In part 1 of this series, I gave an overview of Java agents and bytecode manipulation and provided a simple agent that adds a System.out.println() message to a method. In part 2, I provided an agent that eliminates calls to log4j log methods in designated packages and log levels. In part 3, I'll show how to augment code to help analyze stack traces by introducing an agent that keeps track of how many instrumented methods are called by each thread.

A little-known option to recent Java VMs holds a fair amount of mystery and quite a lot of power. That option is 'javaagent', and it holds the key to monitoring and profiling JVMs and being able to dynamically modify Java classes as they're being loaded.