如何修复“Futures 超时 [5 秒]”和 [akkaDeadLetter] 错误？

Question

I'm trying to realize numbersPrinter class (the code:

package org.example.actors;

import akka.actor.AbstractActor;
import akka.actor.Props;
import akka.event.Logging;
import akka.event.LoggingAdapter;
import akka.japi.Procedure;

import java.util.ArrayList;

public class numbersPrinter extends AbstractActor {
    // fields
    LoggingAdapter log = Logging.getLogger(getContext().system(), this);

    // the method
    public void returnNumber(Object message){
        if (message.equals(String.valueOf(1))){
            getSender().tell(1, getSelf());
        } else if (message instanceof String) {
            log.info((String) message);
        } else {
            unhandled(message);
            log.info("Unhandled message");
        }
    }

    @Override
    public Receive createReceive() {
        return receiveBuilder()
                .create()
                .match(numbersPrinter.class, this::returnNumber)
                .build();
    }

}

Main class code fragment:

package org.example;

import akka.actor.ActorRef;
import akka.actor.ActorSystem;
import akka.actor.Props;
import akka.pattern.Patterns;
import akka.util.Timeout;
import org.example.actors.numbersPrinter;
import scala.concurrent.Await;
import scala.concurrent.duration.Duration;

import java.util.ArrayList;
import java.util.Scanner;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;

// actors system
        final ActorSystem system = ActorSystem.create(
                "ActorsSystem");

        // the actor for numbers output
        ActorRef numbersPrinter = system.actorOf(Props.create(numbersPrinter.class, numbersPrinter::new), "numbersPrinter");
        // number 1 output test
        System.out.println("numbersPrinter actor test");
        final Timeout timeout = new Timeout(Duration.create(5, TimeUnit.SECONDS));
        scala.concurrent.Future<Object> rt = Patterns.ask(numbersPrinter, String.valueOf(1), timeout);
        try {
            String result = (String) Await.result(rt, timeout.duration());
            System.out.println(result);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }

The following errors are occuring:

[INFO] [akkaDeadLetter][12/22/2022 20:10:42.028] [ActorsSystem-akka.actor.default-dispatcher-6] [akka://ActorsSystem/user/numbersPrinter] Message [java.lang.String] from Actor[akka://ActorsSystem/temp/numbersPrinter$a] to Actor[akka://ActorsSystem/user/numbersPrinter#-1051654874] was unhandled. [1] dead letters encountered. This logging can be turned off or adjusted with configuration settings 'akka.log-dead-letters' and 'akka.log-dead-letters-during-shutdown'.

Exception in thread "main" java.lang.RuntimeException:
java.util.concurrent.TimeoutException: Future timed out after [5 seconds]
    at org.example.Main.main(Main.java:63)
Caused by: java.util.concurrent.TimeoutException: Future timed out after [5 seconds]
at scala.concurrent.impl.Promise$DefaultPromise.tryAwait0(Promise.scala:248)
at scala.concurrent.impl.Promise$DefaultPromise.result(Promise.scala:261)
at scala.concurrent.Await$.$anonfun$result$1(package.scala:201)
at scala.concurrent.BlockContext$DefaultBlockContext$.blockOn(BlockContext.scala:62)
at scala.concurrent.Await$.result(package.scala:124)
at scala.concurrent.Await.result(package.scala)
at org.example.Main.main(Main.java:60)

Tell me please, how to fix it.

Answer 1

Futures timed out and messages going to DeadLetter are generally indications that something is wrong, but not what is wrong.

The future timing out is an indication that your actor isn't sending a reply to the asker (more properly, that the temporary actor created for the ask pattern hasn't received the reply by the timeout, but especially in the local case with an application where there aren't a lot of actors "chatting away", this is equivalent to the reply not being sent).

The dead letters indicate that your numbersPrinter actor isn't handling the message of type String .

returnNumber certainly appears to handle that message and reply, but the culprit is the Receive that you're building:

receiveBuilder().create().match(numbersPrinter.class, this::returnNumber)

sets up the Receive to only pass messages of type numbersPrinter to returnNumber . Since that's the only match call in that Receive , messages of all other types are ignored and will go to dead letters.

Since your returnNumber method is total for all Object s, it probably makes sense to replace numbersPrinter.class in the match with Object.class ; replacing with String.class would also work.