The JVM is an excellent platform for monkey-patching.
Monkey patching is a technique used to dynamically update the behavior of a piece of code at run-time. A monkey patch (also spelled monkey-patch, MonkeyPatch) is a way to extend or modify the runtime code of dynamic languages (e.g. Smalltalk, JavaScript, Objective-C, Ruby, Perl, Python, Groovy, etc.) without altering the original source code.