Creating the Actors

So far we have looked at the definitions of Actors and their messages. Now let’s dive a bit deeper into the power of location transparency and see how to create Actor instances.

The power of location transparency

In Akka you can’t create an instance of an Actor using the new keyword. Instead, you create Actor instances using a factory spawn methods. Spawn does not return an actor instance, but a reference, akka.actor.typed.ActorRef, that points to the actor instance. This level of indirection adds a lot of power and flexibility in a distributed system.

In Akka location doesn’t matter. Location transparency means that the ActorRef can, while retaining the same semantics, represent an instance of the running actor in-process or on a remote machine.
If needed, the runtime can optimize the system by changing an Actor’s location or the entire application topology while it is running. This enables the “let it crash” model of failure management in which the system can heal itself by crashing faulty Actors and restarting healthy ones.

The Akka ActorSystem

An ActorSystem is the intial entry point into Akka. Usually only one ActorSystem is created per application. An ActorSystem has a name and a guardian actor. The bootstrap of your application is typically done within the guardian actor.

The guardian actor of this ActorSystem is GreeterMain.

copyfinal ActorSystem<GreeterMain.SayHello> greeterMain = ActorSystem.create(GreeterMain.create(), "helloakka");

It uses Behaviors.setup to bootstrap the application.

copypackage $package$;

import akka.actor.typed.ActorRef;
import akka.actor.typed.Behavior;
import akka.actor.typed.javadsl.*;

public class GreeterMain extends AbstractBehavior<GreeterMain.SayHello> {

    public static class SayHello {
        public final String name;

        public SayHello(String name) {
            this.name = name;
        }
    }

    private final ActorRef<Greeter.Greet> greeter;

    public static Behavior<SayHello> create() {
        return Behaviors.setup(GreeterMain::new);
    }

    private GreeterMain(ActorContext<SayHello> context) {
        super(context);
        //#create-actors
        greeter = context.spawn(Greeter.create(), "greeter");
        //#create-actors
    }

    @Override
    public Receive<SayHello> createReceive() {
        return newReceiveBuilder().onMessage(SayHello.class, this::onSayHello).build();
    }

    private Behavior<SayHello> onSayHello(SayHello command) {
        //#create-actors
        ActorRef<Greeter.Greeted> replyTo =
                getContext().spawn(GreeterBot.create(3), command.name);
        greeter.tell(new Greeter.Greet(command.name, replyTo));
        //#create-actors
        return this;
    }
}

Spawning child actors

Other actors are created using spawn methods on ActorContext. The GreeterMain creates a Greeter actor this way on startup, as well as a new GreeterBot each time it receives a SayHello message.

copygreeter = context.spawn(Greeter.create(), "greeter");
ActorRef<Greeter.Greeted> replyTo =
        getContext().spawn(GreeterBot.create(3), command.name);
greeter.tell(new Greeter.Greet(command.name, replyTo));