如何从正在运行的应用程序的堆转储中提取java类定义？

Question

The need for extracting class definitions from a heap dump comes from the way some classes are loaded dynamically, not from static lib jars, but compiled dynamically or loaded over the network. The heap dump has the same size as the actually heap so I assume all the classes are there, possibly in the permgen. The objective is to extract the definitions in the form of .class files for further examination.

Answer 1

There do not seem to be any tools that readily allow you to retrieve the class bits from a VM, let alone from a heap dump. Nor is it clear that the class definition is even available in the exact same format as the contents of the .class in the VM.

But there are multiple options for saving the class definition before it is loaded in the VM. You could have an agent which can store the class definition, either in heap or external storage. This should also be possible with a custom classloader, but it is possible that it is bypassed by some other customer classloader.

The popular AOP tool AspectJ has an option to save the definition of instrumented class; it can probably be used for your use-case.