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Java Dynamic Management Kit 5.0 Tutorial

NotificationBroadcasterSupport Class

The broadcaster in our example is a very simple MBean with only one attribute. The setter method for this attribute triggers a notification whenever the value actually changes. This policy is specific to our example. You might want to design an MBean that sends an attribute change every time the setter is called, regardless of whether or not the value is modified. Similarly, the fact that the reset operation changes the value of the attribute but does not send a notification is specific to our example. Your management needs might vary.

Example 9–3 shows the code for our SimpleStandard MBean class (the code for its MBean interface has been omitted):

Example 9–3 Broadcaster for Attribute Change Notifications

import javax.management.NotificationBroadcasterSupport;
import javax.management.MBeanNotificationInfo;
import javax.management.AttributeChangeNotification;

public class SimpleStandard
    extends NotificationBroadcasterSupport
    implements SimpleStandardMBean {

    /* "SimpleStandard" does not provide any specific constructors.
     * However, "SimpleStandard" is JMX compliant with regards to 
     * constructors because the default constructor SimpleStandard() 
     * provided by the Java compiler is public.
     */ 

    public String getState() {
        return state;
    }

    // The attribute setter chooses to send a notification only if
    // the value is modified
    public void setState(String s) {
        if (state.equals(s))
            return;
        AttributeChangeNotification acn = new AttributeChangeNotification(
            this, 0, 0, null, "state", "String", state, s);
        sendNotification(acn);
        state = s;
        nbChanges++;
    }

    [...]
    // The reset operation chooses not to send a notification even though
    // it changes the value of the state attribute
    public void reset() {
        state = "initial state";
        nbChanges = 0;
        nbResets++;
    }

    // Provide details about the notification type and class that is sent
    public MBeanNotificationInfo[] getNotificationInfo() {
        
        MBeanNotificationInfo[] ntfInfoArray = new MBeanNotificationInfo[1];
        
        String[] ntfTypes = new String[1];
        ntfTypes[0] = AttributeChangeNotification.ATTRIBUTE_CHANGE;
        
        ntfInfoArray[0] = new MBeanNotificationInfo( ntfTypes,
            "javax.management.AttributeChangeNotification", 
            "Attribute change notification for the 'State' attribute.");
        return ntfInfoArray;
    }

    private String      state = "initial state";
    private int         nbChanges = 0;
    private int         nbResets = 0;
}

This MBean sends its notifications and it implements the NotificationBroadcaster interface by extension of the NotificationBroadcasterSupport class.

This class implements the NotificationBroadcaster interface in order to provide all the mechanisms for adding and removing listeners and sending notifications. It manages an internal list of listeners and their handback objects and updates this list whenever listeners are added or removed. In addition, the NotificationBroadcasterSupport class provides the sendNotification method to send a notification to all listeners currently on its list.

By extending this object, our MBean inherits all of this behavior. Subclassing NotificationBroadcasterSupport is a quick and convenient way to implement notification broadcasters. We do not even have to call a superclass constructor because it has a default constructor. We only need to override the getNotificationInfo method to provide details about all of the notifications that might be sent.