Disassemble/decompile arbitrary JVM bytecode

Question

I have an obfuscated java binary which does runtime code injection using java reflection like this.

Object o = "a long string";
((Method)o).invoke(params..);

(If I write this in a java file, it would compile fine, but I have to edit the disassembly of the class file and remove checkcast instruction to make it run.)

What I want to do is disassemble/decompile the content of the long string as JVM bytecode. Since this is not a full class, but only a method, I can't get any of the available java reverse engineering tools to work with it.

I'm now trying to inject this code into a separate class at runtime and then dump it out of memory. But I'm not sure if it's possible to do. even using a javaagent.

Is there any other easy way to accomplish what I'm trying to do?

Answer 1

It doesn't make sense to talk about the "bytecode for only a method", because Java bytecode can't run in isolation. The instructions contain references to the constant pool, which is part of the classfile and shared between all methods in the class. The smallest executable unit of bytecode is the classfile.

The question as written doesn't make sense, but if you post the actual data you are dealing with, perhaps we can figure out what is going on.