I just discovered Sun's HotSwap Client Tool – a tool for
dynamically updating classes while they are running. This uses the same features of the 1.4.x VMs which Eclipse and other IDEs
use to Hotswap running code while debugging.

Sun's tool is at least partly targeted at updating deployed applications however. It has some nice features like:

• Get the list of classes currently loaded by the VM
• Find which classes have changed
• Compare the source code for the versions

It's under a BSD licence, too, so you are free to use it in your own apps. I'd be interested to hear what
people think of it.