[dalailambda]

Doing this with dynamic libs in C/C++ is done frequently in some areas.

[sievebrain]

Yeah but your program has to be written to do it that way from the start, with a plugin boundary and so on. The JVM can do it for any code, with limitations, but there's a "DCE VM" that is a patched HotSpot. It can do far more kinds of hotswapping.