Some call it hot deploy, some refer to runtime class reload/redefine, but whatsoever….
It is just mechanism to reload compiled class file during jvm runtime, to avoid ordinary java development life cycle of “code change -> build -> deploy”, which will significantly save lot’s of wait time (especially when you have multiple modules dependency project).
Both open source project: HotSwapAgent and Spring-Loaded works pretty well for me, but if you are looking for commercial product, JRebel will be the choice.