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Trail: Java Management Extensions (JMX)
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Lesson: Notifications

> 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.

> - The object name of the source of the notification, namely the `Hello` MBean, represented by `this`
> - A sequence number, namely `sequenceNumber`, that is set to 1 and that increases incrementally
> - A timestamp
> - The content of the notification message
> - The name of the attribute that has changed, in this case, `CacheSize`
> - The type of attribute that has changed
> - The old attribute value, in this case, `oldSize`
> - The new attribute value, in this case, `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.

> #### Running the MBean Notification Example

> Once again, you will use JConsole to interact with the `Hello` MBean, this time to send and receive notifications.

> 1. If you have not done so already, save `jmx_examples.zip` into your `work_dir` directory.

> 2. Unzip the bundle of sample classes by using the following command in a terminal window.

>    > `unzip jmx_examples.zip`

> 3. Compile the example Java classes from within the `work_dir` directory.

>    > `javac com/example/*.java`

> 4. Start the `Main` application.

>    > `java com.example.Main`

>    A confirmation that `Main` is waiting for something to happen is generated.

> 5. Start JConsole in a different terminal window on the same machine.

>    > `jconsole`

>    The New Connection dialog box is displayed, presenting a list of running JMX agents that you can connect to.

> 6. In the New Connection dialog box, select `com.example.Main` from the list and click Connect.

>    A summary of your platform's current activity is displayed.

> 7. Click the MBeans tab.

>    This panel shows all the MBeans that are currently registered in the MBean server.

> 8. In the left frame, expand the `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.

> 9. Expand the Notifications node of the `Hello` MBean in the MBean tree.

>    Note that the panel is blank.

> 10. Click the Subscribe button.

>     The current number of notifications received (0) is displayed in the Notifications node label.

> 11. Expand the Attributes node of the `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.

> 12. Expand again the Notifications node of the `Hello` MBean in the MBean tree.

>     The details of the notification are displayed.

> 13. To close JConsole, select Connection -> Exit.

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