Discovery Monitor

The discovery monitor is a notification broadcaster: when it receives an activation or deactivation message from a discovery responder, it sends a discovery responder notification to its listeners. Once its parameters are configured and the monitor is activated, the discovery is completely passive. You can add or remove listeners at any time.

The DiscoveryMonitor MBean has multicast group and multicast port attributes that determine the multicast socket where it will receive responder messages. Like the other components of the discovery service, the default multicast group is 224.224.224.224 and the default port is 9000. You can specify other values for the group and port either in the constructor or through attribute setters when the monitor is off-line.

The discovery monitor in our example is registered as an MBean. We then add a listener through the MBean server as we would for any other notification broadcaster.

Example 17–4 Instantiating and Starting a Discovery Monitor

// build the DiscoveryMonitor MBean ObjectName
//
ObjectName discoveryMonitorMBeanObjectName =
    new ObjectName(domain + "name=myDiscoveryMonitor");

// Create, register and start the DiscoveryMonitor MBean
//
try {
    ObjectInstance discoveryMonitorObjectInstance =
        myMBeanServer.createMBean(
            "com.sun.jdmk.discovery.DiscoveryMonitor",
            discoveryMonitorMBeanObjectName) ;
    myMBeanServer.invoke (discoveryMonitorMBeanObjectName,
        "start", null, null);

} catch(Exception e) {
    e.printStackTrace();
    System.exit(1);
}

// Add ourselves as a listener to the DiscoveryMonitor MBean
//
try {
    myMBeanServer.addNotificationListener(
        discoveryMonitorMBeanObjectName, this, null, null ) ;

} catch(Exception e) {
    e.printStackTrace();
    System.exit(1);
}

The discovery monitor must be activated with the start operation before it will receive responder messages and send notifications. It will be stopped automatically if it is unregistered from its MBean server. If it is not used as an MBean, you should invoke its stop method before your application exits.