spring-guides/gs-multi-module

You will set up a library jar that exposes a service for simple “Hello, World” messages and then include the service in a web application that uses the library as a dependency.

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This guide walks through building two projects, one of which is a dependency to the other. Consequently, you need to create two child projects under a root project. But first, create the build configuration at the top level. For Maven you will want a pom.xml with <modules> listing the subdirectories:

For Gradle, you will want a settings.gradle including the same directories:

and (optionally) you could include an empty build.gradle (to help IDEs identify the root directory).

Create the Directory Structure

└── library
└── application

One of the two projects serves as a library that the other project (the application) will use.

Create the Directory Structure

└── src
    └── main
        └── java
            └── com
                └── example
                    └── multimodule
                        └── service

Setting up the Library Project

link:initial/library/pom.xml[]

link:initial/library/build.gradle[]

$ mv mvnw* .mvn ..
$ mv gradlew* gradle ..

<build>
  <plugins>
    <plugin>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-maven-plugin</artifactId>
    </plugin>
  </plugins>
</build>

link:complete/library/pom.xml[]

plugins {
  id 'org.springframework.boot' version '2.5.2' apply false
  id 'io.spring.dependency-management' version '1.0.11.RELEASE'
  // ... other plugins
}

dependencyManagement {
  imports {
    mavenBom org.springframework.boot.gradle.plugin.SpringBootPlugin.BOM_COORDINATES
  }
}

link:complete/library/build.gradle[]

The library will provide a MyService class that can be used by applications. The following listing (from library/src/main/java/com/example/multimodule/service/MyService.java) shows the MyService class:

To make it configurable in the standard Spring Boot idiom (with application.properties), you can also add a @ConfigurationProperties class. The ServiceProperties class (from library/src/main/java/com/example/multimodule/service/ServiceProperties.java) fills that need:

You need not do it this way. A library might merely provide pure Java APIs and no Spring features. In that case, the application that consumes the library would need to provide the configuration itself.

You will want to write unit tests for your library components. If you provide re-usable Spring configuration as part of the library, you might also want to write an integration test, to make sure that the configuration works. To do that, you can use JUnit and the @SpringBootTest annotation. The following listing (from library/src/test/java/com/example/multimodule/service/MyServiceTest.java) shows how to do so:

The Application project uses the Library project, which offers a service that other projects can use.

Create the Directory Structure

└── src
    └── main
        └── java
            └── com
                └── example
                    └── multimodule
                        └── application

Setting up the Application Project

link:initial/application/pom.xml[]

link:initial/application/build.gradle[]

$ rm -rf mvnw* .mvn
$ rm -rf gradlew* gradle

<dependency>
  <groupId>com.example</groupId>
  <artifactId>library</artifactId>
  <version>${project.version}</version>
</dependency>

link:complete/application/pom.xml[]

implementation project(':library')

link:complete/application/build.gradle[]

The main class in the application can be a @RestController that uses the Service from the library to render a message. The following listing (from application/src/main/java/com/example/multimodule/application/DemoApplication.java) shows such a class:

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Because DemoApplication is inside a different package (com.example.multimodule.application) than MyService (com.example.multimodule.service), @SpringBootApplication cannot automatically detect it. There are different ways to let `MyService be picked up:

• Import it directly with @Import(MyService.class).

• Fetch everything from its package by using @SpringBootApplication(scanBasePackageClasses={…​}).

• Specifying the parent package by name: com.example.multimodule. (This guide uses this method)

You need to provide a message for the service in the library in application.properties. In the source folder, you need to create a file named src/main/resources/application.properties. The following listing shows a file that would work:

Test the end-to-end result by starting the application. You can start the application in your IDE or use the command line. Once the application is running, visit the client application in the browser, at http://localhost:8080/. There, you should see Hello, World reflected in the response.

If you use Gradle, the following command (really two commands run in sequence) will first build the library and then run the application:

If you use Maven, the following command (really two commands run in sequence) will first build the library and then run the application:

Congratulations! You have used Spring Boot to create a re-usable library and then used that library to build an application.

The following guides may also be helpful:

• Building an Application with Spring Boot

• Centralized Configuration

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