Sun Java System Application Server Enterprise Edition 8.1 2005Q2 Developer's Guide

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## Monitoring Attribute Changes

The `demoJMXMonitor()` demonstrates how to monitor attribute changes.

##### Example 16–13 Monitoring Attribute Changes

    [...]
    demoJMXMonitor() throws InstanceNotFoundException, IOException
    {
        final JMXMonitorMgr mgr = getDomainRoot().getJMXMonitorMgr();
        final String attrName = "SampleString";
        final String attrValue = "hello";
        final SampleListener sampleListener = new SampleListener();
        final MBeanServerConnection conn =
            Util.getExtra( mgr ).getConnectionSource()
            .getExistingMBeanServerConnection();
        conn.addNotificationListener(
            getMBeanServerDelegateObjectName(),
            sampleListener, null, null );
        final Sample sample = (Sample)getDomainRoot()
            .getContainee( XTypes.SAMPLE );
        final String monitorName = "SampleStringMonitor";
        AMXStringMonitor mon = null;
        try
        {
            try { mgr.remove( monitorName ); }
            catch( Exception e ) {}
            mon = mgr.createStringMonitor( monitorName );
            waitMBeanServerNotification( sampleListener,
                MBeanServerNotification.REGISTRATION_NOTIFICATION,
                Util.getObjectName( mon ) );
            sample.addAttribute( attrName, attrValue );
            mon.addNotificationListener( sampleListener, null, null);
            mon.setObservedAttribute( attrName );
            mon.setStringToCompare( attrValue );
            mon.setNotifyDiffer( true );
            mon.setNotifyMatch( true );
            mon.addObservedObject( Util.getObjectName( sample ) );
            final StdAttributesAccess attrs = Util.getExtra( sample);
            attrs.setAttribute( new Attribute(attrName, "goodbye") );
            attrs.setAttribute( new Attribute(attrName, attrValue) );
            sample.removeAttribute( attrName );
            final Map notifs = sampleListener.getNotifsReceived();
            waitNumNotifs( notifs,
                AttributeChangeNotification.ATTRIBUTE_CHANGE, 4 );
        }
        catch( Throwable t )
        {
            t.printStackTrace();
        }
        finally
        {
            try
            {
                mon.removeNotificationListener( sampleListener );
                if ( mon != null )
                {
                    mgr.remove( mon.getName() );
                    waitMBeanServerNotification( sampleListener,
                        MBeanServerNotification
                        .UNREGISTRATION_NOTIFICATION,
                        Util.getObjectName( mon ) );
                }
                conn.removeNotificationListener(
                getMBeanServerDelegateObjectName(),
                sampleListener );
            }
            catch( ListenerNotFoundException e )
            {
            }
        }
    }
    [...]

The `demoJmx()` method demonstrates the implementation of a JMX monitor MBean, that listens for changes in a certain attribute. This is achieved in the following stages:

1. A `com.sun.appserv.management.monitor.JMXMonitorMgr` instance is obtained using the `DomainRoot.getJMXMonitorMgr()` method.

2. A `SampleListener` JMX notification listener that is provided in the sample package is instantiated.

3. A connection to the domain’s MBean server is obtained by calling `com.sun.appserv.management.client.ConnectionSource`. `getExistingMBeanServerConnection()` on the `JMXMonitorMgr` instance’s `Extra` information.

4. The `SampleListener` notification listener is added to the MBean server connection, with an MBean server delegate obtained from `getMBeanServerDelegateObject()`. The notification listener is now in place on the MBean server connection.

5. An AMX MBean, `sample`, of the type `SAMPLE` is obtained by calling the `com.sun.appserv.management.base.Container.getContainee()` method on an instance of the `Sample` interface. The `Sample` interface defines a basic AMX MBean.

6. An `AMXStringMonitor`, an AMX-compatible JMX `StringMonitorMBean`, is instantiated by calling `createStringMonitor` on the `JMXMonitorMgr` instance created above. The `AMXStringMonitor` instance then calls `waitMBeanServerNotification()`. The `waitMBeanServerNotification()` method waits for MBean server notifications of the type `REGISTRATION_NOTIFICATION` from the `SampleListener` instance that is listening on the MBean server connection.

7. An attribute of name `attrName` and value `attrValue` is added to the AMX MBean `sample`.

8. Various methods of the `AMXStringMonitor` instance are called, to add a listener, and to set the value to be observed, the object to be observed, and so on.

9. Access to the `sample` MBean’s attributes is obtained by passing the `sample` MBean’s `Extra` information to an instance of `com.sun.appserv.management.base.StdAttributesAccess`. The `StdAttributesAccess.setAttribute()` method is then called to change the values of these attributes.

10. The `AMXStringMonitor` then calls the sample notification listener’s `getNotifsReceived()` method to retrieve the notifications that resulted from the calls to `setAttribute()` above. The `waitNumNotifs()` method waits until four `ATTRIBUTE_CHANGE` notifications have been received before exiting.

11. The notification listener is then removed and the monitor is closed down.

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