Attach to a Java process that's running?

I'm trying to attach and call methods and edit variables from a java process that's running from a separate java process.

Example: Process 1 has a loop that prints a String variable that's private and defined in the class. Process 2 (when started), changes the String message and then process 1 will continue to print out the changed variable.

This is more of a simple version of what i'm trying, but if i can figure that out, i can easily find out the rest myself.

Thanks! Erouax