Is is possible to instrument with javassist java core classes? Classes loaded by the bootstrap classloader

Question

I want to add an "insertBefore" on a method of a core JDK 5 class. For some reason it´s not working. Here's an example of the code:

ClassPool pool = ClassPool.getDefault();
CtClass ctClass =  pool.get("com.sun.org.apache.xerces.internal.impl.xs.XSMessageFormatter");
CtMethod ctMethod = ctClass.getDeclaredMethods()[0];
ctMethod.insertBefore("System.out.println(\"WORKED\");");
ctClass.toClass();

The com.sun.org.apache.xerces.internal.impl.xs.XSMessageFormatter comes bundled with the JDK, it's inside rt.jar. After the snippet above, I run some code that forces the XSMessageFormatter class to run, but my inserted code never runs. I can only get this to work on my own classes. This code is running as a simple standalone app.

Any ideas?

Answer 1

It is bad idea to modify java core classes. The right way to use Java Instrumentation API (since Java 1.5).