Multiple junit-platform.properties on the classpath

Problem

The test-test JARs from Apache Kafka contain a junit-platform.properties which clashes with the one present in the project.

The JARs are named test-test because their scope in the project is test and they contain the test-classes (as well as the test resources) of the project they come from as described in Apache Maven JAR Plugin documentation.

For Gradle the concept is similar.

As such, JUnit issues a WARNING during the execution of the test(s) claiming that only the first one will be used.

Aug 09, 2024 4:35:50 PM org.junit.platform.launcher.core.LauncherConfigurationParameters loadClasspathResource
WARNING: Discovered 3 'junit-platform.properties' configuration files in the classpath; only the first will be used.
Aug 09, 2024 4:35:51 PM org.junit.platform.launcher.core.LauncherConfigurationParameters loadClasspathResource
WARNING: Discovered 3 'junit-platform.properties' configuration files in the classpath; only the first will be used.
Aug 09, 2024 4:35:51 PM org.junit.platform.launcher.core.LauncherConfigurationParameters loadClasspathResource
WARNING: Discovered 3 'junit-platform.properties' configuration files in the classpath; only the first will be used.

As such, depending on the order used by the classloader, the one present in the project may not be used leading to problems and/or impacting the performances if the test(s) relies on that configuration.

The issue is also tracked on the JUnit issue tracker at junit-team/junit5#2794.

Find the culprits

In order to find the JARs causing the issue, a dummy test has been written that just scans the classpath and prints all the occurrences of the junit-platform.properties file in the classpath.

The code of the test comes from junit-team/junit5#2207 (comment).

To run the test, execute the command:

mvn clean test

An output like the following should be printed, showing that there are multiple junit-platform.properties files on the classpath:

[INFO] Scanning for projects...
[INFO] 
[INFO] ---------------< org.example:kafka-junit-platform-props >---------------
[INFO] Building kafka-junit-platform-props 1.0-SNAPSHOT
[INFO]   from pom.xml
[INFO] --------------------------------[ jar ]---------------------------------
[INFO] 
[INFO] --- clean:3.2.0:clean (default-clean) @ kafka-junit-platform-props ---
[INFO] Deleting C:\Users\Cosimo Damiano Prete\IdeaProjects\kafka-junit-platform-props\target
[INFO]
[INFO] --- resources:3.3.1:resources (default-resources) @ kafka-junit-platform-props ---
[INFO] Copying 0 resource from src\main\resources to target\classes
[INFO]
[INFO] --- compiler:3.13.0:compile (default-compile) @ kafka-junit-platform-props ---
[INFO] Nothing to compile - all classes are up to date.
[INFO]
[INFO] --- resources:3.3.1:testResources (default-testResources) @ kafka-junit-platform-props ---
[INFO] Copying 1 resource from src\test\resources to target\test-classes
[INFO]
[INFO] --- compiler:3.13.0:testCompile (default-testCompile) @ kafka-junit-platform-props ---
[INFO] Recompiling the module because of changed source code.
[INFO] Compiling 1 source file with javac [debug target 21] to target\test-classes
[INFO] 
[INFO] --- surefire:3.2.5:test (default-test) @ kafka-junit-platform-props ---
[INFO] Using auto detected provider org.apache.maven.surefire.junitplatform.JUnitPlatformProvider
[INFO] 
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
Aug 09, 2024 4:50:58 PM org.junit.platform.launcher.core.LauncherConfigurationParameters loadClasspathResource
WARNING: Discovered 3 'junit-platform.properties' configuration files in the classpath; only the first will be used.
Aug 09, 2024 4:50:59 PM org.junit.platform.launcher.core.LauncherConfigurationParameters loadClasspathResource
WARNING: Discovered 3 'junit-platform.properties' configuration files in the classpath; only the first will be used.
Aug 09, 2024 4:50:59 PM org.junit.platform.launcher.core.LauncherConfigurationParameters loadClasspathResource
WARNING: Discovered 3 'junit-platform.properties' configuration files in the classpath; only the first will be used.
[INFO] Running ClassPathTest

loader = jdk.internal.loader.ClassLoaders$AppClassLoader@70dea4e
loader.parent = jdk.internal.loader.ClassLoaders$PlatformClassLoader@10163d6
loader.parent.parent = null

single: file:.../IdeaProjects/kafka-junit-platform-props/target/test-classes/junit-platform.properties

multi: file:.../IdeaProjects/kafka-junit-platform-props/target/test-classes/junit-platform.properties
multi: jar:file:.../.m2/repository/org/apache/kafka/kafka-clients/3.7.1/kafka-clients-3.7.1-test.jar!/junit-platform.properties
multi: jar:file:.../.m2/repository/org/apache/kafka/kafka-server-common/3.7.1/kafka-server-common-3.7.1-test.jar!/junit-platform.properties

distinct: jar:file:.../.m2/repository/org/apache/kafka/kafka-server-common/3.7.1/kafka-server-common-3.7.1-test.jar!/junit-platform.properties
distinct: file:.../IdeaProjects/kafka-junit-platform-props/target/test-classes/junit-platform.properties
distinct: jar:file:.../.m2/repository/org/apache/kafka/kafka-clients/3.7.1/kafka-clients-3.7.1-test.jar!/junit-platform.properties
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.061 s -- in ClassPathTest
[INFO] 
[INFO] Results:
[INFO]
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  4.774 s
[INFO] Finished at: 2024-08-09T16:50:59+02:00
[INFO] ------------------------------------------------------------------------

The starting point of the paths has been removed for privacy reasons.

The output shows that the additional properties files are coming from the test-test JARs since the JAR names follow the pattern <name>-<version>-test.jar.

Possible solutions

The ideal solution would be to remove the test-test JAR files.
If this is not possible, it would already be beneficial to remove the junit-platform.properties file from them.