How can i check if an Akka actor exists (akka 2.2)?

Question

I have a java object which is not an actor which selects actors from an actor system with actorSelection(Path)). It is possible, that the selected actor does not exist in the system.

In the Java Api ask() does not exist for ActorSelection, so I can not send and Identify message to the actor selection and use the sender of the response.

I tried to solve the problem by sending the message to the actor anyway via the actor selection and then reacting to the deadletter. But I don't get any deadletters.

How can I check with the ActorSelection if the actor is alive or does not exist?

ActorSystem system = ActorSystem.create("test");  //create test actor system.actorOf(Props.create(TestActor.class), "testActor");  //add dead letter listener to the system ActorRef eventBusActor = asys.actorOf(Props.create(EventBusActor.class), "eventbusactor"); system.eventStream().subscribe(eventBusActor, DeadLetter.class);   //This works. The test actor receives the message       ActorSelection a1 = asys.actorSelection("/user/testActor"); a1.tell("hello", ActorRef.noSender());  //This does not work and does not send dead letters       ActorSelection a2 = asys.actorSelection("/user/doesnotexist"); a2.tell("hello", ActorRef.noSender());  //Does not compile, because ask needs an ActorRef as first argument ActorSelection a3 = asys.actorSelection("/user/test"); Future f = Patterns.ask(a3, new Identify(), 1000); 

Answer 1

I recently found the ActorSelection.resolveOne method:

val name = "myActor" implicit val timeout = 5000 // Timeout for the resolveOne call system.actorSelection(name).resolveOne().onComplete {   case Success(actor) => actor ! message    case Failure(ex) =>     val actor = system.actorOf(Props(classOf[ActorClass]), name)     actor ! message } 

One problem I'm still investigating is, the method where this is defined might be called concurrently (from other actors). Therefore it's possible to get a race condition where you try to create the actor twice if the resolveOne call fails because the actor is still being created. This might or might not be an issue for your use case

Answer 2

It looks like Akka left off support for ActorSelection on the java api for ask. I played with the code a little and I found something that works though. See if this code works for you:

import java.util.concurrent.TimeUnit;  import scala.concurrent.Await; import scala.concurrent.Future;  import akka.actor.ActorIdentity; import akka.actor.ActorRef; import akka.actor.ActorSelection; import akka.actor.ActorSystem; import akka.actor.Identify; import akka.actor.Props; import akka.pattern.AskableActorSelection; import akka.util.Timeout;  public class AskTest {    public static void main(String[] args) throws Exception{     ActorSystem sys = ActorSystem.apply("test");     sys.actorOf(Props.create(TestActor.class), "mytest");      ActorSelection sel = sys.actorSelection("/user/mytest");      Timeout t = new Timeout(5, TimeUnit.SECONDS);     AskableActorSelection asker = new AskableActorSelection(sel);     Future<Object> fut = asker.ask(new Identify(1), t);     ActorIdentity ident = (ActorIdentity)Await.result(fut, t.duration());     ActorRef ref = ident.getRef();     System.out.println(ref == null);   } } 

I just looked at how the scala ask support worked and hooked into it via java. This worked for me; I'm hoping it works for you.