bytecode instrumentation using ClassFileTransformer.transform

Question

I have written a Class with premain method and inside that method I have added a ClassFileTransformer to Instrumentation ( Instrumentation.addTransformer() ).

I have invoked a application using

java -javaagent:<path_to_agnet.jar> <application>

However ClassFileTransformer.transform() method is not being invoked.
I have observed that premain is being invoked.
I have also observed that if I call Instrumentation.retransformClasses() , then ClassFileTransformer.transform() is being invoked.
On first definition ( Classloader.defineClass() ), transform() method is not being invoked.

Any clue what could be wrong?

Note: I can post the source code if that is of any help.

Regards, Rejeev.

Answer 1

Possible causes

.

Incorrect MANIFEST

Did you follow all the steps required to define an Instrumentation class ?

Especially the "packaging" step, which involve specifing a slightly different set of attributes in the JAR's manifest:

• Instead of Main-Class , you must specify a Premain-Class attribute that gives the full class-name of the class that implements premain() in your agent.

Premain-Class: my.package.MyAgentClass

• If your agent uses any class-libraries, then you should specify the Boot-Class-Path attribute, since your instrumentation agent will need its libraries to be visible from the bootstrap classloader.
  If you don't do this, you will probably have to use the wacky -Xbootclasspath/a:... argument to the JVM.
  The command-line argument is a decent way to get things going, but you want to use this attribute in the long-run, because the command-line is already growing from having to specify the Java instrumentation agent. Might as well keep it as simple as possible.

• Finally, there is the Can-Redefine-Classes attribute.
  If it is set to true, then the Java instrumentation agent is able to redefine the classes that the agent itself uses.
  This is a pretty odd circumstance, and certainly won't arise much.

.

Silent Exception

( Rejeev Divakaran got that one).

I was using classBeingRedefined.getName() to print the class name.
classBeingRedefined is null when it is loaded first time .

The bottom line is if there is uncaught exception in transform method .
It will be silently eaten up .