Question

I’m trying to print out debug statements when some third party code changes a variable. For example, consider the following:

public final class MysteryClass {

private int secretCounter;

public synchronized int getCounter() {
    return secretCounter;
}

public synchronized void incrementCounter() {
    secretCounter++;
}

}

public class MyClass {

public static void main(String[] args) {
    MysteryClass mysteryClass = new MysteryClass();
    // add code here to detect calls to incrementCounter and print a debug message
}

I don’t have the ability to change the 3rd party MysteryClass, so I thought that I could use PropertyChangeSupport and PropertyChangeListener to detect changes to the secretCounter:

public class MyClass implements PropertyChangeListener {

    private PropertyChangeSupport propertySupport = new PropertyChangeSupport(this);

    public MyClass() {
        propertySupport.addPropertyChangeListener(this);
    }

    public void propertyChange(PropertyChangeEvent evt) {
        System.out.println("property changing: " + evt.getPropertyName());
    }

    public static void main(String[] args) {
        MysteryClass mysteryClass = new MysteryClass();
        // do logic which involves increment and getting the value of MysteryClass
    }
}

Unfortunately, this did not work and I have no debug messages printed out. Does anyone see what is wrong with my implementation of the PropertyChangeSupport and Listener interfaces? I want to print a debug statement whenever incrementCounter is called or the value of secretCounter changes.

Answer 1

I’m pretty sure the PropertyChangeListener mechanism only works if you set the properties through the PropertyEditor mechanis, not through getters and setters

I’d probably try it with AspectJ, but you can only advise the method calls, not the execution, as the third party class is final.