using guice injection with actor throws null point

问题:

I'm getting null pointer exception on the field injection of a server which is started as an akka actor.

Schedular part:

private ActorRef myActor = Akka.system().actorOf(
        new Props(Retreiver.class));

@Override
public void onStart(Application app) {
    log.info("Starting schedular.....!");
    Akka.system()
            .scheduler()
            .schedule(Duration.create(0, TimeUnit.MILLISECONDS),
                    Duration.create(30, TimeUnit.MINUTES), myActor, "tick",
                    Akka.system().dispatcher());

}

Retreiver class part:

public class Retreiver extends UntypedActor {

private Logger.ALogger log = Logger.of(Retreiver .class);

@Inject
private myDataService dataService;

@Override
public void onReceive(Object arg0) throws Exception {

    if (0 != dataService.getDataCount()) {
    ....
    ....
    ....
    }

}

I'm getting null for dataService. Please advice me on this.

Thanks.

回答1:

You're getting the NullPointerException because Akka is instantiating your Retriever actor and not Guice. You need to get Guice to construct your instance and then pass that to Akka, IndirectActorProducer can help you achieve this, e.g.:

class RetrieverDependencyInjector implements IndirectActorProducer {
    final Injector injector;

    public RetrieverDependencyInjector(Injector injector) {
        this.injector = injector;
    }

    @Override
    public Class<? extends Actor> actorClass() {
        return Retriever.class;
    }

    @Override
    public Retriever produce() {
        return injector.getInstance(Retriever.class);
    }
}

Note that produce() must create a new Actor instance each time it is invoked, it cannot return the same instance.

You can then get Akka to retrieve your actor through the RetrieverDependencyInjector, e.g.:

ActorRef myActor = Akka.system().actorOf(
    Props.create(RetrieverDependencyInjector.class, injector)
);

UPDATE

I thought about you comment further, you might be able to turn RetrieverDependencyInjector into a GenericDependencyInjector by providing the class of the Actor you want as a constructor parameter, that perhaps will allow you to do something like:

Props.create(GenericDependencyInjector.class, injector, Retriever.class)

I haven't tried this, but it might give you a starting point.

回答2:

For anyone who needs this:

public class GuiceInjectedActor implements IndirectActorProducer {

final Injector injector;
final Class<? extends Actor> actorClass;

public GuiceInjectedActor(Injector injector, Class<? extends Actor> actorClass) {
    this.injector = injector;
    this.actorClass = actorClass;
}

@Override
public Class<? extends Actor> actorClass() {
    return actorClass;
}

@Override
public Actor produce() {
    return injector.getInstance(actorClass);
}

}

AND

Akka.system().actorOf(Props.create(GuiceInjectedActor.class, INJECTOR,Retreiver.class))

Thats it...!!!

回答3:

There can be other ways, eg, you can put a static injector handle in Boot or Some-Injector-Holder-class, and inject when you create the actor by call the inherited method: preStart ()

public class Retreiver extends UntypedActor {

    private Logger.ALogger log = Logger.of(Retreiver .class);

    @Override
    public void preStart () {
        super.preStart ();
        Boot.injector.injectMembers (this);
        //Some-Injector-holder.injectMembers (this);
    }

    @Inject
    private myDataService dataService;

    @Override
    public void onReceive(Object arg0) throws Exception {

        if (0 != dataService.getDataCount()) {
        ....
        ....
        ....
        }
    }

and also, you can also inject the injector in actor provider to initialize the actor by the injector in a scope of UntypedActorFactory:

public class InjectingActorProvider implements Provider<ActorRef> {
    @Inject
    private Injector injector;

    @SuppressWarnings("serial")
    @Override
    public final ActorRef get() {
            Props props = new Props(new UntypedActorFactory() {
                @Override
                public Actor create() {
                    return injector.getInstance(actorClass);
                }
            });

            props = props.withRouter(...);
            props = props.withDispatcher(...);
            actor = system.actorOf(props, actorName);
            return actor;
    }
}