Question

In my web app, during some change over the object, i need to send a mail about the changes happened in the object.
My question is how to write a listener for this.
Please give me some article regarding this.
Thanks

Answer 1

A typical implementation could be like this: your object is observable. So every time, one of the (observed) values changes, an event is fired and all registered listeners are notified. One of those listeners now would be designed to take the notification and create and send an EMail (Java Mail API)

Let’s take a sample bean which we make observable:

public class Bean implements Observable{

  // code to maintain listeners
  private List<Listener> listeners = new ArrayList<Listener>();
  public void add(Listener listener) {listeners.add(listener);}
  public void remove(Listener listener) {listeners.remove(listener);}

  // a sample field
  private int field;
  public int getField() {return field;}
  public int setField(int value) {
    field = value;
    fire("field");        
  }

  // notification code
  private void fire(String attribute) {
    for (Listener listener:listeners) {
      fieldChanged(this, attribute);
    }
  }
}

The Listener interface:

public interface Listener {
  public void fieldChanged(Object source, String attrbute);
}

The Observable interface:

public interface Observable {
  public void add(Listener listener);
  public void remove(Listener listener);
}

And the EMailer:

public class Sender implements Listener {

  public void register(Observable observable) {observable.add(this);}
  public void unregister(Observable observable) {observable.remove(this);}

  public void fieldChanged(Object source, String attribute) {
    sendEmail(source, attribute); // this has to be implemented
  }

}

EDIT
Corrected an ugly mistake in the setter method – now the event is fired after the property has been set. Was the other way round, with the side effect, that if a listener read the changed property, he still saw the old, unchanged value…