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The JMX API defines a mechanism to enable MBeans to generate notifications, for example, to signal a state change, a detected event, or a problem.
To generate notifications, an MBean must implement the interface
NotificationEmitter
or extend
NotificationBroadcasterSupport
. To send a notification, you need to construct an instance of the class
javax.management.Notification
or a subclass (such as
AttributeChangedNotification
), and pass the instance to
NotificationBroadcasterSupport.sendNotification
.
Every notification has a source. The source is the object name of the MBean that generated the notification.
Every notification has a sequence number. This number can be used to order notifications coming from the same source when order matters and there is a risk of the notifications being handled in the wrong order. The sequence number can be zero, but preferably the number increments for each notification from a given MBean.
The
Hello
MBean implementation in
Standard MBeans
actually implements the notification mechanism. However, this code was omitted in that lesson for the sake of simplicity. The complete code for Hello
follows:
package com.example; import javax.management.*; public class Hello extends NotificationBroadcasterSupport implements HelloMBean { public void sayHello() { System.out.println("hello, world"); } public int add(int x, int y) { return x + y; } public String getName() { return this.name; } public int getCacheSize() { return this.cacheSize; } public synchronized void setCacheSize(int size) { int oldSize = this.cacheSize; this.cacheSize = size; System.out.println("Cache size now " + this.cacheSize); Notification n = new AttributeChangeNotification(this, sequenceNumber++, System.currentTimeMillis(), "CacheSize changed", "CacheSize", "int", oldSize, this.cacheSize); sendNotification(n); } @Override public MBeanNotificationInfo[] getNotificationInfo() { String[] types = new String[]{ AttributeChangeNotification.ATTRIBUTE_CHANGE }; String name = AttributeChangeNotification.class.getName(); String description = "An attribute of this MBean has changed"; MBeanNotificationInfo info = new MBeanNotificationInfo(types, name, description); return new MBeanNotificationInfo[]{info}; } private final String name = "Reginald"; private int cacheSize = DEFAULT_CACHE_SIZE; private static final int DEFAULT_CACHE_SIZE = 200; private long sequenceNumber = 1; }
This Hello
MBean implementation extends the NotificationBroadcasterSupport
class. NotificationBroadcasterSupport
implements the NotificationEmitter
interface.
The operations and attributes are set in the same way as in the standard MBean example, with the exception that the CacheSize
attribute's setter method now defines a value of oldSize
. This value records the CacheSize
attribute's value prior to the set operation.
The notification is constructed from an instance, n
, of the JMX class AttributeChangeNotification
, which extends javax.management.Notification
. The notification is constructed within the definition of the setCacheSize()
method from the following information. This information is passed to AttributeChangeNotification
as parameters.
Hello
MBean, represented by this
sequenceNumber
, that is set to 1 and that increases incrementallyCacheSize
oldSize
this.cacheSize
The notification n
is then passed to the NotificationBroadcasterSupport.sendNotification()
method.
Finally, the
MBeanNotificationInfo
instance is defined to describe the characteristics of the different notification instances generated by the MBean for a given type of notification. In this case the type of notifications sent is AttributeChangeNotification
notifications.
Once again, you will use JConsole to interact with the Hello
MBean, this time to send and receive notifications. This example requires version 6 of the Java SE platform.
jmx_examples.zip
into your work_dir
directory.unzip jmx_examples.zip
work_dir
directory.
javac com/example/*.java
Main
application.
java com.example.Main
A confirmation that Main
is waiting for something to happen is generated.
jconsole
The New Connection dialog box is displayed, presenting a list of running JMX agents that you can connect to.
com.example.Main
from the list and click Connect.
A summary of your platform's current activity is displayed.
This panel shows all the MBeans that are currently registered in the MBean server.
com.example
node in the MBean tree.
You see the example MBean Hello
that was created and registered by Hello
. If you click Hello
, you see its Notifications node in the MBean tree.
Hello
MBean in the MBean tree.
Note that the panel is blank.
The current number of notifications received (0) is displayed in the Notifications node label.
Hello
MBean in the MBean tree, and change the value of the CacheSize
attribute to 150.
In the terminal window in which you started Main
, a confirmation of this attribute change is displayed. Note that the number of received notifications displayed in the Notifications node has changed to 1.
Hello
MBean in the MBean tree.
The details of the notification are displayed.