Java 6 Hotpatching

I already knew of the ability for a java process to attach itself to another process for monitoring purposes. Java 6 comes with a nice graphical console (jconsole) that shows several statistics about the running process, like memory usage, etc.
What I didn't know was that the API for that allowed to do this. According to the article we can substitute a class for a new version of it in runtime!! Although I don't see this sort of thing being used very often, it's still very cool :)