17.3 Sending notifications

MBeans can push notifications to interested JMX clients with Spring’s NotificationPublisher. NotificationPublisher has a single sendNotification() method that, when given a Notification object, publishes the notification to any JMX clients that have subscribed to the MBean.

For an MBean to be able to publish notifications, it must implement the NotificationPublisherAware interface, which requires that a setNotificationPublisher() method be implemented. For example, suppose you want to publish a notification for every 100 tacos that are created. You can change the TacoCounter class so that it implements NotificationPublisherAware and uses the injected NotificationPublisher to send notifications for every 100 tacos that are created.

The following listing shows the changes that must be made to TacoCounter to enable such notifications.

Listing 17.2 Sending notifications for every 100 tacos

package tacos.jmx;

import java.util.concurrent.atomic.AtomicLong;
import org.springframework.data.rest.core.event.AbstractRepositoryEventListener;
import org.springframework.jmx.export.annotation.ManagedAttribute;
import org.springframework.jmx.export.annotation.ManagedOperation;
import org.springframework.jmx.export.annotation.ManagedResource;
import org.springframework.stereotype.Service;

import org.springframework.jmx.export.notification.NotificationPublisher;
import org.springframework.jmx.export.notification.NotificationPublisherAware;
import javax.management.Notification;

import tacos.Taco;
import tacos.data.TacoRepository;

@Service
@ManagedResource
public class TacoCounter
        extends AbstractRepositoryEventListener<Taco>
        implements NotificationPublisherAware {

  private AtomicLong counter;
  private NotificationPublisher np;

  @Override
  public void setNotificationPublisher(NotificationPublisher np) {
    this.np = np;
  }

  ...

  @ManagedOperation
  public long increment(long delta) {
    long before = counter.get();
    long after = counter.addAndGet(delta);
    if ((after / 100) > (before / 100)) {
      Notification notification = new Notification(
              "taco.count", this,
              before, after + "th taco created!");
      np.sendNotification(notification);
    }
    return after;
  }
}

In the JMX client, you’ll need to subscribe to the TacoCounter MBean to receive notifications. Then, as tacos are created, the client will receive notifications for each century count. Figure 17.5 shows how the notifications may appear in JConsole.

Figure 17.5 JConsole, subscribed to the TacoCounter MBean, receives a notification for every 100 tacos that are created

Notifications are a great way for an application to actively send data and alerts to a monitoring client without requiring the client to poll managed attributes or invoke managed operations.