获取现有的或创建新的Akka演员

Question

I'm trying to get an existing ActorRef with ActorFor or create a new one if it does not exists. I have the following code but it doesn't seem to work as expected. .isTerminated() is always true.

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

            ActorRef subscriberCandidate = system.actorFor("akka://System/user/"+name);

            if (subscriberCandidate.isTerminated())
            {
                ActorRef subscriber = system.actorOf(new Props(new UntypedActorFactory() {
                      public UntypedActor create() {
                        return new Sub(name,link);
                      }
                    }), name);
                System.out.println(subscriber.path().toString() + " created");
            }
            else
                System.out.println("already exists"); 

What am I missing here? Thanks in advance.

Answer 1

Get-or-create can only be performed by the parent of the designated actor, since only that parent can create the actor if it does not exist, and only the parent can do so consistently (ie without race conditions). Within an actor you can do

// assuming a String name like "fred" or "barney", i.e. without "/"
final Option<ActorRef> child = child(name);
if (child.isDefined())
  return child.get();
else
  return getContext().actorOf(..., name);

Do not do this at the top-level (ie using system.actorOf ), because then you cannot be sure who “wins” in requesting creation and also relying on the user guardian is not good a good supervision strategy.

Answer 2

Change your lookup to be:

system.actorFor("/user/" + name)

You don't need the "akka://System" part if this is a local actor you are looking up. This is assuming that this actor was already started up elsewhere in your code though. If not it won't work.

Answer 3

Based on the given code you are calling actorFor to look up a non-existent actor. The actor doesn't exist until actorOf is called.