Error: test loader org.eclipse.jdt.internal.junit6.runner.JUnit6TestLoader not found

[LorenzoBettini]

In a Maven project using junit6, when I try to run the tests, I get

java.lang.IllegalArgumentException: Error: test loader org.eclipse.jdt.internal.junit6.runner.JUnit6TestLoader not found:
java.lang.reflect.InvocationTargetException
	at java.base/jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:74)
	at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:502)
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:486)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.createRawTestLoader(RemoteTestRunner.java:372)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.createLoader(RemoteTestRunner.java:367)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.defaultInit(RemoteTestRunner.java:311)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.init(RemoteTestRunner.java:226)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:210)
Caused by: java.lang.NoClassDefFoundError: org/junit/platform/engine/ConfigurationParameters
	at org.eclipse.jdt.internal.junit6.runner.JUnit6TestLoader.<init>(JUnit6TestLoader.java:37)
	at java.base/jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:62)
	... 7 more
Caused by: java.lang.ClassNotFoundException: org.junit.platform.engine.ConfigurationParameters
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
	... 9 more

	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.createRawTestLoader(RemoteTestRunner.java:377)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.createLoader(RemoteTestRunner.java:367)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.defaultInit(RemoteTestRunner.java:311)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.init(RemoteTestRunner.java:226)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:210)

I seem to recall that JUnit 6 is supported in 2025-12

[trancexpress]

Similar to this maybe? #2695, eclipse-m2e/m2e-core#2102

If you can reproduce with a Java project, then it should be something else. If not, its probably the same.

[LorenzoBettini]

@trancexpress I confirm that adding to the .classpath

<classpathentry kind="con" path="org.eclipse.jdt.junit.JUNIT_CONTAINER/6"/>

I should have thought about that myself because I have the same problem with JUnit 5 in Tycho projects.

I seem to understand that the problem is due to the Maven integration not to JDT itself, right?

[LorenzoBettini]

Though I'm quite sure it used to work in 2025-09 without this explicit classpath entry.

[trancexpress]

I seem to understand that the problem is due to the Maven integration not to JDT itself, right?

Hard to be sure without an attached project, but likely yes.

Though I'm quite sure it used to work in 2025-09 without this explicit classpath entry.

It really depends on what m2e is doing, I'm not familiar with that. Generally, specifying JUnit 6 libraries on the classpath and telling JDT to use a JUnit 5 type launch no longer works. It used to work in 2025-09, see: #2654

And explanation why it no longer works: #2655 (comment)

Essentially JDT will automatically add some JUnit libraries, if it finds (I think) junit-jupiter-api on the classpath. What is added is now version constrained, i.e. JUnit 5 libraries for a JUnit 5 launch kind. If there is also JUnit 6 on the classpath, this can result in odd errors at runtime:

• https://eclipse.dev/eclipse/markdown/?f=news/4.38/jdt.md#junit-5-and-junit-6-conflicts
• https://eclipse.dev/eclipse/markdown/?f=news/4.38/pde.md#junit-plug-in-test-launch

[LorenzoBettini]

Some further experiments: I removed the JUnit 6 classpath entry, reproduced the problem, then I replaced:

		<dependency>
			<groupId>org.junit.jupiter</groupId>
			<artifactId>junit-jupiter-api</artifactId>
			<scope>test</scope>
		</dependency>

with

		<dependency>
			<groupId>org.junit.jupiter</groupId>
			<artifactId>junit-jupiter</artifactId>
			<scope>test</scope>
		</dependency>

And this allowed me to run JUnit 6 tests from Eclipse without problems.

Thus, junit-jupiter-api is not enough. I guess that junit-jupiter works because it also drags the engine deps.

[trancexpress]

Thus, junit-jupiter-api is not enough. I guess that junit-jupiter works because it also drags the engine deps.

I double-checked what we do with junit-jupiter-api, since I thought we add extra JUnit libraries if we see junit-jupiter-api... But this is probably just in PDE and I misremembered JDT doing the same. Still, I found: #2721

It shouldn't be relevant here, since the code block in question adds junit-jupiter-api if its missing.