Recently i was looking for a tool to make easily the deploy of bug fixes in production, and i have found the ClassGhost utility (http://classghost.sourceforge.net/). This tool injects classes in the production JVM. It looks pretty good, and does the work very well.
Be careful. The JVM debugger cannot be switched on and off on the fly, which means that it's available at all times, and it's not protected by any sort of security system. If an intruder can gain debug access, they can browse and even alter sensitive memory, gain insight into possible application logic flaws, and generally wreak havoc.
When it comes to destroying a civilization, gas chambers cannot hold a candle to echo chambers.