问akka乒乓球不工作

EN

public class TestMain {
    public static void main(String[] args){
        ActorSystem actorSystem = ActorSystem.create("TestActorSystem");
        ActorRef ping = actorSystem.actorOf(Props.create(PingActor.class), "pingActor");
        ping.tell("start", ActorRef.noSender());
    }
}


public class PingActor extends AbstractActor{

    private static Logger log = LoggerFactory.getLogger(PingActor.class);
    private ActorRef pong;

    @Override
    public void preStart() throws Exception {
        this.pong = getContext().actorOf(Props.create(PongActor.class, getSelf()), "pongActor");
    }

    @Override
    public Receive createReceive() {
        return receiveBuilder()
            .match(PingCallTell.class, this::tell)
            .matchAny(this::unhandled)
            .build();
    }

    public class PingCallTell{}

    private void tell(PingCallTell call) throws Exception{
        log.info("Ping received..");
        pong.tell("ping", getSelf());
    }
}

public class PongActor extends AbstractActor{

    private ActorRef ping;

    public PongActor(ActorRef ping) { 
        this.ping = ping;
    }

    //same PingActor createReceive() 

    private void tell(PongCallTell call) throws Exception {
        log.info("Pong received..");
        ping.tell("pong", getSelf()); 
        Thread.sleep(1000);
    }
}