Launch configuration does not use the project JRE but Java 17

[nxglhez]

Hello,

I am trying to figure what's gone wrong in my Eclipse 2025-09: my project are all using Java 21 however, when I try to run a JUnit test, Eclipse pick up Java 17: it warns me about "class incompatibility", select the wrong JDK (17):

This happens whenever I want to run a unit test.
And the default workspace JRE is set to java 21

Note: I can not use 2025-12 due to #4687
And I did have the same issue with eclipse-m2e/m2e-core#2091 (comment) but I though it was due to a conflict (now resolved) between EclEmma and m2e.

[trancexpress]

Can you attach a minimal reproducer?

The code in question should be in JDT debug, JavaJRETab:342:

https://github.com/eclipse-jdt/eclipse.jdt.debug/blob/2e3d5c81b72e043c94c03ecc133d4958e6eaccac/org.eclipse.jdt.debug.ui/ui/org/eclipse/jdt/debug/ui/launchConfigurations/JavaJRETab.java#L342

It seems to complain that compliance (JLS) version is higher than the JRE version of the launch.

Might also be worth checking the installed JREs preferences page, maybe the 21 JDK/JRE is misconfigured.

[nxglhez]

Can you attach a minimal reproducer?

Finding a culprit will be hard : we use Oomph and perhaps a set of plugins conflict with one another.
At least, I started having this problem with #4687

But I can take some time to debug my instance of Eclipse using the JDT Debug dev environment.

The code in question should be in JDT debug, JavaJRETab:342:
https://github.com/eclipse-jdt/eclipse.jdt.debug/blob/2e3d5c81b72e043c94c03ecc133d4958e6eaccac/org.eclipse.jdt.debug.ui/ui/org/eclipse/jdt/debug/ui/launchConfigurations/JavaJRETab.java#L342

This part is what show the "error about bytecode compliance", but where is the part that says "create a junit launch with this JRE" ?

Can m2e contribute to these selected JRE ? (I say that because it configured the VM properties based on my pom file).

Might also be worth checking the installed JREs preferences page, maybe the 21 JDK/JRE is misconfigured.

I've seen two cases:

• In some case, there is no JRE selected for the execution environment but one is in bold with "Perfect match", so I assume Eclipse select it.
• When I restart, I think the task that "find installed JRE" do something because 1) the JRE is selected when it was not 2) the launch file is gone back to Java 17 even after my change.

[nxglhez]

Thank for the debugging points: I think I've found part of the problem.

While the JavaRuntime.computeVMInstall breakpoint was not triggered, I used the other (JRETab) as a basic since it was triggered: org.eclipse.jdt.debug.ui.launchConfigurations.JavaJRETab.initializeFrom(ILaunchConfiguration) is being called from JUnitTabGroup and in the ILaunchConfiguration, the attributes are already invalid:

• org.eclipse.jdt.junit.TEST_KIND=org.eclipse.jdt.junit.loader.junit5
• org.eclipse.jdt.launching.JRE_CONTAINER=org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/openjdk-17-hotspot

This is interesting: I am using JUnit 6 and the loader is junit5 (I assume it is available since 2025-12).

The (truncated) trace:

Thread [main] (Suspended (breakpoint at line 1231 in LaunchManager))	
	owns: LaunchConfigurationWorkingCopy  (id=24719)	
	owns: LaunchConfigurationWorkingCopy  (id=24691)	
	LaunchManager.getInfo(LaunchConfiguration) line: 1231	
	LaunchConfiguration.getInfo() line: 465	
	LaunchConfiguration.getType() line: 621	
	LaunchManager.isValid(ILaunchConfiguration) line: 1947	
	LaunchManager.launchConfigurationChanged(ILaunchConfiguration) line: 2002	
	LaunchConfigurationWorkingCopy.writeNewFile(IProgressMonitor) line: 380	
	LaunchConfigurationWorkingCopy.doSave0(IProgressMonitor) line: 259	
	LaunchConfigurationWorkingCopy.doSave(IProgressMonitor) line: 229	
	LaunchConfigurationWorkingCopy.doSave() line: 187	
	UnitTestSupport$ConfigurationManager.defineConfigurationValues(IProject, ILaunchConfiguration, TestLaunchArguments, Optional<MavenExecutionJre>) line: 302	
	UnitTestSupport$ConfigurationManager.setupLaunchConfiguration(ILaunchConfiguration) line: 243	
	UnitTestSupport.setupLaunchConfigurationFromMavenConfiguration(ILaunchConfiguration) line: 169	
	MavenLaunchConfigurationListener.launchConfigurationAdded(ILaunchConfiguration) line: 47	
	LaunchManager$ConfigurationNotifier.run() line: 214	
	SafeRunner.run(ISafeRunnable) line: 47	
	LaunchManager$ConfigurationNotifier.notify(ILaunchConfiguration, int) line: 204	
	LaunchManager.launchConfigurationAdded(ILaunchConfiguration) line: 1980	
	LaunchConfigurationWorkingCopy.writeNewFile(IProgressMonitor) line: 378	
	LaunchConfigurationWorkingCopy.doSave0(IProgressMonitor) line: 259	
	LaunchConfigurationWorkingCopy.doSave(IProgressMonitor) line: 229	
	LaunchConfigurationWorkingCopy.doSave() line: 187	
	CreateLaunchConfigurationAction.performAction() line: 84	

The UnitTestSupport is the part that may be setting the JRE:

      mavenExecutionJre.ifPresent(jre -> {
        jre.getBestMatchingJreContainerPath()
            .ifPresent(jreId -> copy.setAttribute(IJavaLaunchConfigurationConstants.ATTR_JRE_CONTAINER_PATH, jreId));
      });

I don't have m2e-core but mavenExecutionJre is set to Optional[org.eclipse.m2e.jdt.MavenExecutionJre@22b510bf] with executionJreVersionRange"[17,)" : the value comes from my parent pom since we allow to use Maven with Java 17 while compiling with Java 8 / 21 / 25 / whatever (our project was still compiling in Java 8 no so long ago).

The part I copied here was changed 2 month ago here: eclipse-m2e/m2e-core@4b69dfc and ... we allow to use Maven with Java 17 while compiling with Java 21.

-> Due to issues in m2e (eclipse-m2e/m2e-core#2091 (comment)), I've added the latest repo in Eclipse installation but I think the issue is present in 2025-12/2026-03 as well.

I'll try to create a simpler reproducer and comment on eclipse-m2e/m2e-core#2059 when possible.

PS: why Eclipse is not able to go into classes that are in the workspace ? (I added -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=*:31666 to the Eclipse under debug and I've done remote debug with all project in the workspace as source folder from on Eclipse installed via the installer in jdt debug repo README).

[trancexpress]

I assume org.eclipse.jdt.launching.JRE_CONTAINER is IJavaLaunchConfigurationConstants.ATTR_JRE_CONTAINER_PATH... I'm not sure where it can be set to 17 in your case. You could debug into ILaunchConfigurationWorkingCopy.setAttribute() with that key.

But probably simplest would be to provide a reproducer.

I am using JUnit 6 and the loader is junit5 (I assume it is available since 2025-12).

How do you specify JUnit 6 for the launch? If the launch is also coming from m2e, m2e doesn't know the JUnit 6 kind yet: eclipse-m2e/m2e-core#2102

PS: why Eclipse is not able to go into classes that are in the workspace ?

When the launch stops at a breakpoint you don't see a source file? Do you see an editor open, indicating no sources are found? You can add projects/folders with sources via the link in that editor.

Generally attaching to a remote launch will require to specify where the sources are found for that launch.

[nxglhez]

I assume org.eclipse.jdt.launching.JRE_CONTAINER is IJavaLaunchConfigurationConstants.ATTR_JRE_CONTAINER_PATH... I'm not sure where it can be set to 17 in your case. You could debug into ILaunchConfigurationWorkingCopy.setAttribute() with that key.

I can say for sure now that eclipse-m2e/m2e-core#2059 is responsible here: I changed enforcer to Java 25 and Java 25 was selected.

I'll reproduce in a simpler project and reopen eclipse-m2e/m2e-core#2059 when I can.

How do you specify JUnit 6 for the launch? If the launch is also coming from m2e, m2e doesn't know the JUnit 6 kind yet: eclipse-m2e/m2e-core#2102

I did not configure anything special: I am simply using JUnit 6 in the pom.

And this JUnit 6 issue is unrelated: as I said, we enforce Java 17 to run Maven with Toolchain to build in Java 8: JUnit 6 requires Java >= 17, so this Java 8 is still using JUnit 5 and Java 17 is still selected as runtime when executing the test.

When the launch stops at a breakpoint you don't see a source file? Do you see an editor open, indicating no sources are found? You can add projects/folders with sources via the link in that editor.

Generally attaching to a remote launch will require to specify where the sources are found for that launch.

First, the workspace is configured as such:

• I created the installation using Oomph from the setup file found in eclipse.jdt.debug
• I cloned eclipse.jdt.ui because I've seen some classes in there
• I imported all project (using the Import Java Project option)
• I created a new Remote debug application launch
• In the Launch configuration > Source, I added all project as "source folder" with the "Search nested folder" checkbox checked.

I've seen two kind of issues with debugging in Eclipse:

• When debugging this JRE problem: Eclipse open the class file even if the source file is present. I don't want to generalize but it happened with class in internal in the package name.
• When debugging other project, mainly Tomcat but JUnit test as well: Eclipse opens the classes but it is unable to evaluate anything. In such case, only the method is seen. I had created an issue in the past: Issue with JAR in debug mode with m2e ("no java context") eclipse.jdt.debug#86

The second issue is (in my opinion) far more annoying than the first because you can't even evaluate any breakpoint.
The other, is annoying because you need to manually open the class to view the code but you can still evaluate expressions.

[trancexpress]

In the Launch configuration > Source, I added all project as "source folder" with the "Search nested folder" checkbox checked.

If you have Eclipse projects with the sources, best to add them as Java projects in the sources tab (Add -> Java Project).

[nxglhez]

I always does that whenever Eclipse does not find sources.

[iloveeclipse]

Can m2e contribute to these selected JRE ? (I say that because it configured the VM properties based on my pom file).

Change the pom to use Java 25 and see what happens.

[nxglhez]

While I could, I don't think the project would (yet) build with Java 25.
And also, due to bug with annotation processing in 2025-12 (#4687) which have support for Java 25, I can't use it and 2026-03 was not present last week.

[ahoehma]

I have the same effect.

Maven reactor project ... complete on jdk25 source/target.

The project is configured with javase-25

The eclipse global setting defines jdk25 as default

But whenever I try to start a junit-test the first time (ALT+SHIFT+X T) the launcher is using java-17?!

[nxglhez]

Hello,

That's a feature recently implemented in m2e which forgot toolchains in the process: eclipse-m2e/m2e-core#2127

You use enforcer to force a minimal version (Java 17) using maven-enforcer-plugin but you want to compile with Java 25: that's means either you use Java 25 to build and should enforce Java 25, either you use a toolchains which is not supported.

You can match the valid enforcer for Eclipse only assuming you use requireJavaVersion for that:

    <profile>
      <id>force-jdk25-in-m2e</id>
      <activation> <property> <name>m2e.version</name> </property> </activation>
      <properties> <requireJavaVersion>25</requireJavaVersion> </properties>
    </profile>