite simple and fast to write it.

Key points:

• the fastest execution is provided by Chunk.java class which relies on the built-in List.subList() method

Time-performance trend graphic for chunking 500, 1_000_000, 10_000_000 and 20_000_000 items in lists of 5 items:

228. How To Implement Complex Data Integrity Constraints And Rules

Description: Consider the Book and Chapter entities. A book has a maximum accepted number of pages (book_pages) and the author should not exceed this number. When a chapter is ready for review, the author is submitting it. At this point, the publisher should check that the currently total number of pages doesn't exceed the allowed book_pages:

This kind of checks or constraints are easy to implement via database triggers. This application relies on a MySQL trigger to empower our complex contraint (check_book_pages).

Key points:

• define a MySQL trigger that run after each insert (if you want to run it after each update as well then extract the trigger logic into a function and call it from two triggers - this is specific to MySQL, while is PostgreSQL we have AFTER INSERT OR AFTER UPDATE)

229. How To Check If A Transient Entity Exists In The Database Via Spring Query By Example (QBE)

Description: This application is an example of using Spring Data Query By Example (QBE) to check if a transient entity exists in the database. Consider the Book entity and a Spring controller that exposes an endpoint as: public String checkBook(@Validated @ModelAttribute Book book, ...). Beside writting an explicit JPQL, we can rely on Spring Data Query Builder mechanism or, even better, on Query By Example (QBE) API. In this context, QBE API is quite useful if the entity has a significant number of attributes and:

• for all attributes, we need a head-to-head comparison of each attribute value to the corresponding column value
• for a subset of attributes, we need a head-to-head comparison of each attribute value to the corresponding column value
• for a subset of attributes, we return true at first match between an attribute value and the corresponding column value
• any other scenario

Key points:

• the repository, BookRepository extends QueryByExampleExecutor
• the application uses <S extends T> boolean exists(Example<S> exmpl) with the proper probe (an entity instance populated with the desired fields values)
• moreover, the probe relies on ExampleMatcher which defines the details on how to match particular fields

Note: Do not conclude that Query By Example (QBE) defines only the exists() method. Check out all methods here.

230. Best Way To Use @Transactional

Description: This application is meant to highlight that the best place to use @Transactional for user defined query-methods is in repository interface, and afterwards, depending on situation, on service-methods level.

Key points:

• this application is dissected in my book, Spring Boot Persistence Best Practices.

231. How To Use JPA JOINED Inheritance Strategy And Visitor Design Pattern

Description: This application is an example of using JPA JOINED inheritance strategy and Visitor pattern.

Key points:

• this application allows us to define multiple visitors and apply the one that we want

232. How To Use JPA JOINED Inheritance Strategy And Strategy Design Pattern

Description: This application is an example of using JPA JOINED inheritance strategy and Strategy pattern.

Key points:

• this application allows us to define multiple strategies and apply the one that we want

233. How Spring Transaction Propagation Work

Description: This folder holds several applications that shows how each Spring transaction propagation works.

Key points:

• it is strongly recommended to follow these examples in the context of my book, Spring Boot Persistence Best Practices.

234. How To Use JPA GenerationType.AUTO And UUID Identifiers

Description: This application is an example of using the JPA GenerationType.AUTO for assigning automatically UUID identifiers.

Key points:

• store UUID in a BINARY(16) column

235. How To Manually Assign UUID Identifiers

Description: This application is an example of manually assigning UUID identifiers.

Key points:

• store UUID in a BINARY(16) column

236. How To Use Hibernate uuid2 For Generating UUID Identifiers

Description: This application is an example of using the Hibernate RFC 4122 compliant UUID generator, uuid2.

Key points:

• store UUID in a BINARY(16) column

237. How Hibernate Session-Level Repeatable Reads Works

Description: This Spring Boot application is a sample that reveals how Hibernate session-level repeatable reads works. Persistence Context guarantees session-level repeatable reads. Check out how it works.

Key points:

• rely on two transactions implemented via TransactionTemplate

Note: For a detailed explanation of this application consider my book, Spring Boot Persistence Best Practices

238. Why To Avoid Hibernate-specific hibernate.enable_lazy_load_no_trans

Description: This application is an example of using Hibernate-specific hibernate.enable_lazy_load_no_trans. Check out the application log to see how transactions and database connections are used.

Key points:

• always avoid Hibernate-specific hibernate.enable_lazy_load_no_trans

239. The Best Way To Clone Entities

Description: This application is an example of cloning entities. The best way to achieve this goal relies on copy-constructors. This way we can control what we copy. Here we use a bidirectional-lazy @ManyToMany association between Author and Book.

Key points:

• clone an Author (only the genre) and associate the corresponding books
• clone an Author (only the genre) and clone the books as well

240. How To Include In The UPDATE Statement Only The Modified Columns Via Hibernate @DynamicUpdate

Description: This application is an example of using the Hibernate-specific, @DynamicUpdate. By default, even if we modify only a subset of columns, the triggered UPDATE statements will include all columns. By simply annotating the corresponding entity at class-level with @DynamicUpdate the generated UPDATE statement will include only the modified columns.

Key points:

• pro: avoid updating unmodified indexes (useful for heavy indexing)
• con: cannot reuse the same UPDATE for different subsets of columns via JDBC statements caching (each triggered UPDATE string will be cached and reused accordingly)

241. How To Log Spring Data JPA Repository Query-Method Execution Time

Description: This application is an example of logging execution time for a repository query-method.

Key points:

• write an AOP component (see RepositoryProfiler)

242. How To Take Control Before/After Transaction Commits/Completes Via Callbacks

Description: This application is an example of using the TransactionSynchronizationAdapter for overriding beforeCommit(), beforeCompletion(), afterCommit() and afterCompletion() callbacks globally (application-level) and at method-level.

Key points:

• application-level: write an AOP component (see TransactionProfiler)
• method-level: use TransactionSynchronizationManager.registerSynchronization()

243. How To Fetch DTO Via SqlResultSetMapping And NamedNativeQuery Using {EntityName}.{RepositoryMethodName} Naming Convention

Description: Fetching more data than needed is prone to performance penalities. Using DTO allows us to extract only the needed data. In this application we rely on SqlResultSetMapping, NamedNativeQuery and the {EntityName}.{RepositoryMethodName} naming convention. This convention allows us to create in the repository interface methods with the same name as of native named query.

Key points:

• use SqlResultSetMapping, NamedNativeQuery
• for using Spring Data Projections check this item

244. How To Fetch Entity Via SqlResultSetMapping And NamedNativeQuery Using {EntityName}.{RepositoryMethodName} Naming Convention

Description: This is a sample application of using SqlResultSetMapping, NamedNativeQuery and EntityResult for fetching single entity and multiple entities as List<Object[]>. In this application we rely on the {EntityName}.{RepositoryMethodName} naming convention. This convention allows us to create in the repository interface methods with the same name as of native named query.

Key points:

• use SqlResultSetMapping, NamedNativeQuery and EntityResult

245. How To Use JPA Named Queries @NamedQuery And Spring Projection (DTO)

Description: This application is an example of combining JPA named queries @NamedQuery and Spring projections (DTO). For queries names we use the {EntityName}.{RepositoryMethodName} naming convention. This convention allows us to create in the repository interface methods with the same name as of named query.

Key points:

• define the named queries
• define the proper Spring projection

246. How To Use JPA Named Native Queries @NamedNativeQuery And Spring Projection (DTO)

Description: This application is an example of combining JPA named native queries @NamedNativeQuery and Spring projections (DTO). For queries names we use the {EntityName}.{RepositoryMethodName} naming convention. This convention allows us to create in the repository interface methods with the same name as of named native query.

Key points:

• define the named native queries
• define the proper Spring projection

247. How To Use JPA Named Queries Via a Properties File

Description: JPA named (native) queries are commonly written via @NamedQuery and @NamedNativeQuery annotations in entity classes. Spring Data allows us to write our named (native) queries in a typical *.properties file inside the META-INF folder of your classpath. This way, we avoid modifying our entities. This application shows you how to do it.

Warning: Cannot use native queries with dynamic sorting (Sort). Nevertheless, using Sort in named queries works fine. Moreover, using Sort in Pageable works fine for both, named queries and named native queries. At least this is how it behave in Spring Boot 2.2.2. From this point of view, this approach is better than using @NamedQuery/@NamedNativeQuery or orm.xml file.

Key points:

• define the named (native) queries in a file, META-INF/jpa-named-queries.properties
• follow the Spring {EntityName}.{RepositoryMethodName} naming convention for a quick and slim implementation

248. How To Use JPA Named Queries Via The orm.xml File

Description: JPA named (native) queries are commonly written via @NamedQuery and @NamedNativeQuery annotations in entity classes. Spring Data allows us to write our named (native) queries in a typical orm.xml file inside the META-INF folder of your classpath. This way, we avoid modifying our entities. This application shows you how to do it.

Warning: Pay attention that, via this approach, we cannot use named (native) queries with dynamic sorting (Sort). Using Sort in Pageable is ignored, therefore you need to explicitly add ORDER BY in the queries. At least this is how it behave in Spring Boot 2.2.2. A better approach relies on using a properties file for listing the named (native) queries. In this case, dynamic Sort works for named queries, but not for named native queries. Using Sort in Pageable works as expected in named (native) queries.

Key points:

• define the named (native) queries in a file, META-INF/orm.xml
• follow the Spring {EntityName}.{RepositoryMethodName} naming convention for a quick and slim implementation

249. How To Use JPA Named Queries Via Annotations

Description: JPA named (native) queries are commonly written via @NamedQuery and @NamedNativeQuery annotations in entity classes. This application shows you how to do it.

Warning: Pay attention that, via this approach, we cannot use named (native) queries with dynamic sorting (Sort). Using Sort in Pageable is ignored, therefore you need to explicitly add ORDER BY in the queries. At least this is how it behave in Spring Boot 2.2.2. A better approach relies on using a properties file for listing the named (native) queries. In this case, dynamic Sort works for named queries, but not for named native queries. Using Sort in Pageable works as expected in named (native) queries. And, you don't need to modify/pollute entitites with the above annotations.

Key points:

• use @NamedQuery and @NamedNativeQuery annotations in entity classes
• follow the Spring {EntityName}.{RepositoryMethodName} naming convention for a quick and slim implementation
• avoid Sort and Pageable

250. How To Use JPA Named Queries Via Properties File And Spring Projection (DTO)

Description: This application is an example of combining JPA named queries listed in a properties file and Spring projections (DTO). For queries names we use the {EntityName}.{RepositoryMethodName} naming convention. This convention allows us to create in the repository interface methods with the same name as of named query.

Key points:

• define the named queries in a properties file (e.g., jpa-named-queries.properties) in a folder named META-INF the application classpath
• define the proper Spring projection

251. How To Use JPA Named Native Queries Via Properties File And Spring Projection (DTO)

Description: This application is an example of combining JPA named native queries listed in a properties file and Spring projections (DTO). For queries names we use the {EntityName}.{RepositoryMethodName} naming convention. This convention allows us to create in the repository interface methods with the same name as of named native query.

Key points:

• define the named native queries in a properties file (e.g., jpa-named-queries.properties) in a folder named META-INF the application classpath
• define the proper Spring projection

252. How To Use JPA Named Queries Via orm.xml File And Spring Projection (DTO)

Description: This application is an example of combining JPA named queries listed in orm.xml file and Spring projections (DTO). For queries names we use the {EntityName}.{RepositoryMethodName} naming convention. This convention allows us to create in the repository interface methods with the same name as of named query.

Key points:

• define the named queries in orm.xml file in a folder named META-INF the application classpath
• define the proper Spring projection

253. How To Use JPA Named Native Queries Via orm.xml File And Spring Projection (DTO)

Description: This application is an example of combining JPA named native queries listed in orm.xml file and Spring projections (DTO). For queries names we use the {EntityName}.{RepositoryMethodName} naming convention. This convention allows us to create in the repository interface methods with the same name as of named native query.

Key points:

• define the named native queries in orm.xml file in a folder named META-INF the application classpath
• define the proper Spring projection

254. How To Dto Via Named Native Query And Result Set Mapping Via orm.xml

Description: Fetching more data than needed is prone to performance penalities. Using DTO allows us to extract only the needed data. In this application we rely on named native queries and result set mapping via orm.xml and the {EntityName}.{RepositoryMethodName} naming convention. This convention allows us to create in the repository interface methods with the same name as of native named query.

Key points:

• use <named-native-query/> and <sql-result-set-mapping/> to map the native query to AuthorDto class

255. How To Use Spring Projections(DTO) And Cross Joins

Description: This application is a proof of concept for using Spring Projections(DTO) and cross joins written via JPQL and native SQL (for MySQL).

Key points:

• define two entities (e.g., Book and Format
• populate the database with some test data (e.g., check the file resources/data-mysql.sql)
• write interfaces (Spring projections) that contains getters for the columns that should be fetched from the database (e.g., check BookTitleAndFormatType.java)
• write cross joins queries using JPQL/SQL

256. Calling Stored Procedure That Returns A Result Set Via JdbcTemplate And BeanPropertyRowMapper

Description: This application is an example of calling a MySQL stored procedure that returns a result set via JdbcTemplate and BeanPropertyRowMapper.

Key points:

• rely on JdbcTemplate, SimpleJdbcCall and BeanPropertyRowMapper

257. Defining Entity Listener Class Via @EntityListeners

Description: This application is a sample of using the JPA @MappedSuperclass and @EntityListeners with JPA callbacks.

Key points:

• the base class ,Book, is not an entity, it can be abstract, and is annotated with @MappedSuperclass and @EntityListeners(BookListener.class)
• BookListener defines JPA callbacks (e.g., @PrePersist)
• subclasses of the base class are mapped in tables that contains columns for the inherited attributes and for their own attibutes
• when any entity that is a subclass of Book is persisted, loaded, updated, etc the corresponding JPA callbacks are called

258. Improper Usage Of @Fetch(FetchMode.JOIN) May Causes N+1 Issues

Advice: Always evaluate JOIN FETCH and entities graphs before deciding to use FetchMode.JOIN. The FetchMode.JOIN fetch mode always triggers an EAGER load so the children are loaded when the parents are. Beside this drawback, FetchMode.JOIN may return duplicate results. You’ll have to remove the duplicates yourself (e.g. storing the result in a Set). But, if you decide to go with FetchMode.JOIN at least pay attention to avoid N+1 issues discussed below.

Note: Let's assume three entities, Author, Book and Publisher. Between Author and Book there is a bidirectional-lazy @OneToMany association. Between Author and Publisher there is a unidirectional-lazy @ManyToOne. Between Book and Publisher there is no association.

Now, we want to fetch a book by id (BookRepository#findById()), including its author, and the author's publisher. In such cases, using Hibernate fetch mode, @Fetch(FetchMode.JOIN) works as expected. Using JOIN FETCH or entity graph is also working as expected.

Next, we want to fetch all books (BookRepository#findAll()), including their authors, and the authors publishers. In such cases, using Hibernate fetch mode, @Fetch(FetchMode.JOIN) will cause N+1 issues. It will not trigger the expected JOIN. In this case, using JOIN FETCH or entity graph should be used.

Key points:

• using Hibernate fetch mode, @Fetch(FetchMode.JOIN) doesn't work for query-methods
• Hibernate fetch mode, @Fetch(FetchMode.JOIN) works in cases that fetches the entity by id (primary key) like using EntityManager#find(), Spring Data, findById(), findOne().

259. How To Efficiently Assign A Database Temporary Ranking Of Values To Rows via RANK()

Description: This application is an example of assigning a database temporary ranking of values to rows via the window function, RANK(). This window function is available in almost all databases, and starting with version 8.x is available in MySQL as well.

Key points:

• commonly, you don't need to fetch in the result set the temporary ranking of values produced by RANK() (you will use it internally, in the query, usually in the WHERE clause and CTEs), but, this time, let's write a Spring projection (DTO) that contains a getter for the column generated by RANK() as well
• write several native querys relying on RANK() window function

Output sample:

260. How To Efficiently Assign A Database Temporary Ranking Of Values To Rows via DENSE_RANK()

Description: This application is an example of assigning a database temporary ranking of values to rows via the window function, DENSE_RANK(). In comparison with the RANK() window function, DENSE_RANK() avoid gaps within partition. This window function is available in almost all databases, and starting with version 8.x is available in MySQL as well.

Key points:

• commonly, you don't need to fetch in the result set the temporary ranking of values produced by DENSE_RANK() (you will use it internally, in the query, usually in the WHERE clause and CTEs), but, this time, let's write a Spring projection (DTO) that contains a getter for the column generated by DENSE_RANK() as well
• write several native querys relying on DENSE_RANK() window function

Output sample:

261. How To Efficiently Distribute The Number Of Rows In The Specified (N) Number Of Groups Via NTILE(N)

Description: This application is an example of distributing the number of rows in the specified (N) number of groups via the window function, NTILE(N). This window function is available in almost all databases, and starting with version 8.x is available in MySQL as well.

Key points:

• commonly, you don't need to fetch in the result set the temporary ranking of values produced by NTILE() (you will use it internally, in the query, usually in the WHERE clause and CTEs), but, this time, let's write a Spring projection (DTO) that contains a getter for the column generated by NTILE() as well
• write several native querys relying on NTILE() window function

Output sample:

262. How To Write Derived Count And Delete Queries

Description: Spring Data comes with the Query Builder mechanism for JPA that is capable to interpret a query method name (known as a derived query) and convert it into a SQL query in the proper dialect. This is possible as long as we respect the naming conventions of this mechanism. Beside the well-known query of type find..., Spring Data supports derived count queries and derived delete queries.

Key points:

• a derived count query starts with count... (e.g., long countByGenre(String genre)) - Spring Data will generate a SELECT COUNT(...) FROM ... query
• a derived delete query can return the number of deleted records or the list of the deleted records
• a derived delete query that returns the number of deleted records starts with delete... or remove... and returns long (e.g., long deleteByGenre(String genre)) - Spring Data will trigger first a SELECT to fetch entities in the Persistence Context, and, afterwards, it triggers a DELETE for each entity that must be deleted
• a derived delete query that returns the list of deleted records starts with delete... or remove... and returns List<entity> (e.g., List<Author> removeByGenre(String genre)) - Spring Data will trigger first a SELECT to fetch entities in the Persistence Context, and, afterwards, it triggers a DELETE for each entity that must be deleted

263. Working With Spring Data Property Expressions

Description: Property expressions can refer to a direct property of the managed entity. However, you can also define constraints by traversing nested properties. This application is a sample of traversing nested properties for fetching entities and DTOs.

Key points:

• Assume an Author has several Book and each book has several Review (between Author and Book there is a bidirectional-lazy @oneToMany association, and, between Book and Review there is also a bidirectional-lazy @OneToMany association)
• Assume that we fetched a Review and we want to know the Author of the Book that has received this Review
• via property expressions, we can write in AuthorRepository the following query that will be processed by the Spring Data Query Builder mechanism: Author findByBooksReviews(Review review);
• Behind the scene Spring Data will produce a SELECT with two LEFT JOIN
• In this case, the method creates the property traversal books.reviews. The algorithm starts by interpreting the entire part (BooksReviews) as the property and checks the domain class for a property with that name (uncapitalized). If the algorithm succeeds, it uses that property. If not, the algorithm splits up the source at the camel case parts from the right side into a head and a tail and tries to find the corresponding property — in our example, Books and Reviews. If the algorithm finds a property with that head, it takes the tail and continues building the tree down from there, splitting the tail up in the way just described. If the first split does not match, the algorithm moves the split point to the left and continues.
• Although this algorithm should work for most cases, it is possible for the algorithm to select the wrong property. Suppose the Author class has an booksReview property as well. The algorithm would match in the first split round already, choose the wrong property, and fail (as the type of booksReview probably has no code property). To resolve this ambiguity you can use _ inside your method name to manually define traversal points. So our method name would be as follows: Author findByBooks_Reviews(Review review);
• More examples (including DTOs) are available in the application

264. The Best Way To Fetch Parent And Children In Different Queries

Note: Fetching read-only data should be done via DTO, not managed entities. But, there is no tragedy to fetch read-only entities in a context as follows:

• we need all attributes of the entity (so, a DTO just mirrors an entity)
• we manipulate a small number of entities (e.g., an author with several books)
• we use @Transactional(readOnly = true)

Under these circumstances, let's tackle a common case that I saw quite a lot. There is even an SO answer about it (don't do this):

Description: Let's assume that Author and Book are involved in a bidirectional-lazy @OneToMany association. Imagine an user that loads a certain Author (without the associated Book). The user may be interested or not in the Book, therefore, we don't load them with the Author. If the user is interested in the Book then he will click a button of type, View books. Now, we have to return the List<Book> associated to this Author.

So, at first request (query), we fetch an Author. The Author is detached. At second request (query), we want to load the Book associated to this Author. But, we don't want to load the Author again (for example, we don't care about lost updates of Author), we just want to load the associated Book in a single SELECT. A common (not recommended) approach is to load the Author again (e.g., via findById(author.getId())) and call the author.getBooks(). But, this end up in two SELECT statements. One SELECT for loading the Author, and another SELECT after we force the collection initialization. We force collection initialization because it will not be initialize if we simply return it. In order to trigger the collection initialization the developer call books.size() or he rely on Hibernate.initialize(books);.

But, we can avoid such solution by relying on an explicit JPQL or Query Builder property expressions. This way, there will be a single SELECT and no need to call size() or Hibernate.initialize();

Key points:

• use an explicit JPQL
• use Query Builder propery expressions

This item is detailed in my book, Spring Boot Persistence Best Practices.

265. How To Optimize The Merge Operation Using Update

Description: Behind the built-in Spring Data save() there is a call of EntityManager#persist() or EntityManager#merge(). It is important to know this aspect in several cases. Among this cases, we have the entity update case (simple update or update batching).

Consider Author and Book involved in a bidirectional-lazy @OneToMany association. And, we load an Author, detach it, update it in the detached state, and save it to the database via save() method. Calling save() will come with the following two issues resulting from calling merge() behind the scene:

• there will be two SQL statements, one SELECT (merge) and one UPDATE
• the SELECT will contain a LEFT OUTER JOIN to fetch the associated Book as well (we don't need the books!)

How about triggering only the UPDATE instead of this? The solution relies on calling Session#update(). Calling Session.update() requires to unwrap the Session via entityManager.unwrap(Session.class).

Key points:

• calling Session.update() will trigger only the UPDATE (there is no SELECT)
• Session.update() works with versioned optimistic locking mechanism as well (so, lost updates are prevented)

266. How To NOT Use Spring Data Streamable

Description: This application is a sample of fetching Streamable<entity> and Streamable<dto>. But, more important, this application contains three examples of how to not use Streamable. It is very tempting and comfortable to fetch a Streamable result set and chop it via filter(), map(), flatMap(), and so on until we obtain only the needed data instead of writing a query (e.g., JPQL) that fetches exactly the needed result set from the database. Mainly, we just throw away some of the fetched data to keep only the needed data. But, is not advisable to follow such practices because fetching more data than needed can cause significant performance penalties.

Moreover, pay attention to combining two or more Streamable via the and() method. The returned result may be different from what you are expecting to see. Each Streamable produces a separate SQL statement and the final result set is a concatenation of the intermediate results sets (prone to duplicate values).

Key points:

• don't fetch more columns than needed just to drop a part of them (e.g., via map())
• don't fetch more rows than needed just to throw away a part of it (e.g., via filter())
• pay attention on combining Streamable via and(); each Streamable produces a separate SQL statement and the final result set is a concatenation of the intermediate results sets (prone to duplicate values)

267. How To Return Custom Streamable Wrapper Types

Description: A common practice consists of exposing dedicated wrappers types for collections resulted after mapping a query result set. This way, on a single query execution, the API can return multiple results. After we call a query-method that return a collection, we can pass it to a wrapper class by manually instantiation of that wrapper-class. But, we can avoid the manually instantiation if the code respects the following key points.

Key points:

• the type implements Streamable
• the type exposes a constructor (used in this example) or a static factory method named of(…) or valueOf(…) taking Streamable as argument

268. How To Use In Spring Boot JPA 2.1 Schema Generation And Data Loading

Description: JPA 2.1 come with schema generation features. This feature can setup the database or export the generated commands to a file. The parameters that we should set are:

• spring.jpa.properties.javax.persistence.schema-generation.database.action: Instructs the persistence provider how to setup the database. Possible values include: none, create, drop-and-create, drop

• javax.persistence.schema-generation.scripts.action: Instruct the persistence provider which scripts to create. Possible values include: none, create, drop-and-create, drop.

• javax.persistence.schema-generation.scripts.create-target: Indicate the target location of the create script generated by the persistence provider. This can be as a file URL or a java.IO.Writer.

• javax.persistence.schema-generation.scripts.drop-target: Indicate the target location of the drop script generated by the persistence provider. This can be as a file URL or a java.IO.Writer.

Moreover, we can instruct the persistence provider to load data from a file into the database via: spring.jpa.properties.javax.persistence.sql-load-script-source. The value of this property represents the file location and it can be a file URL or a java.IO.Writer.

Key points:

• the settings are available in application.properties

269. How To Return A Map Result From A Spring Data Query Method

Description: Sometimes, we need to write in repositories certain query-methods that return a Map instead of a List or a Set. For example, when we need a Map<Id, Entity> or we use GROUP BY and we need a Map<Group, Count>. This application shows you how to do it via default methods directly in repository.

Key points:

• rely on default methods and Collectors.toMap()

270. How To Handle Entities Inheritance With Spring Data Repositories

Description: Consider one of the JPA inheritance strategies (e.g., JOINED). Handling entities inheritance With Spring Data repositories can be done as follows:

• Single Table
• Joined
• Table per class
• Mapped Superclass