ASM (from ObjectWeb) not calculating MaxStack correctly even though ClassWriter( COMPUTE_MAX + COMPUTE_STACK ) is set

Question

I am getting expected ClassVerifyErrors when attempting to load a class i have generated using ASM. On further inspection i can see that the jvm is correct and that the method is talking about has an invalid MAX_STACK value. THe strange thing is am using the auto calculate the stack and max local options so this should not be a problem... The method with the invalid option is very simple and yet the result is bad bytecode.

I have written a class with the intended method and compared my asm generated class against what javac produces and the byte codes matchup with the only error being the max stack is 0 which is wrong while javac sets a value of 2.

Id like to avoid having to calculate tha max stack/locals myself.

Answer 1

Max stack and variable calculation can produce the wrong results if bytecode is not valid. You can verify that by running generated code trough the CheckClassAdapter.

For example,

  ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
  // generate code into cw instance...

  PrintWriter pw = new PrintWriter(System.out);
  CheckClassAdapter.verify(new ClassReader(cw.toByteArray()), true, System.out);