什么Akka Java类用来实现Future回调？

Question

Akka Java here. My classes:

class SomeActor extends UntypedActor {
    // ...
}

class SomeMessage {
    // ...
}

// Inside onReceive method for another actor:
Future<Fizz> fizzFut = Patterns.ask(someActor, someMsg, 500)
fizzFut.onComplete(new FizzHandler())

class FizzHandler extends akka.dispatch.OnComplete<Fizz> {
    @Override
    void onComplete(Throwable error, Fizz result) {
        if(error != null) {
            // Handle error.
        } else {
            // Handle success.
        }

        // TODO: Now how do I send a message back "inside" the
        // actor system?
    }
}

At runtime I get the following exception:

[ERROR] [08/23/2015 05:55:09.490] [myapp-akka.actor.default-dispatcher-4]
[akka://myapp/user/AnotherActor] No signature of method: 
scala.concurrent.impl.Promise$DefaultPromise.onComplete() is applicable for argument
types: (com.me.myapp.FizzHandler) values: [<function1>]
Possible solutions: onComplete(scala.Function1, scala.concurrent.ExecutionContext),
isCompleted(), complete(scala.util.Try), tryComplete(scala.util.Try)

So I tried implementing a scala.Function subclass but apparently that class is final and cannot be extended.

So two concerns here:

• I can't decipher what Java API classes I should be using for my onComplete handler. Any ideas?; and
• Once I get this working, and the FizzHandler#onComplete(...) method is running, how do I send a message back "inside" my actor system? The Inbox ?

Answer 1

Using Akka 2.3.8 and scala 2.10 :

I use the separate onFailure and onSuccess methods which take an akka.dispatch.OnFailure and akka.dispatch.OnSuccess<T> respectively. For example:

future.onFailure(new OnFailure() {
    @Override
    public void onFailure(Throwable failure) throws Throwable {
        // handle failure
    }
}, context().dispatcher());

and

future.onSuccess(new OnSuccess<Object>() {
    @Override
    public void onSuccess(Object result) throws Throwable {
        // handle successful value return
    }
}, context().dispatcher());

(if you're calling this outside an actor you'll have to change where you get the dispatcher)

Answer 2

In your example using onComplete you're missing the dispatcher. onComplete takes 2 parameters - the handler function and the dispatcher which should be used to run the call-back on once it's ready. So the fixed sample is:

    import akka.actor.ActorSystem;
    import akka.dispatch.Futures;
    import akka.dispatch.OnComplete;
    import scala.concurrent.Future;
    import scala.runtime.BoxedUnit;

    final ActorSystem sys = ActorSystem.create();

    final Future<String> successful = Futures.successful("");
    successful.onComplete(new OnComplete<String>() {
        @Override
        public void onComplete(Throwable failure, String success) throws Throwable {

        }
    }, sys.dispatcher());

Note that once Scala 2.12 hits it's stable release and Akka compiles using it (soon) it will emit FunctionN classes in a compatible way with Java lambda expressions, then you'll be able to write onComplete(d -> {}, dispatcher) .