Files in this tutorial

Emitter and Listener

Listener

A listener implements NotificationListener to receive Notification events. The notification contains information for the type of the notification, the sender of the notification, and any notification-specific information.

The listener implements the single handleNotification method. It's parameters are the notification and an opaque handback object. The handback is specified during the listener registration and can be any information the listener wants.

package example;

import javax.management.NotificationListener;
import javax.management.Notification;

public class Listener
  implements NotificationListener, ListenerMBean {
  private int _count;

  public void handleNotification(Notification notif,
                                 Object handback)
  {
    _count++;
  }

  public int getNotificationCount()
  {
    return _count;
  }
}

Emitter

The Emitter sends notifications. Any managed bean which implements the NotificationEmitter interface can be an emitter. Many Emitters will extend the NotificationBroadcasterSupport, although this is not required.

NotificationBroadcasterSupport will handle the logic for adding and removing listeners as well as sending notifications to the proper listener. By extending NotificationBroadcasterSupport, the emitter only needs to call sendNotification to send the notification.

The first argument for the Notification is the notification type. Because each emitter can send multiple notifications, the type tells the listener which event has happened.

The second argument is typically the ObjectName for the emitter. Often, emitters will use the MBeanRegistration interface to find out the ObjectName.

package example;

import javax.management.NotificationBroadcasterSupport;
import javax.management.Notification;

/**
 * Implements an MBean which sends notifications.
 */
public class Emitter extends NotificationBroadcasterSupport
  implements EmitterMBean {
  private long _sequence;
  
  /**
   * Sends a notification.
   */
  public void send()
  {
    Notification notif;

    notif = new Notification("example.send", this, _sequence++);

    sendNotification(notif);
  }
}

web.xml configuration

The web.xml (or resin.conf) configures the resource with the <resource> tag just as with other resources. The resources is registered as an MBean by specifying an mbean-name.

<web-app xmlns="http://caucho.com/ns/resin">
  <resource mbean-name="example:name=emitter"
            type="example.Emitter">
  </resource>

  <resource mbean-name="example:name=listener"
            type="example.Listener">
    <listener mbean-name="example:name=emitter" handback="tutorial"/>
  </resource>
</web-app>

Using the listener

This example provides a send() method to trigger a notification, but most notifications occuring when specific events occur, e.g. when a pool fills up.

In this case, invoking the send() method triggers the notification which will be sent to any waiting listeners. Calling listener.getNotificationCount() checks that the listener is getting called back.

public class ListenerServlet extends GenericServlet {
  private EmitterMBean _emitter;
  private ListenerMBean _listener;

  public void setEmitter(EmitterMBean emitter)
  {
    _emitter = emitter;
  }

  public void setListener(ListenerMBean listener)
  {
    _listener = listener;
  }

  public void service(ServletRequest request,
		      ServletResponse response)
    throws ServletException, IOException
  {
    PrintWriter out = response.getWriter();
    
    _emitter.send();

    out.println("listener count: " + _listener.getNotificationCount());
  }
}

count: 15

[15:37:15.545] notification(type=example.send,handback=tutorial)
[15:37:16.624] notification(type=example.send,handback=tutorial)
[15:37:17.453] notification(type=example.send,handback=tutorial)

Configuration with Dependency Injection

The ListenerServlet example follows the Dependency Injection pattern. Resin's web.xml will assemble the correct EmitterMBean and ListenerMBean. Using the Dependency Injection pattern simplifies the servlet, makes it more configurable, and more testable.

The configuration takes advantage of the "mbean:" JNDI scheme in Resin. The name following "mbean:" is used to lookup the mbean instance. The "mbean:" scheme then constructs a proxy for the mbean. The proxy of the JNDI lookup is then passed to setEmitter and setListener.

<servlet-mapping url-pattern="/listener"
                 servlet-class="example.ListenerServlet">
  <init>
    <emitter>\${jndi:lookup("mbean:example:name=emitter")}</emitter>
    <listener>\${jndi:lookup("mbean:example:name=listener")}</listener>
  </init>
</servlet-mapping>

Compatibility