Overview

This article demonstrates how to use akka.testkit.TestActorRef in Akka TestKit. After reading this article, you will understand the motivation of using TestActorRef<T>, its usage and its limit.

Motivation

Here is a simple actor MyActor: it contains a state value and has different behaviors based on the message of its mailbox. When receiving an “increment” message, it increments its value; when receiving a “decrement” message, it decrements its value; when receiving a “reply” message, it replies to the sender with the value received.

class MyActor extends AbstractActor {
  final AtomicInteger value = new AtomicInteger(0);

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals("increment", msg -> value.incrementAndGet())
        .matchEquals("decrement", msg -> value.decrementAndGet())
        .matchEquals("reply", msg -> sender().tell(value.get(), self()))
        .build();
  }
}

Testing this with Akka TestKit is not easy. You have to send a message from a test kit and assert the reply. It could be something like:

@Test
public void normalReplyTesting() {
  // Given an actor under test
  Props props = Props.create(MyActor.class);
  TestActorRef<MyActor> myActor = TestActorRef.create(system, props);
  // And a test kit
  TestKit probe = new TestKit(system);

  // When asking for reply
  myActor.tell("reply", probe.getRef());

  // Then the reply is returned
  probe.expectMsgEquals(Duration.ofSeconds(2), 0);
}

It works for message “reply” but it does not work for message “increment” or “decrement” because when those messages are received, myActor will not reply to the sender — only the state of the actor is changed. Also, the test kit cannot initialize the actor with a predefined state. While this is good in production for its strong encapsulation, it makes unit testing hard. That’s why TestActorRef can be a good alternative for unit testing.

Create Actor

The following code snippet demonstrates how to create an actor using actor configuration object Props for the actor under test and an existing actor system:

Props props = Props.create(MyActor.class);
TestActorRef<MyActor> ref = TestActorRef.create(system, props);

There are also overloaded methods available such as:

TestActorRef<T> create(ActorSystem system, Props props);
TestActorRef<T> create(ActorSystem system, Props props, String name);
TestActorRef<T> create(ActorSystem system, Props props, ActorRef supervisor);
TestActorRef<T> create(ActorSystem system, Props props, ActorRef supervisor, String name);

Get Underlying Actor

The underlying actor T can be retrieved from TestActorRef#underlyingActor. Then, you can access to its states (class attributes) and its methods for testing purpose.

MyActor a = ref.underlyingActor();

Note that when using TestActorRef, the messages sent to the actor are process synchronously on the current thread and answers may be sent back as usual. One command use-case is setting up the actor into a specific internal state before sending the test message. Another command use-case is to verify correct internal state transitions after having sent the test message.

@Test
public void decrement() {
  Props props = Props.create(MyActor.class);
  TestActorRef<MyActor> ref = TestActorRef.create(system, props);
  /*
   * Note: one common use case is setting up the actor into a
   * specific internal state before sending the test message.
   */
  ref.underlyingActor().value.set(1);

  /*
   * Note: messages sent to the actor are process synchronously on
   * the current thread and answers may be sent back as usual.
   */
  ref.tell("decrement", ActorRef.noSender());

  /*
   * Note: another is to verify correct internal state transitions
   * after having sent the test message.
   */
  assertEquals(0, ref.underlyingActor().value.get());
}

TestActorRef extends class ActorRef, so you can use the methods defined by ActorRef as in other Akka tests. The limit of this solution is that it cannot test the communication between actors, scheduling, etc. You need to think about asynchronicity again and use TestKit for that.

Conclusion

In this article, I demonstrated how to use TestActorRef to test your actor, including creation and retrieving the underlying reference. The source code is available on GitHub. I also recommend you to read the official Akka documentation: Testing Classic Actors for more information. Interested to know more? You can subscribe to the feed of my blog, follow me on Twitter or GitHub. Hope you enjoy this article, see you the next time!

References