The Agent-Side Broadcaster

As in most management architectures, the notification broadcasters are agent-side entities. The broadcasters are MBeans registered in an agent's MBean server to which our management application will need to connect. Only notifications sent by registered MBeans can be forwarded to manager applications, and a manager-side listener can receive them only by registering through a connector object.

Since the notification support classes do not rely on the MBean server, they could be reused in the manager or any other Java-based application that requires a notification mechanism. In this case, listeners will need to register directly with local broadcasters and there is no longer any mechanism for forwarding notifications to a remote listener. Because the same classes are used everywhere, it would be possible to have a listener in a manager that is added to both a broadcaster MBean in a remote agent and directly to a local broadcaster object. However, none of these situations are covered in our example.

Here we give the code of the MBean that we will use to send notifications (the code for its MBean interface has been omitted). It extends the NotificationBroadcasterSupport class to reuse all of its listener registration facilities. It only contains one operation which can be called by our manager to trigger any number of notifications.

Example 7-1 The Agent-Side Broadcaster MBean

import javax.management.MBeanNotificationInfo;
import javax.management.NotificationBroadcasterSupport;
import javax.management.Notification;

public class NotificationEmitter
    extends NotificationBroadcasterSupport
    implements NotificationEmitterMBean {

    // Just to make the inheritance explicit
    public NotificationEmitter() {
        super();
    }

    // Provide details about the notification type and class that is sent
    public MBeanNotificationInfo[] getNotificationInfo() {
        
        MBeanNotificationInfo[] ntfInfoArray = new MBeanNotificationInfo[1];
        
        String[] ntfTypes = new String[1];
        ntfTypes[0] = myType;
        
        ntfInfoArray[0] = new MBeanNotificationInfo( ntfTypes,
            "javax.management.Notification", 
            "Notifications sent by the NotificationEmitter");
        return ntfInfoArray;
    }  

    // The only operation: sends nb number of notifications
    // whose sequence numbers go from 1 to nb
    public void sendNotifications(Integer nb) {

        for (int i=1; i<=nb.intValue(); i++) {
            sendNotification(new Notification(myType, this, i));
        }
    }
    private String myType = "notification.my_notification";
}

Our MBean invents a notification type string and exposes this information through the getNotificationInfo method. To demonstrate the forwarding mechanism, we are more interested in the sequence number: this will allow us to identify the notifications as they are received in the manager.

This MBean demonstrates that the broadcaster has total control over the contents of its notifications. Constructors for the Notification object allow you to specify all of the fields, even ones such as the time stamp. In this example, we control the sequence number, and our chosen policy is to reset the sequence number to 1 with every call to the operation. Of course, you are free to choose the notification contents, including the time-stamping and sequence-numbering policies that fit your management solution.