class org.eclipse.jdt.internal.core.JavaNature cannot be cast to class org.eclipse.jdt.core.IJavaProject

[Cavva79]

Hi all,
I've a strange error with a new maven project in eclipse 2025-06.

I found this error in Tomcat server plugin: eclipse-pde/eclipse.pde#1215

Errors occurred during the build.
Errors running builder 'Maven Project Builder' on project 'demo'.
class org.eclipse.jdt.internal.core.JavaNature cannot be cast to class org.eclipse.jdt.core.IJavaProject (org.eclipse.jdt.internal.core.JavaNature and org.eclipse.jdt.core.IJavaProject are in unnamed module of loader org.eclipse.osgi.internal.loader.EquinoxClassLoader @3859fe49)

Anyone faced it or solved?

[HannesWell]

Can you provide a full stack-trace?
Because I cannot find any location that could cause this with the provided information.

[Cavva79]

Hi @HannesWell,

here the stacktrace:

!ENTRY org.eclipse.core.resources 4 75 2025-06-27 15:08:59.000
!MESSAGE Errors occurred during the build.
!SUBENTRY 1 org.eclipse.m2e.core 4 75 2025-06-27 15:08:59.000
!MESSAGE Errors running builder 'Maven Project Builder' on project 'demo'.
!STACK 0
java.lang.ClassCastException: class org.eclipse.jdt.internal.core.JavaNature cannot be cast to class org.eclipse.jdt.core.IJavaProject (org.eclipse.jdt.internal.core.JavaNature and org.eclipse.jdt.core.IJavaProject are in unnamed module of loader org.eclipse.osgi.internal.loader.EquinoxClassLoader @3859fe49)
	at org.jboss.tools.quarkus.lsp4e.internal.qute.QuteProjectListener.configureProject(QuteProjectListener.java:101)
	at org.jboss.tools.quarkus.lsp4e.internal.qute.QuteProjectListener.handleEvent(QuteProjectListener.java:152)
	at org.eclipse.core.internal.resources.Workspace.broadcastEvent(Workspace.java:460)
	at org.eclipse.core.internal.resources.Resource.refreshLocal(Resource.java:1600)
	at org.eclipse.m2e.core.internal.builder.MavenBuilderImpl.refreshResources(MavenBuilderImpl.java:325)
	at org.eclipse.m2e.core.internal.builder.MavenBuilderImpl.build(MavenBuilderImpl.java:196)
	at org.eclipse.m2e.core.internal.builder.MavenBuilder$1.method(MavenBuilder.java:164)
	at org.eclipse.m2e.core.internal.builder.MavenBuilder$1.method(MavenBuilder.java:1)
	at org.eclipse.m2e.core.internal.builder.MavenBuilder$BuildMethod.lambda$1(MavenBuilder.java:109)
	at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.executeBare(MavenExecutionContext.java:458)
	at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.execute(MavenExecutionContext.java:292)
	at org.eclipse.m2e.core.internal.builder.MavenBuilder$BuildMethod.lambda$0(MavenBuilder.java:100)
	at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.executeBare(MavenExecutionContext.java:458)
	at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.execute(MavenExecutionContext.java:339)
	at org.eclipse.m2e.core.internal.embedder.MavenExecutionContext.execute(MavenExecutionContext.java:278)
	at org.eclipse.m2e.core.internal.builder.MavenBuilder$BuildMethod.execute(MavenBuilder.java:83)
	at org.eclipse.m2e.core.internal.builder.MavenBuilder.build(MavenBuilder.java:192)
	at org.eclipse.core.internal.events.BuildManager$2.run(BuildManager.java:1077)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
	at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:296)
	at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:352)
	at org.eclipse.core.internal.events.BuildManager$1.run(BuildManager.java:441)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
	at org.eclipse.core.internal.events.BuildManager.basicBuild(BuildManager.java:444)
	at org.eclipse.core.internal.events.BuildManager.basicBuildLoop(BuildManager.java:555)
	at org.eclipse.core.internal.events.BuildManager.basicBuildLoop(BuildManager.java:503)
	at org.eclipse.core.internal.events.BuildManager.build(BuildManager.java:585)
	at org.eclipse.core.internal.events.AutoBuildJob.doBuild(AutoBuildJob.java:207)
	at org.eclipse.core.internal.events.AutoBuildJob.run(AutoBuildJob.java:300)
	at org.eclipse.core.internal.jobs.Worker.run(Worker.java:63)

it is weird, I could reproduce the error just after a restart of eclipse itself.

Afert that it throws only those errors that does not imply any functionalities:

!ENTRY org.eclipse.ui.workbench 4 2 2025-06-27 15:14:49.724
!MESSAGE Problems occurred when invoking code from plug-in: "org.eclipse.ui.workbench".
!STACK 0
java.lang.ClassCastException: class org.eclipse.jdt.internal.core.JavaNature cannot be cast to class org.eclipse.jdt.core.IJavaProject (org.eclipse.jdt.internal.core.JavaNature and org.eclipse.jdt.core.IJavaProject are in unnamed module of loader org.eclipse.osgi.internal.loader.EquinoxClassLoader @29f4cfb8)
	at org.jboss.tools.quarkus.lsp4e.internal.qute.QuteProjectListener.configureProject(QuteProjectListener.java:101)
	at org.jboss.tools.quarkus.lsp4e.internal.qute.QuteProjectListener.<init>(QuteProjectListener.java:49)
	at org.jboss.tools.quarkus.lsp4e.internal.startup.qute.QuteStartup.earlyStartup(QuteStartup.java:24)
	at org.eclipse.ui.internal.EarlyStartupRunnable.runEarlyStartup(EarlyStartupRunnable.java:79)
	at org.eclipse.ui.internal.EarlyStartupRunnable.run(EarlyStartupRunnable.java:55)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
	at org.eclipse.ui.internal.Workbench$39.run(Workbench.java:2785)
	at org.eclipse.core.internal.jobs.Worker.run(Worker.java:63)

!ENTRY org.eclipse.ui 2 2 2025-06-27 15:14:49.725
!MESSAGE Invalid preference category path: org.jboss.tools.common.model.ui.MainPreferencePage (bundle: org.jboss.tools.foundation.ui, page: org.jboss.tools.foundation.ui.internal.credentials.CredentialPreferencePage)

!ENTRY org.eclipse.ui 4 0 2025-06-27 15:14:49.725
!MESSAGE Unable to execute early startup code for the org.eclipse.ui.IStartup extension contributed by the 'org.jboss.tools.quarkus.lsp4e' plug-in.
!STACK 0
java.lang.ClassCastException: class org.eclipse.jdt.internal.core.JavaNature cannot be cast to class org.eclipse.jdt.core.IJavaProject (org.eclipse.jdt.internal.core.JavaNature and org.eclipse.jdt.core.IJavaProject are in unnamed module of loader org.eclipse.osgi.internal.loader.EquinoxClassLoader @29f4cfb8)
	at org.jboss.tools.quarkus.lsp4e.internal.qute.QuteProjectListener.configureProject(QuteProjectListener.java:101)
	at org.jboss.tools.quarkus.lsp4e.internal.qute.QuteProjectListener.<init>(QuteProjectListener.java:49)
	at org.jboss.tools.quarkus.lsp4e.internal.startup.qute.QuteStartup.earlyStartup(QuteStartup.java:24)
	at org.eclipse.ui.internal.EarlyStartupRunnable.runEarlyStartup(EarlyStartupRunnable.java:79)
	at org.eclipse.ui.internal.EarlyStartupRunnable.run(EarlyStartupRunnable.java:55)
	at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:47)
	at org.eclipse.ui.internal.Workbench$39.run(Workbench.java:2785)
	at org.eclipse.core.internal.jobs.Worker.run(Worker.java:63)

Maybe quarkus tools does conflicts with m2e in some ways, doesn't it?