When packing a jar file with custom java agent, you can add the following properties:
Can-Redefine-ClassesCan-Retransform-ClassesWhat is the difference between those two?
If redefinition happens before class is loaded and retransformation after, then when does exactly retransformation happen?
Redefinition means that at an arbitrary point of time an agent will invoke Instrumentation. redefineClasses to change the actual definition of existing (and already loaded) classes. The agent will provide the bytecode for the new definition.
Retransformation refers to the process of class file transformation which is normally applied at class loading time. Agents can register ClassFileTransformers which are called one after another to apply transformations to the byte code before the class will be initialized. So Retransformation refers to the capability of the JVM to repeat this process for already loaded classes. In this case an agent may invoke Instrumentation.retransformClasses specifying which classes to retransform but no bytecode. Instead the JVM will call all registered retransforming capable ClassFileTransformers providing the actual bytecode (or the result of the previous transformer for a chained transformer).