Error Prone Support is a Picnic-opinionated extension of Google's Error Prone. It aims to improve code quality, focussing on maintainability, consistency and avoidance of common pitfalls.

Error Prone is a static analysis tool for Java that catches common programming mistakes at compile-time.

To learn more about Error Prone (Support), how you can start using Error Prone in practice, and how we use it at Picnic, watch the conference talk Automating away bugs with Error Prone in practice. Also consider checking out the blog post Picnic loves Error Prone: producing high-quality and consistent Java code.

Getting started • Developing Error Prone Support • How it works • Contributing

This library is built on top of Error Prone. To use it, read the installation guide for Maven or Gradle below. The library requires that your build is executed using JDK 17 or above, but supports builds that target older versions of Java.

1. First, follow Error Prone's installation guide.

2. Next, edit your pom.xml file to add one or more Error Prone Support modules to the annotationProcessorPaths of the maven-compiler-plugin:

   <build>
       <pluginManagement>
           <plugins>
               <plugin>
                   <groupId>org.apache.maven.plugins</groupId>
                   <artifactId>maven-compiler-plugin</artifactId>
                   <!-- Prefer using the latest release. -->
                   <version>3.12.0</version>
                   <configuration>
                       <annotationProcessorPaths>
                           <!-- Error Prone itself. -->
                           <path>
                               <groupId>com.google.errorprone</groupId>
                               <artifactId>error_prone_core</artifactId>
                               <version>${error-prone.version}</version>
                           </path>
                           <!-- Error Prone Support's additional bug checkers. -->
                           <path>
                               <groupId>tech.picnic.error-prone-support</groupId>
                               <artifactId>error-prone-contrib</artifactId>
                               <version>${error-prone-support.version}</version>
                           </path>
                           <!-- Error Prone Support's Refaster rules. -->
                           <path>
                               <groupId>tech.picnic.error-prone-support</groupId>
                               <artifactId>refaster-runner</artifactId>
                               <version>${error-prone-support.version}</version>
                           </path>
                       </annotationProcessorPaths>
                       <compilerArgs>
                           <arg>
                               -Xplugin:ErrorProne
                               <!-- Add other Error Prone flags here. See
                               https://errorprone.info/docs/flags. -->
                           </arg>
                           <arg>-XDcompilePolicy=simple</arg>
                       </compilerArgs>
                       <!-- Enable this if you'd like to fail your build upon warnings. -->
                       <!-- <failOnWarning>true</failOnWarning> -->
                   </configuration>
               </plugin>
           </plugins>
       </pluginManagement>
   </build>

1. First, follow the installation guide of the gradle-errorprone-plugin.

2. Next, edit your build.gradle file to add one or more Error Prone Support modules:

   dependencies {
       // Error Prone itself.
       errorprone("com.google.errorprone:error_prone_core:${errorProneVersion}")
       // Error Prone Support's additional bug checkers.
       errorprone("tech.picnic.error-prone-support:error-prone-contrib:${errorProneSupportVersion}")
       // Error Prone Support's Refaster rules.
       errorprone("tech.picnic.error-prone-support:refaster-runner:${errorProneSupportVersion}")
   }
   
   tasks.withType(JavaCompile).configureEach {
       options.errorprone.disableWarningsInGeneratedCode = true
       // Add other Error Prone flags here. See:
       // - https://github.com/tbroyer/gradle-errorprone-plugin#configuration
       // - https://errorprone.info/docs/flags
   }

Consider the following example code:

import com.google.common.collect.ImmutableSet;
import java.math.BigDecimal;

public class Example {
  static BigDecimal getNumber() {
    return BigDecimal.valueOf(0);
  }

  public ImmutableSet<Integer> getSet() {
    ImmutableSet<Integer> set = ImmutableSet.of(1);
    return ImmutableSet.copyOf(set);
  }
}

If the installation was successful, then building the above code with Maven should yield two compiler warnings:

$ mvn clean install
...
[INFO] Example.java:[9,34] [Refaster Rule] BigDecimalRules.BigDecimalZero: Refactoring opportunity
    (see https://error-prone.picnic.tech/refasterrules/BigDecimalRules#BigDecimalZero)
  Did you mean 'return BigDecimal.ZERO;'?
...
[WARNING] Example.java:[13,35] [IdentityConversion] This method invocation appears redundant; remove it or suppress this warning and add a comment explaining its purpose
    (see https://error-prone.picnic.tech/bugpatterns/IdentityConversion)
  Did you mean 'return set;' or '@SuppressWarnings("IdentityConversion") public ImmutableSet<Integer> getSet() {'?
...

Two things are kicking in here:

1. An Error Prone BugChecker that flags unnecessary identity conversions.
2. A Refaster rule capable of rewriting expressions of the form BigDecimal.valueOf(0) and new BigDecimal(0) to BigDecimal.ZERO.

Be sure to check out all bug checks and refaster rules.

This is a Maven project, so running mvn clean install performs a full clean build and installs the library to your local Maven repository.

Once you've made changes, the build may fail due to a warning or error emitted by static code analysis. The flags and commands listed below allow you to suppress or (in a large subset of cases) automatically fix such cases. Make sure to carefully check the available options, as this can save you significant amounts of development time!

Relevant Maven build parameters:

• -Dverification.warn makes the warnings and errors emitted by various plugins and the Java compiler non-fatal, where possible.
• -Dverification.skip disables various non-essential plugins and compiles the code with minimal checks (i.e. without linting, Error Prone checks, etc.).
• -Dversion.error-prone=some-version runs the build using the specified version of Error Prone. This is useful e.g. when testing a locally built Error Prone SNAPSHOT.
• -Perror-prone-fork runs the build using Picnic's Error Prone fork, hosted on Jitpack. This fork generally contains a few changes on top of the latest Error Prone release.
• -Pself-check runs the checks defined by this project against itself. Pending a release of google/error-prone#3301, this flag must currently be used in combination with -Perror-prone-fork.

Other highly relevant commands:

• mvn fmt:format formats the code using google-java-format.
• ./run-full-build.sh builds the project twice, where the second pass validates compatbility with Picnic's Error Prone fork and compliance of the code with any rules defined within this project. (Consider running this before opening a pull request, as the PR checks also perform this validation.)
• ./apply-error-prone-suggestions.sh applies Error Prone and Error Prone Support code suggestions to this project. Before running this command, make sure to have installed the project (mvn clean install) and make sure that the current working directory does not contain unstaged or uncommited changes.
• ./run-branch-mutation-tests.sh uses Pitest to run mutation tests against code that is modified relative to the upstream default branch. The results can be reviewed by opening the respective target/pit-reports/index.html files. One can use ./run-mutation-tests.sh to run mutation tests against all code in the current working directory. For more information check the PIT Maven plugin.

When running the project's tests in IntelliJ IDEA, you might see the following error:

java: exporting a package from system module jdk.compiler is not allowed with --release

If this happens, go to Settings -> Build, Execution, Deployment -> Compiler -> Java Compiler and deselect the option Use '--release' option for cross-compilation (Java 9 and later). See IDEA-288052 for details.

This project provides additional BugChecker implementations.

Want to report or fix a bug, suggest or add a new feature, or improve the documentation? That's awesome! Please read our contribution guidelines.

If you want to report a security vulnerability, please do so through a private channel; please see our security policy for details.