How to create or retrieve a StackMapTable in Javassist

Question

In the javassist API it seems there is no direct way to add anything by hand in the CodeAttribute property.

Given this example:

CtMethod m;
m.getMethodInfo().getCodeAttribute().setAttribute(???);

this seems to be the only way to alter attributes. But, neither is possible to retrieve the original attributes and modify them (it returns a list od AttributeInfo), nor a public constructor exists for StackMapTable, which seems to be the only accepted input.

Anyone knows how to do something like this?

Answer 1

The CodeAttribute object is responsible for holding the bytecode that represents the method flow. You can use it to iterate the bytecode that represents the method logic.

This basically means that it represents the compiled method itself. I know you can use this object to manually modify the bytecode itself for example deleting all the instruction between two lines:

// erase from line 4 to 6
int startingLineNumber= 6;
// Access the code attribute
CodeAttribute codeAttribute = mymethod.getMethodInfo().getCodeAttribute();

LineNumberAttribute lineNumberAttribute = (LineNumberAttribute)      codeAttribute.getAttribute(LineNumberAttribute.tag);

// Index in bytecode array where we start to delete
int firstIndex = lineNumberAttribute.toStartPc(startingLineNumber);

// Index in the bytecode where the first instruction that we keep starts
int secondIndex = lineNumberAttribute.toStartPc(startingLineNumber+2);

// go through the bytecode array
byte[] code = codeAttribute.getCode();
for (int i = firstIndex ; i < secondIndex ; i++) {
   // set the bytecode of this line to No OPeration
   code[i] = CodeAttribute.NOP;
}

However this could be really tricky and can get out of hands quite easily. What I suggest to do is to just create a new CodeAttribute rom scratch and substitute it into you method in order to avoid unplesant surprises:

 ClassFile cf = ...;
 MethodInfo minfo = cf.getMethod("yourMethod");
 Bytecode code = new Bytecode(cf.getConstPool());
 // insert your logic here
 code.addAload(0);
 code.addInvokespecial("java/lang/Object", MethodInfo.nameInit, "()V");
 code.addReturn(null);
 code.setMaxLocals(1);

 MethodInfo minfo = new MethodInfo(cf.getConstPool(), MethodInfo.nameInit, "()V");
 minfo.setCodeAttribute(code.toCodeAttribute());

However thi method also may be quite risky since you are handling directly bytecode. Probably the best would be just to delete your method and add a new one with your logic instead.