How to automatically migrate from JUnit 4 to JUnit 5?

Question

In the spirit of this question from JUnit 3 to JUnit 4, are there any list of regular expressions to efficiently migrate from the junit 4 API to the junit 5 API, regardless of the code size?

Answer 1

The tooling at the moment is not great, but improving:

• IntelliJ: Migrates most annotations to JUnit 5 versions. Hovewer, does not do anything if your test file contains @Rules (e.g., ExpectedException) as of v2018.2.
• Error Prone: contains built-in refactorings to automatically migrate various JUnit 4 exception-testing idioms (try/fail/catch/assert, ExpectedExceptions, @Test(expected = …)) to assertThrows, perfectly augmenting IntelliJ.

I recommend the following steps:

1. Add JUnit 5 artefacts to your test dependencies.
2. Install Error Prone compiler.
3. Configure it to produce a patch to migrate to assertThrows, enabling the following checks (the example Maven configuration is below):
   • TryFailRefactoring,
   • ExpectedExceptionRefactoring,
   • TestExceptionRefactoring.
4. Use IntelliJ built-in refactorings to migrate JUnit 4 annotations and methods to JUnit 5 counterparts.

I am not aware of any tools that help to automatically migrate Parameterized tests, or rules other than ExpectedException.

Here is an example Error Prone configuration:

<build>
  <plugins>
    <plugin>
      <artifactId>maven-compiler-plugin</artifactId>
      <version>3.8.0</version>
      <configuration>
        <compilerId>javac-with-errorprone</compilerId>
        <showWarnings>true</showWarnings>
        <forceJavacCompilerUse>true</forceJavacCompilerUse>
        <source>${java.compiler.source}</source>
        <target>${java.compiler.target}</target>
        <compilerArgs>        
          <arg>-XepPatchChecks:TryFailRefactoring,ExpectedExceptionRefactoring,TestExceptionRefactoring</arg>
          <arg>-XepPatchLocation:${project.basedir}</arg>
        </compilerArgs>
      </configuration>
      <dependencies>
        <dependency>
          <groupId>org.codehaus.plexus</groupId>
          <artifactId>plexus-compiler-javac-errorprone</artifactId>
          <version>2.8.3</version>
        </dependency>
        <dependency>
          <groupId>com.google.errorprone</groupId>
          <artifactId>error_prone_core</artifactId>
          <version>2.3.2</version>
        </dependency>
      </dependencies>
    </plugin>
  </plugins>
</build>

Answer 2

With openrewrite you can migrate automagically.

Just run:

mvn org.openrewrite.maven:rewrite-maven-plugin:4.36.0:run -Drewrite.recipeArtifactCoordinates=org.openrewrite.recipe:rewrite-testing-frameworks:1.28.0 -Drewrite.activeRecipes=org.openrewrite.java.testing.junit5.JUnit5BestPractices