How to test Akka Actor functionality by mocking one or more methods in it

Question

I\'m interested to know about how to test Akka Actor functionality, by mocking some methods (substitute real object\'s/actor\'s method implementation by moc

Answer 1

I have no experience in using Akka with Java, but I guess the solution for this I use in Scala can also apply to Java. There is no need at all to mock anything. In Java mocking is sometimes useful for testing, but my personal experience/opinion is that whenever you need PowerMock you're doing something wrong.

Here's how I try to test using Akka:

In Scala I use a trait (aka interface) in which the actor methods are defined.

trait ToBeTested {
  def getHelloMessage(msg: String, replyTarget: ActorRef): String = 
      replyTarget ! s"Hello $msg"
}

This way, this functionality can be unit tested very easy. For the real actor I try to stick to implement the receive method only.

class ToBeTestedActor extends Actor with ToBeTested {
  def receive: Receive = {
    case msg: String => getHelloMessage(msg, sender())
  }
}

Then when testing the actor, you can override the getHelloMessage implementation to do whatever you want.

class ToBeTestedActorTest extends TestKit(ActorSystem("toBeTested") with .... {
  trait MyToBeTested extends ToBeTested {
    // do something predictable for testing or defer to a TestProbe which you can
    // either define globally in the test class or provide one in a constructor.
    override def getHelloMessage(msg: String, replyTarget: ActorRef): String = ??? 
  }

  val toBeTestedActor = TestActorRef(Probe(new ToBeTestedActor with MyToBeTested))

  // ... (test cases)
}

In Java you can do pretty much the same thing. Since Java 8 you can provide default method implementations in interfaces, which you can override in a sub-interface for testing. Another way would be to subclass the actor in your test to override some methods to provide predictable behaviour.

// An easy unit testable interface
public interface ToBeTested {

  public ActorRef self();

  default public void getHelloMessage(String msg, ActorRef replyTarget) {
    replyTarget.tell(String.format("Hello %s", msg), self());
  }
}

public class ToBeTestedActor extends UntypedActor implements ToBeTested {

  // self() already implemented by Actor class

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

    if (message instanceof String) {
        getHelloMessage((String)message, getSender());
    }
  }
}

public class ToBeTestedActorTest {

  @Test
  public void test() throws Exception {
    ActorSystem system = ActorSystem.create();

    TestActorRef testActorRef = TestActorRef.create(system, Props.create(TestActor.class));

    Future