22.1.2 Manager-Side Listener

In our simple example, the Client class itself is the listener object. Usually, a listener would be a separate instance of a special listener class and depending on the complexity of the manager, there might be several classes of listeners, each for a specialized category of notifications.

Example 22–2 The Manger-Side Listener

public class Client implements NotificationListener {

    [...] // Constructor omitted

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

        System.out.println("Client: received a notification of type "
            + notif.getType() + "\nwith the sequence number "
            + notif.getSequenceNumber());
    }
    [...]  // main omitted
}

As explained in the notification mechanism 8.1 Overview of Notifications, a listener on the agent side is typically an MBean that receives notifications about the status of other MBeans and then processes or exposes this information in some manner. Only if a key value or some management event is observed will this information be passed to a listening manager, probably by sending a different notification.

In this manner, the notification model reduces the communication that is necessary between agents and managers. Your management solution determines how much decisional power resides in the agent and when situations are escalated. These parameters will affect your design of the notification flow between broadcasters, listeners, agents, and managers.

The usual role of a manager-side listener is to process the important information in a notification and take the appropriate action. As we shall see, our notification example is much simpler. Our goal is not to construct a real-world example, but to demonstrate the mechanisms that are built into the Java DMK.