WatchActor.java

package akka.common;

import akka.actor.ActorRef;
import akka.actor.Terminated;
import akka.actor.UntypedActor;

public class WatchActor extends UntypedActor {

	public static final class WatchMe {
		final ActorRef actor;

		public WatchMe(ActorRef actor) {
			this.actor = actor;
		}
	}

	private int running;

	@Override
	public void onReceive(Object message) throws Exception {
		if (message instanceof WatchMe) {
			ActorRef actor = ((WatchMe) message).actor;
			context().watch(actor);
			running++;
		} else if (message instanceof Terminated) {
			running--;
			if (running == 0) {
				context().system().shutdown();
			}
		} else {
			unhandled(message);
		}
	}

}