8.2.3 Adding a Listener Through the MBean Server

Now that we have identified the objects involved, we need to add the listener to the notification broadcaster. Our example does this in the main method of the agent application.

Example 8–2 Registering for MBean Server Delegate Notifications

AgentListener agentListener = null;
[...]

echo("\nRegistering the MBean server delegate listener...");
try {
    agentListener = new AgentListener();
    myAgent.server.addNotificationListener(
        new ObjectName(ServiceName.DELEGATE),
        agentListener, null, null);
} catch(Exception e) {
    e.printStackTrace();
    System.exit(1);
}
echo("done");

In Example 8–2, the agent application adds the AgentListener instance to the delegate MBean, which is known to be a broadcaster. The object name of the MBean server delegate is given by the DELEGATE constant in the ServiceName class. The listener is added through the addNotificationListener method of the MBean server. This method preserves the management architecture by adding listeners to MBeans while referring only to the MBean object names.

If an MBean implements the listener interface and needs to receive certain notifications, it can add itself to a broadcaster. For example, an MBean could use its preregistration method in order to add itself as a notification listener or it could expose a method that takes the object name of the notification broadcaster MBean. In both cases, its notification handler method must be designed to process all expected notification types.

The last two parameters of the addNotificationListener methods of both the MBeanServer and the NotificationBroadcaster interfaces define a filter and a handback object, respectively. Filter objects are defined by the NotificationFilter interface and provide a callback method that the broadcaster calls before calling the notification handler. If the filter is defined by the entity that adds the listener, it prevents the handler from receiving unwanted notifications.

Handback objects are added to a broadcaster along with a listener and are returned to the designated handler with every notification. The handback object is completely untouched by the broadcaster and can be used to transmit context information from the entity that adds the listener to the handler method. The functionality of filters and handback objects is beyond the scope of this tutorial; refer to the JMX specification for their full description.