Java Dynamic Management Kit 4.2 Tutorial

The Attribute Change Listener

Like our listener for MBean server notifications, our listener for attribute change notifications is a trivial class consisting of just the handler method.

Example 7-4 The Listener for Attribute Change Notifications

import javax.management.Notification;
import javax.management.NotificationListener;
import javax.management.AttributeChangeNotification;

public class SimpleStandardListener implements NotificationListener {
    [...]

    // Implementation of the NotificationListener interface
    //
    public void handleNotification(Notification notification,
                                   Object handback) {

        // Process the different types of notifications fired by the
        // simple standard MBean.
        String type = notification.getType();

        System.out.println(
            "\n\t>> SimpleStandardListener received notification:" +
            "\n\t>> ---------------------------------------------");
        try {
            if (type.equals(AttributeChangeNotification.ATTRIBUTE_CHANGE)) {

  System.out.println("\t>> Attribute \"" +
      ((AttributeChangeNotification)notification).getAttributeName()
      + "\" has changed");
  System.out.println("\t>> Old value = " +
      ((AttributeChangeNotification)notification).getOldValue());
  System.out.println("\t>> New value = " +
      ((AttributeChangeNotification)notification).getNewValue());

            }                
            else {
                System.out.println("\t>> Unknown event type (?)\n");
            }
        } catch (Exception e) {
            e.printStackTrace();
            System.exit(1);
        }
    }
}

Again, we are handling a subclass of the Notification class, this one specific to attribute change notifications. The AttributeChangeNotification class provides methods for extracting the information about the attribute, notably its name, its type and its values before and after the modification. Our handler does nothing more than display these to the user. If this handler were part of an MBean in a larger management solution, it would undoubtedly want to take some action, depending upon the change in value of the attribute.

As demonstrated by the broadcaster's code (see Example 7-3), the subclass can easily be instantiated and sent instead of a Notification object. Its constructor provides parameters for initializing all of the attribute-related values. In our example, we do not use significant values for the sequenceNumber and timeStamp parameters because our listener has no need for them. This is one great advantage of the Java Dynamic Management Kit: you only need to implement the level of functionality that you require for your management solution.