How to observe when a custom object changes even after it's extended

Question

So let's say I have a custom object in java:

   public class TestObject {

    private int value = 0;

        public TestObject(int value) {
                this.value = value;
        }

    public void increaseValue() {
        value++;
    }

    }

Now I want to know when this object is modified. Or more specifically, I want to know when any of it's fields have changed (in this case value). Furthermore, if I extend TestObject, I still want to be able to listen to any field changes that might happen to that object, including if that change is to a new field new fields.

I've done some research and found of variety of listeners that come with java, but they all seem to fail in the area that they require you to put calls to the listeners at the end of you're methods. For example, increaseValue() would also have to notify all of the listeners to the TestObject that value had changed. Obviously this doesn't work for me because extensibility is a must have and I do not know that if people who inherit from that object will adhere to the requirement that they must notify listeners. Not to mention, it seems like a pain to have to program that for each mutator method

If any light could be shed on this subject it would be greatly appreciated.

Answer 1

You can use approach that Hibernate uses i.e. instrument/proxy your classes to add additional (listener) logic around your POJOs' getters and setters. But then you need to ensure that all use only proxied classes.