test

Java Dynamic Management Kit 5.1 Tutorial

8.3.1 NotificationBroadcasterSupport Class

The broadcaster in our example is a very simple MBean. The reset method triggers a notification whenever it is called. This policy is specific to our example. You might want to design an MBean that sends an attribute change every time the setter is called, regardless of whether or not the value is modified. Your management needs might vary.

Example 8–3 shows the code for our SimpleStandard MBean class (the code for its MBean interface has been omitted).

Example 8–3 Broadcaster for Attribute Change Notifications

import javax.management.NotificationBroadcasterSupport;
import javax.management.MBeanNotificationInfo;
import javax.management.AttributeChangeNotification;

public class SimpleStandard
    extends NotificationBroadcasterSupport
    implements SimpleStandardMBean {

    public String getState() {
        return state;
    }

    public void setState(String s) {
        state = s;
        nbChanges++;
    }

    [...]
   
    public int getNbChanges() {
        return nbChanges;
    }

    public void reset() {
	         AttributeChangeNotification acn =
	             new AttributeChangeNotification(this,
					                                0,
					                                0,
					                                "NbChanges reset",
					                                "NbChanges",
					                                "Integer",
					                                new Integer(nbChanges),
					                                new Integer(0));
	         state = "initial state";
          nbChanges = 0;
	         nbResets++;
	         sendNotification(acn);
    }    

   public int getNbResets() {
       	return nbResets;
   }

   public MBeanNotificationInfo[] getNotificationInfo() {
        return new MBeanNotificationInfo[] {
	          new MBeanNotificationInfo(
	          new String[] { AttributeChangeNotification.ATTRIBUTE_CHANGE },
	          AttributeChangeNotification.class.getName(),
	          "This notification is emitted when the reset() method is 
            called.")
	   };
   
    private String      state = "initial state";
    private int         nbChanges = 0;
    private int         nbResets = 0;
}

This MBean sends its notifications and it implements the NotificationBroadcaster interface by extension of the NotificationBroadcasterSupport class, in order to provide all the mechanisms for adding and removing listeners and sending notifications. It manages an internal list of listeners and their handback objects and updates this list whenever listeners are added or removed. In addition, the NotificationBroadcasterSupport class provides the sendNotification method to send a notification to all listeners currently on its list.

By extending this object, our MBean inherits all of this behavior. Subclassing NotificationBroadcasterSupport is a quick and convenient way to implement notification broadcasters. We do not even have to call a superclass constructor because it has a default constructor. We only need to override the getNotificationInfo method to provide details about all of the notifications that might be sent.