Communication with MBeans using notifications - JMX in Action - page 124

The NotificationListener interface contains a single method: handleNotifica-
tion() . It takes two arguments: an instance of Notification and an instance of
Object . Notification broadcasters invoke this method when they are ready to
deliver a notification to the listener. The instance of Notification is the notifica-
tion being sent, and the Object instance is the handback object registered by the
listener. Recall that when a listener registers to receive notifications using the add-
NotificationListener() method, it passes in an Object instance named handback .

When implementing the handleNotification() method from the
NotificationListener interface, you should have it return as soon as
possible. Notification broadcasters invoke this method synchronously,
meaning it will block until the method invocation is complete. Depend-
ing on the implementation and purpose of an MBean, this behavior
could present a problem. You can avoid it by implementing the method
that sends notifications (discussed later), which invoked the handleNo-
tification() method asynchronously.

NOTE

The only component remaining is the notification filter. Because MBeans can
emit an infinite number of notification types, listeners can use a filter to ensure
they receive only the specific notification types in which they are interested.
Remember that listeners register once to receive all notifications from an MBean,
and therefore need to be able to filter out notifications that are unwanted.

6.2.5 Filtering notifications

As stated earlier, notification filters give notification listeners a way to sort
through a potential barrage of notifications to receive only those notifications
that are important to them. To be more accurate, notification broadcasters use a
registered listener's filter to determine whether to send a notification to a lis-
tener. Recall from section 6.2.1 that one of the arguments passed to the addNoti-
ficationListener() method is an implementation of the javax.management.
NotificationFilter interface.
The NotificationFilter interface declares only one method: isNotification-
Enabled() . This method accepts a Notification object that is about to be sent and
returns a boolean value indicating whether the listener associated with this filter
wants to receive the notification. Implementers are free to choose any way of fil-
tering through notifications, but probably the best way to filter notifications is by
their type. JMX contains a class called NotificationFilterSupport that does just