[tycho-user] Help debug classloader conflict loading plugin in standalon

["David M. Karr" <davidmichaelkarr@xxxxxxxxx>]

I'm working on an eclipse plugin codebase that uses Tycho to build 
itself.  The application requires several artifacts that aren't 
available in a p2 repo.  To resolve this, I created one plugin project 
whose only purpose is to use the "copy-dependencies" goal from the 
"maven-dependency-plugin" to specify the artifacts that are needed and 
make them available in the "Bundle-ClassPath" property of the same 
plugin.  I then set other plugins that require these libraries to depend 
on this plugin.

This had been working fine, although there was one point where I saw the 
following error after installing the plugin from a local update site 
constructed from the build:

    java.lang.LinkageError: org/antlr/v4/runtime/TokenStream

After some fiddling, this went away, although I wasn't certain exactly 
what I did to make it go away.

I'm aware of the fact that this indicates a conflict with loading a 
class from multiple classloaders.

Today, this error came back.

Curiously, if I instead test this with an "Eclipse Application" run 
configuration, loading all plugins in the workspace, I don't see the error.

I changed the command line of the Eclipse instance (not the one from the 
launch configuration) to add "-verbose:class" to see where this is 
loaded from.  I extracted the following relevant lines from the output:
----------------------
[Loaded org.antlr.v4.runtime.TokenStream from 
file:/home/opnfv/frameworks/eclipse-neon/java-mars/eclipse/../../../../.p2/pool/plugins/net.sf.eclipsecs.checkstyle_6.16.0.201603042321/checkstyle-6.16.1-all.jar]
[Loaded org.antlr.v4.runtime.TokenStream from 
file:/home/opnfv/frameworks/eclipse-neon/java-mars/eclipse/configuration/org.eclipse.osgi/605/0/.cp/libs/antlr4-runtime-4.5.1.jar]
java.lang.LinkageError: loader constraint violation: loader (instance of 
org/eclipse/osgi/internal/loader/EquinoxClassLoader) previously 
initiated loading for a different type with name 
"org/antlr/v4/runtime/TokenStream"
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
    at 
org.eclipse.osgi.internal.loader.ModuleClassLoader.defineClass(ModuleClassLoader.java:272)[Loaded 
com.google.common.collect.Collections2$FilteredCollection from 
file:/home/opnfv/frameworks/eclipse-neon/java-mars/eclipse/../../../../.p2/pool/plugins/com.google.guava_15.0.0.v201403281430.jar]

    at 
org.eclipse.osgi.internal.loader.classpath.ClasspathManager.defineClass(ClasspathManager.java:632)
    at 
org.eclipse.osgi.internal.loader.classpath.ClasspathManager.findClassImpl(ClasspathManager.java:588)
    at 
org.eclipse.osgi.internal.loader.classpath.ClasspathManager.findLocalClassImpl(ClasspathManager.java:540)
    at 
org.eclipse.osgi.internal.loader.classpath.ClasspathManager.findLocalClass(ClasspathManager.java:527)
    at 
org.eclipse.osgi.internal.loader.ModuleClassLoader.findLocalClass(ModuleClassLoader.java:324)
    at 
org.eclipse.osgi.internal.loader.BundleLoader.findLocalClass(BundleLoader.java:327)
    at 
org.eclipse.osgi.internal.loader.BundleLoader.findClassInternal(BundleLoader.java:402)
    at 
org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:352)
    at 
org.eclipse.osgi.internal.loader.BundleLoader.findClass(BundleLoader.java:344)
    at 
org.eclipse.osgi.internal.loader.ModuleClassLoader.loadClass(ModuleClassLoader.java:160)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at org.antlr.v4.runtime.Parser.setInputStream(Parser.java:530)
-----------------

I could use some ideas or suggestions for troubleshooting and fixing this.