java - 创建一个 ActorSelection 以获取在父 actor 中创建的 ActorRef

Question

我有一个带有 ParentOrderActor 的 Akka 系统，它接收 Create 和 Read 消息以创建和读取子级 OrderActor分别。

如果我在 main "/user/..." 路径中创建所有 Actor，一切顺利:

// Inside the ParentOrderActor ("/user/orders")

// Create in "/user"
getContext().system().actorOf(new Props(OrderActor.class), "ABC");

// Read via an ActorSelection
ActorSelection actorSelection = getContext().system().actorSelection("/user/ABC");

当一切顺利时，日志会显示:

ParentOrderActor -> created: Actor[akka://system/user/ABC#1132819541]
ParentOrderActor -> received: Message[name=Read, id=ABC]
ParentOrderActor -> actorSelection: ActorSelection[Actor[akka://system/]/user/ABC]
ParentOrderActor -> for id=Message[name=Read, id=ABC], we retrieved ActorRef: Actor[akka://system/user/ABC#1132819541]

但是当尝试在 ParentOrderActor 中创建和读取子 actor 时:

// Inside the ParentOrderActor ("/user/orders")

// Create in "/user/orders"
getContext().actorOf(new Props(OrderActor.class), "ABC");

// Read via an ActorSelection
ActorSelection actorSelection = getContext().actorSelection("ABC");

正在打印:

ParentOrderActor -> created: Actor[akka://system/user/orders/$a/ABC#-436492577]
ParentOrderActor -> received: Message[name=Read, id=ABC]
ParentOrderActor -> actorSelection: ActorSelection[Actor[akka://system/user/orders/$a#478613574]/ABC]
Oops: java.util.concurrent.TimeoutException: Futures timed out after [2 seconds]
ParentOrderActor -> for id=Message[name=Read, id=ABC], we retrieved ActorRef: null

我已经在 actorSelection(...) 中尝试了所有类型的路径，但 ActorRef 始终为 null。

为了完整起见，这里有一个简短的 SSCCE 来演示上述内容:

public class Main {

    private static boolean createChildrenInUser = false;
    private static LoggingAdapter log;

    static abstract class Message {

        final String id;

        Message(String id) {
            this.id = id;
        }

        @Override
        public String toString() {
            return "Message[name=" + getClass().getSimpleName() + ", id=" + id + "]";
        }
    }

    static class Create extends Message {
        Create(String id) {
            super(id);
        }
    }

    static class Read extends Message {
        Read(String id) {
            super(id);
        }
    }

    static class ParentOrderActor extends UntypedActor {

        @Override
        public void preStart() {
            log.debug("ParentOrderActor -> starting...");
        }

        @Override
        public void onReceive(Object message) throws Exception {

            log.debug("ParentOrderActor -> received: {}", message);

            if(message instanceof Create) {
                if(createChildrenInUser) {
                    // Create an Actor in the root context ("/user/id").
                    ActorRef created = getContext().system().actorOf(new Props(OrderActor.class), ((Create)message).id);
                    log.debug("ParentOrderActor -> created: {}", created);
                }
                else {
                    // Create an Actor in this ParentOrderActor's context ("/user/orders/id")
                    ActorRef created = getContext().actorOf(new Props(OrderActor.class), ((Create)message).id);
                    log.debug("ParentOrderActor -> created: {}", created);
                }
            }
            else if(message instanceof Read) {
                ActorRef ref = select((Message)message);
                log.debug("ParentOrderActor -> for id={}, we retrieved ActorRef: {}", message, ref);
                getContext().system().shutdown();
            }
            else {
                unhandled(message);
            }
        }

        private ActorRef select(Message message) {
            try {
                ActorSelection actorSelection;
                if(createChildrenInUser) {
                    // Select the Actor with a certain id in the root context ("/user/id").
                    actorSelection = getContext().system().actorSelection("/user/" + message.id);
                }
                else {
                    // Create an Actor in this ParentOrderActor's context ("/user/orders/id")
                    actorSelection = getContext().actorSelection(message.id);
                }
                log.debug("ParentOrderActor -> actorSelection: {}", actorSelection);
                Timeout timeout = new Timeout(2, TimeUnit.SECONDS);
                AskableActorSelection askableActorSelection = new AskableActorSelection(actorSelection);
                Future<Object> future = askableActorSelection.ask(new Identify(1), timeout);
                ActorIdentity actorIdentity = (ActorIdentity) Await.result(future, timeout.duration());
                return actorIdentity.getRef();
            }
            catch(Exception e) {
                log.debug("Oops: {}", e);
                return null;
            }
        }
    }

    static class OrderActor extends UntypedActor {

        @Override
        public void preStart() {
            log.debug("OrderActor -> starting...");
        }

        @Override
        public void onReceive(Object message) throws Exception {
            log.debug("OrderActor -> received: {}", message);
            unhandled(message);
        }
    }

    public static void main(String[] args) {

        ActorSystem system = ActorSystem.create("system");

        log = Logging.getLogger(system, Main.class);

        ActorRef orders = system.actorOf(new Props(ParentOrderActor.class).withRouter(new RoundRobinRouter(1)), "orders");

        orders.tell(new Create("ABC"), orders);

        orders.tell(new Read("ABC"), orders);
    }
}

运行上面的代码将导致 Read 返回一个 ActorRef，它是 null。将 boolean 标志 createChildrenInUser 更改为 true 将在 "/user" 中创建子 actor，一切顺利。

问题:如何创建 ActorSelection 并获取在 /user/orders 中创建的 actor 的 ActorRef (ParentOrderActor)?

Answer 1

如果我正确理解你的问题并且你想要从父 actor 中加载子 actor 的最佳方式，那么你可以使用:

getContext().child("ABC")

这将返回一个 scala.Option<ActorRef>虽然不是 ActorSelection , 所以如果你想用通配符查找多个 child 并向他们发送一条消息，这种方法将不起作用。