Detecting field mutation using ASM

Question

I want to detect if one of the methods of a class mutates a certain instance field using ASM. For example:

public class Box {
    public Object o;

    public void mutate() {
        o = new Object();
    }
}

Question: is instance field o mutated by one of the methods of the class Box? In this case, yes.

Using the MethodNode class from the ASM tree library I can get the opcodes of the methods which look like this

-1 -1 25 187 89 183 181 -1 -1 177 -1

This array contains the opcode 181 for putfield, but how can I tel that it is the field Box.o that is assigned?

BTW: Why does the array contain the -1 values?

Tnx

Answer 1

Not sure why you are looking at the raw byte code. I would look at the individual instructions.

public class Box {
    public Object o;

    public void mutate() {
        o = new Object();
    }

    public static void main(String... args) throws IOException {
        ClassReader cr = new ClassReader(Box.class.getName());
        ASMifierClassVisitor acv = new ASMifierClassVisitor(new PrintWriter(System.out));
        cr.accept(acv, 0);
    }
}

prints

... lots of code ...
{
mv = cw.visitMethod(ACC_PUBLIC, "mutate", "()V", null, null);
mv.visitCode();
Label l0 = new Label();
mv.visitLabel(l0);
mv.visitLineNumber(11, l0);
mv.visitVarInsn(ALOAD, 0);
mv.visitTypeInsn(NEW, "java/lang/Object");
mv.visitInsn(DUP);
mv.visitMethodInsn(INVOKESPECIAL, "java/lang/Object", "<init>", "()V");
mv.visitFieldInsn(PUTFIELD, "Box", "o", "Ljava/lang/Object;");
Label l1 = new Label();
mv.visitLabel(l1);
mv.visitLineNumber(12, l1);
mv.visitInsn(RETURN);
Label l2 = new Label();
mv.visitLabel(l2);
mv.visitLocalVariable("this", "LBox;", null, l0, l2, 0);
mv.visitMaxs(3, 1);
mv.visitEnd();
}
... more code ...

you can see that the method visitor is called with

mv.visitFieldInsn(PUTFIELD, "Box", "o", "Ljava/lang/Object;");

and this should tell you what you want to know.

---

If you have a constructor (and I suggest you do)

private Object o;

public Box(Object o) {
    this.o = o;
}

you may want to treat this "mutation" differently because its in the constructor.

Answer 2

Also found another solution using the tree API, which is more suitable for my application:

private boolean methodMutatesField(List<MethodNode> methods, FieldNode field, ClassNode fieldOwner) {

    for(MethodNode methodNode : methods) {
        Iterator<AbstractInsnNode> instructionIterator = methodNode.instructions.iterator();

        while (instructionIterator.hasNext()) {
            AbstractInsnNode abstractInsNode = instructionIterator.next();

            if(abstractInsNode.getType() != AbstractInsnNode.FIELD_INSN) {  
                continue;
            } 

            FieldInsnNode fieldInsnNode = (FieldInsnNode) abstractInsNode;

            if(fieldInsnNode.getOpcode() != Opcodes.PUTFIELD) {
                continue;
            }

            if(fieldInsnNode.name.equals(field.name) && fieldInsnNode.owner.equals(fieldOwner.name)) {
                return true;
            }
        }
    }

    return false;
}