I was attempting to run hudson inside a copy of Tomcat 6 using JDK 1.6 on Ubuntu
8.10 and run a Maven 2 build with JDK 1.5. As soon as hudson tried to archive
the first pom encountered it blows up with the stack trace:

[INFO] Trace
java.lang.UnsupportedClassVersionError: Bad version number in .class file
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at java.lang.ClassLoader.defineClass(ClassLoader.java:465)
at hudson.remoting.RemoteClassLoader.findClass(RemoteClassLoader.java:96)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
at hudson.FilePath.act(FilePath.java:434)
at hudson.FilePath.copyTo(FilePath.java:869)
at hudson.FilePath.copyTo(FilePath.java:857)
at hudson.maven.reporters.MavenArtifact.archive(MavenArtifact.java:184)
at
hudson.maven.reporters.MavenArtifactArchiver.postBuild(MavenArtifactArchiver.java:107)
at
hudson.maven.MavenModuleSetBuild$Builder.postModule(MavenModuleSetBuild.java:612)
at hudson.maven.MavenBuilder$Adapter.fireLeaveModule(MavenBuilder.java:307)
at hudson.maven.MavenBuilder$Adapter.postBuild(MavenBuilder.java:265)
at
org.apache.maven.lifecycle.LifecycleExecutorInterceptor.execute(LifecycleExecutorInterceptor.java:68)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:336)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:129)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:301)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.codehaus.classworlds.Launcher.launchEnhanced(Launcher.java:315)
at org.codehaus.classworlds.Launcher.launch(Launcher.java:255)
at hudson.maven.agent.Main.launch(Main.java:158)
at hudson.maven.MavenBuilder.call(MavenBuilder.java:162)
at hudson.maven.MavenModuleSetBuild$Builder.call(MavenModuleSetBuild.java:579)
at hudson.maven.MavenModuleSetBuild$Builder.call(MavenModuleSetBuild.java:525)
at hudson.remoting.UserRequest.perform(UserRequest.java:92)
at hudson.remoting.UserRequest.perform(UserRequest.java:46)
at hudson.remoting.Request$2.run(Request.java:236)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:417)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:269)
at java.util.concurrent.FutureTask.run(FutureTask.java:123)
at
java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:650)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:675)
at java.lang.Thread.run(Thread.java:595)

The reason is that on Ubuntu 8.10 they compile Tomcat 6 on JDK 1.6 so all the
tomcat classfiles have a version requirement of 50 (JDK 1.6).

It seems that the classloader of the servlet container is used during the maven
build which causes this issue. I wouldn't have expected the build to have access
to any classes of the container that is was running in.