Does JRebel use Javassist or some kind of bytecode manipulation? I'm asking this out of pure interest, I don't actually "need" to know :)
JRebel uses class rewriting (both ASM and Javassist) and JVM integration to version individual classes. Plus it integrates with app servers to redirect class/resource and web server lookups back to the workspace. And it also integrates with most app servers and frameworks to propagate changes to the configuration (metadata or files). That's the short of it. The long of it takes 10 world-class engineers to develop and support and is our commercial secret :)