Question

I am trying to debug some C/Java bindings that use some custom refcounting/locking. I would like to have the JVM print a message every time a given object has its monitor entered or exited. Is there any way to do this? Basically, I want this:

synchronized(lock) {
   ...
   System.out.println("hi");
   ...
}

to print this:

*** "lock" monitorenter
hi
*** "lock" monitorexit

I have looked at the XX options and found nothing. This is OpenJDK 6.

Answer 1

Good question. The only solution I could come up with is basically this:

Use a custom class-loader and preprocess the files with using a bytecode manipulation library such as ASM. (ASM has a good example of how to work with bytecode rewriting in class loaders.)

Then simply add a call to System.out.println before each monitorenter and monitorexit.

Thanks to the nice visitor pattern in the ASM library, this shouldn’t be more than a screen or two of code.