Java NoClassDefFoundError when calling own class from instrumented method

Question

I am working on a kit of simple Java agents to help me (and hopefully others) troubleshoot Java applications. One of the agents I would like to create instruments the JComponent.getToolTipText() method to quickly identify any GUI class by just hovering the mouse cursor over it.

You can find the code of my transformer and the rest of the project here:

http://sfn.cvs.sourceforge.net/viewvc/sfn/core/src/main/java/org/leplus/sfn/transformer/JComponentTransformer.java?view=markup

I launch my test GUI with the agent attached as follow:

$ java -javaagent:target/jars/sfn-0.1-agent.jar=JComponent -cp lib/jars/bcel-5.2.jar:target/jars/sfn-0.1-test.jar:target/jars/sfn-0.1-agent.jar org.leplus.sfn.test.Main

sfn-0.1-agent.jar contains the org.leplus.sfn.transformer.JComponentTransformer class. sfn-0.1-test.jar contains the org.leplus.sfn.test.Main class.

Here is what the application prints when I launch it and I put the mouse over it:

Loading agent: JComponent
Instrumentation ready!
Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: org/leplus/sfn/tracer/ComponentTracer
 at javax.swing.JComponent.getToolTipText(JComponent.java)
 at javax.swing.ToolTipManager$insideTimerAction.actionPerformed(ToolTipManager.java:662)
...

What is surprising to me is that if I change my transformer to call any class from the JRE, it works. But it doesn't work when I call my own class org.leplus.sfn.tracer.ComponentTracer. My first guess was a classpath issue but the ComponentTracer is both in the classpath and in the agent's jar. So I am lost.

If any of you see where I am missing something.

Cheers,

Tom

Answer 1

It's a class loader issue. You're instrumenting a class (javax.swing.JComponent) managed by the bootstrap class loader, and having it refer to a class (org.leplus.sfn.tracer.ComponentTracer) managed by the system class loader.

If you put your ComponentTracer class in the bootstrap class loader, the problem should disappear.

java -Xbootclasspath/p:<path/to/jar/containing/ComponentTracer> -javaagent:...

Answer 2

Try running with -DDEBUG as it might show you more information.

Also, I see the target directory here. http://sfn.cvs.sourceforge.net/viewvc/sfn/core/target/ It contains a classes folder, but no jars folder? Check to make sure the jar paths are relative to the project root.