How to watch a variable for changes

Question

I want to know if there's any way so I could watch for variable value change when the program is running. Of course not using debugger I wanna do it Programmatically. For example:

class A
{
   public static int valueToBeWatched;
}

So at runtime if in any method of any class in my project modifies this value MyValueChangeListner event should get called.

Answer 1

You cannot. There is no watch-on-modification hook built into Java itself. Obviously, you could do polling, though. But then it won't be "live".

AspectJ may allow such a think, but I'm not sure whether it holds for primitive variables, or only when you are using getters and setters.

The clean Java-way is to make the variable private and use getters and setters.

private valueToBeWatched;

public void setValue(int newval) {
  valueToBeWatched = newval;
  notifyWatchers();
}

public int getValue() {
  return valueToBeWatched;
}

On a side note, avoid static whenever possible. In particular public but not final.

Answer 2

You need to replace the type int with a class which will invoke your listener whenever the values changes. You might like to ignore setting the value when it hasn't actually changed.

e.g.

 private int value;
 private final MyValueChangeListener listener;

 public void setValue(int value) {
    if(this.value == value) return;
    listener.changed(this, this.value, value);
    this.value = value;
 }

You can perform this replace using byte code injection, but it is much simple to change the original code.

An alternative is to monitor the value to look for changes periodically. Unless the value changes very slowly you might not see every change. You can use the Debugger API to do this but it not simple and I wouldn't suggest you use it unless you are familiar with the API already.

Java Debugger API Links

http://java.sun.com/javase/technologies/core/toolsapis/jpda/

http://docs.oracle.com/javase/6/docs/technotes/guides/jpda/

http://docs.oracle.com/javase/6/docs/jdk/api/jpda/jdi/