“Hello World,” the JMX way - JMX in Action - page 43

2.6 Using MBean notifications

After creating and registering your own MBean in the previous section, you
already have enough knowledge to start working with JMX . You have learned
how to create a Standard MBean, how to add it to a simple JMX agent, and how
to manage that agent by using the HTML adapter. However, you are still missing
a key ingredient: notifications.
JMX notifications are Java objects used to send information from MBeans and
agents to other objects that have registered to receive them (see figure 2.9).
Objects interested in receiving events are notification listeners—they implement
the javax.management.NotificationListener interface.
Notifications are an important piece of JMX because they allow for the trans-
mission of events. JMX events can be anything from the changing of an MBean
attribute to the registration of a new MBean on an MBean server.
To give you a quick introduction to notifications, you'll add them to the
HelloWorld MBean in this section. In chapter 6, we'll cover the notification
model in depth.

2.6.1 Adding notification code to the HelloWorld MBean

Fo r t h e HelloWorld MBean to send notifications, it needs to allow objects inter-
ested in receiving notifications to register for them. JMX supports two mecha-
nisms for MBeans to provide listeners to register for notifications:
■ Implement the javax.management.NotificationBroadcaster interface
■ Extend the javax.management.NotificationBroadcasterSupport class (which
in turn implements the NotificationBroadcaster interface)
The advantage of implementing the interface is that it frees your class from being
tied to a particular super class. The advantage of extending the broadcaster

Figure 2.9
Notification being sent
to a registered listener
from an MBean