Hello everybody, classes that are loaded via Unsafe::defineAnonymousClass are not transformed by a registered ClassFileTransformer. At the same time, it is possible to retransform / redefine such an anonymous classes using the instrumentation API.
Here is a rather confusing bug that I encountered when working with an internally used Java agent after upgrading a code base to Java 8: The Java agent attaches at runtime (agentmain). It then registers a ClassFileTransformer that also applies for retransformation. Afterwards, all loaded classes that fullfil a given condition are explicitly registered to be retransformed by the agent. (Doing so, anonymous classes are returned by Instrumentation::getAllLoadedClasses.) This resulted in the following behavior of the agent: a) If the agent was attached after an anonymous class was loaded, the retransformation would apply to an anonymous class. b) If the agent was attached "too early", the (non-re-)transformation would not apply to an anonymous class. I wonder if it is intentional that a ClassFileTransformer is not applied when an anonymous class is loaded. Personally, I find this counter-intuitive, especially when converting anonymous inner classes to lambda expressions where many users of instrumentation do not forsee the behavioral change. It also puts a very unforseeable limit to the instrumentation API. I would therefore like to suggest that ClassFileTransformers are also applied to anonymous classes when Unsafe::defineAnonymousClass is called just as when going via ClassLoader::defineClass. I can tell that this behavior has not only affected me as I had this question comming up by multiple users of my open-source code generation library. What is your view on this? Thank you for your feedback! Best regards, Rafael
