Testing Actor Systems

Dependency

To use Akka Testkit, you must add the following dependency in your project:

sbt

libraryDependencies += "com.typesafe.akka" %% "akka-testkit" % "2.5.32" % Test

Maven

<dependencies>
  <dependency>
    <groupId>com.typesafe.akka</groupId>
    <artifactId>akka-testkit_2.12</artifactId>
    <version>2.5.32</version>
    <scope>test</scope>
  </dependency>
</dependencies>

Gradle

dependencies {
  testImplementation "com.typesafe.akka:akka-testkit_2.12:2.5.32"
}

Introduction

As with any piece of software, automated tests are a very important part of the development cycle. The actor model presents a different view on how units of code are delimited and how they interact, which has an influence on how to perform tests.

Akka comes with a dedicated module akka-testkit for supporting tests.

Asynchronous Testing: TestKit

Testkit allows you to test your actors in a controlled but realistic environment. The definition of the environment depends very much on the problem at hand and the level at which you intend to test, ranging from simple checks to full system tests.

The minimal setup consists of the test procedure, which provides the desired stimuli, the actor under test, and an actor receiving replies. Bigger systems replace the actor under test with a network of actors, apply stimuli at varying injection points and arrange results to be sent from different emission points, but the basic principle stays the same in that a single procedure drives the test.

The TestKit class contains a collection of tools which makes this common task easy.

Scala

copysourceimport akka.actor.ActorSystem
import akka.testkit.{ ImplicitSender, TestActors, TestKit }
import org.scalatest.{ BeforeAndAfterAll, Matchers, WordSpecLike }

class MySpec()
    extends TestKit(ActorSystem("MySpec"))
    with ImplicitSender
    with WordSpecLike
    with Matchers
    with BeforeAndAfterAll {

  override def afterAll: Unit = {
    TestKit.shutdownActorSystem(system)
  }

  "An Echo actor" must {

    "send back messages unchanged" in {
      val echo = system.actorOf(TestActors.echoActorProps)
      echo ! "hello world"
      expectMsg("hello world")
    }

  }
}

Java

copysourceimport jdocs.AbstractJavaTest;
import akka.testkit.javadsl.TestKit;
import org.junit.AfterClass;
import org.junit.Assert;
import org.junit.BeforeClass;
import org.junit.Test;

import akka.actor.ActorRef;
import akka.actor.ActorSystem;
import akka.actor.Props;
import akka.actor.AbstractActor;

import java.time.Duration;

public class TestKitSampleTest extends AbstractJavaTest {

  public static class SomeActor extends AbstractActor {
    ActorRef target = null;

    @Override
    public Receive createReceive() {
      return receiveBuilder()
          .matchEquals(
              "hello",
              message -> {
                getSender().tell("world", getSelf());
                if (target != null) target.forward(message, getContext());
              })
          .match(
              ActorRef.class,
              actorRef -> {
                target = actorRef;
                getSender().tell("done", getSelf());
              })
          .build();
    }
  }

  static ActorSystem system;

  @BeforeClass
  public static void setup() {
    system = ActorSystem.create();
  }

  @AfterClass
  public static void teardown() {
    TestKit.shutdownActorSystem(system);
    system = null;
  }

  @Test
  public void testIt() {
    /*
     * Wrap the whole test procedure within a testkit constructor
     * if you want to receive actor replies or use Within(), etc.
     */
    new TestKit(system) {
      {
        final Props props = Props.create(SomeActor.class);
        final ActorRef subject = system.actorOf(props);

        // can also use JavaTestKit “from the outside”
        final TestKit probe = new TestKit(system);
        // “inject” the probe by passing it to the test subject
        // like a real resource would be passed in production
        subject.tell(probe.getRef(), getRef());
        // await the correct response
        expectMsg(Duration.ofSeconds(1), "done");

        // the run() method needs to finish within 3 seconds
        within(
            Duration.ofSeconds(3),
            () -> {
              subject.tell("hello", getRef());

              // This is a demo: would normally use expectMsgEquals().
              // Wait time is bounded by 3-second deadline above.
              awaitCond(probe::msgAvailable);

              // response must have been enqueued to us before probe
              expectMsg(Duration.ZERO, "world");
              // check that the probe we injected earlier got the msg
              probe.expectMsg(Duration.ZERO, "hello");
              Assert.assertEquals(getRef(), probe.getLastSender());

              // Will wait for the rest of the 3 seconds
              expectNoMessage();
              return null;
            });
      }
    };
  }
}

The TestKit contains an actor named testActor which is the entry point for messages to be examined with the various expectMsg... assertions detailed below. When mixing in the trait ImplicitSender this test actor is implicitly used as sender reference when dispatching messages from the test procedure. The test actor’s reference is obtained using the getRef() method as demonstrated above. The testActor may also be passed to other actors as usual, usually subscribing it as notification listener. There is a whole set of examination methods, e.g. receiving all consecutive messages matching certain criteria, receiving a whole sequence of fixed messages or classes, receiving nothing for some time, etc.

The ActorSystem passed in to the constructor of TestKit is accessible via the system membergetSystem() method.

Built-In Assertions

The above mentioned expectMsgexpectMsgEquals is not the only method for formulating assertions concerning received messages, the full set is this:

Scala

copysourceval hello: String = expectMsg("hello")
val any: String = expectMsgAnyOf("hello", "world")
val all: immutable.Seq[String] = expectMsgAllOf("hello", "world")
val i: Int = expectMsgType[Int]
expectNoMessage(200.millis)
val two: immutable.Seq[AnyRef] = receiveN(2)

Java

copysourcefinal String hello = expectMsgEquals("hello");
final String any = expectMsgAnyOf("hello", "world");
final List<String> all = expectMsgAllOf("hello", "world");
final int i = expectMsgClass(Integer.class);
final Number j = expectMsgAnyClassOf(Integer.class, Long.class);
expectNoMessage();
final List<Object> two = receiveN(2);

In these examples, the maximum durations you will find mentioned below are left out, in which case they use the default value from configuration item akka.test.single-expect-default which itself defaults to 3 seconds (or they obey the innermost enclosing Within as detailed below). The full signatures are:

• expectMsg[T](d: Duration, msg: T): Tpublic <T> T expectMsgEquals(Duration max, T msg) The given message object must be received within the specified time; the object will be returned.
• expectMsgPF[T](d: Duration)(pf: PartialFunction[Any, T]): Tpublic <T> T expectMsgPF(Duration max, String hint, Function<Object, T> f) Within the given time period, a message must be received and the given partial function must be defined for that message; the result from applying the partial function to the received message is returned. The duration may be left unspecified (empty parentheses are required in this case) to use the deadline from the innermost enclosing within block instead.
• expectMsgClass[T](d: Duration, c: Class[T]): Tpublic <T> T expectMsgClass(Duration max, Class<T> c) An object which is an instance of the given Class must be received within the allotted time frame; the object will be returned. Note that this does a conformance check; if you need the class to be equal, have a look at expectMsgAllClassOf with a single given class argumentyou need to verify that afterwards.

• expectMsgAnyOf[T](d: Duration, obj: T*): Tpublic Object expectMsgAnyOf(Duration max, Object... msg) An object must be received within the given time, and it must be equal ( compared with ==equals()) to at least one of the passed reference objects; the received object will be returned.
• expectMsgAnyClassOf[T](d: Duration, obj: Class[_ <: T]*): Tpublic <T> T expectMsgAnyClassOf(Duration max, Class<? extends T>... c) An object must be received within the given time, and it must be an instance of at least one of the supplied Class objects; the received object will be returned. Note that this does a conformance check, if you need the class to be equal you need to verify that afterwards.
• expectMsgAllOf[T](d: Duration, obj: T*): Seq[T]public List<Object> expectMsgAllOf(Duration max, Object... msg) A number of objects matching the size of the supplied object array must be received within the given time, and for each of the given objects there must exist at least one among the received ones which equals (compared with ==equals()) it. The full sequence of received objects is returned in the order received.

• expectNoMessage(d: Duration)public void expectNoMessage(Duration max) No message must be received within the given time. This also fails if a message has been received before calling this method which has not been removed from the queue using one of the other methods.
• receiveN(n: Int, d: Duration): Seq[AnyRef]List<Object> receiveN(int n, Duration max) n messages must be received within the given time; the received messages are returned.

In addition to message reception assertions there are also methods which help with message flows:

• receiveWhile[T](max: Duration, idle: Duration, messages: Int)(pf: PartialFunction[Any, T]): Seq[T]public <T> List<T> receiveWhile(Duration max, Duration idle, Int messages, Function<Object, T> f) Collect messages as long as

  • they are matching the given partial function
  • the given time interval is not used up
  • the next message is received within the idle timeout
  • the number of messages has not yet reached the maximum All collected messages are returned. The maximum duration defaults to the time remaining in the innermost enclosing within block and the idle duration defaults to infinity (thereby disabling the idle timeout feature). The number of expected messages defaults to Int.MaxValue, which effectively disables this limit.

• awaitCond(p: => Boolean, max: Duration, interval: Duration)public void awaitCond(Duration max, Duration interval, Supplier<Boolean> p) Poll the given condition every interval until it returns true or the max duration is used up. The interval defaults to 100 ms and the maximum defaults to the time remaining in the innermost enclosing within block.

• awaitAssert(a: => Any, max: Duration, interval: Duration)public void awaitAssert(Duration max, Duration interval, Supplier<Object> a) Poll the given assert function every interval until it does not throw an exception or the max duration is used up. If the timeout expires the last exception is thrown. The interval defaults to 100 ms and the maximum defaults to the time remaining in the innermost enclosing within block. The interval defaults to 100 ms and the maximum defaults to the time remaining in the innermost enclosing within block. Return an arbitrary value that would be returned from awaitAssert if successful, if not interested in such value you can return null.

• ignoreMsg(pf: PartialFunction[AnyRef, Boolean])public void ignoreMsg(Function<Object, Boolean> f) ignoreMsgpublic void ignoreMsg() There are also cases where not all messages sent to the test kit are actually relevant to the test, but removing them would mean altering the actors under test. For this purpose it is possible to ignore certain messages. The internal testActor contains a partial function for ignoring messages: it will only enqueue messages which do not match the function or for which the function returns false. This function can be set and reset using the methods given above; each invocation replaces the previous function, they are not composed. This feature is useful e.g. when testing a logging system, where you want to ignore regular messages and are only interested in your specific ones.

Expecting Log Messages

Since an integration test does not allow to the internal processing of the participating actors, verifying expected exceptions cannot be done directly. Instead, use the logging system for this purpose: replacing the normal event handler with the TestEventListener and using an EventFilter allows assertions on log messages, including those which are generated by exceptions:

Scala

copysourceimport akka.testkit.EventFilter
import com.typesafe.config.ConfigFactory

implicit val system = ActorSystem(
  "testsystem",
  ConfigFactory.parseString("""
  akka.loggers = ["akka.testkit.TestEventListener"]
  """))
try {
  val actor = system.actorOf(Props.empty)
  EventFilter[ActorKilledException](occurrences = 1).intercept {
    actor ! Kill
  }
} finally {
  shutdown(system)
}

Java

copysourcenew TestKit(system) {
  {
    assertEquals("TestKitDocTest", system.name());
    final ActorRef victim = system.actorOf(Props.empty(), "victim");

    final int result =
        new EventFilter(ActorKilledException.class, system)
            .from("akka://TestKitDocTest/user/victim")
            .occurrences(1)
            .intercept(
                () -> {
                  victim.tell(Kill.getInstance(), ActorRef.noSender());
                  return 42;
                });
    assertEquals(42, result);
  }
};

If a number of occurrences is specific—as demonstrated above—then intercept will block until that number of matching messages have been received or the timeout configured in akka.test.filter-leeway is used up (time starts counting after the passed-in block of code returns). In case of a timeout the test fails.

akka.loggers = [akka.testkit.TestEventListener]

Overriding behavior

Sometimes you want to ‘hook into’ your actor to be able to test some internals. Usually it is better to test an actors’ external interface, but for example if you want to test timing-sensitive behavior this can come in handy. Say for instance you want to test an actor that schedules a task:

Scala

copysourcecase class TriggerScheduling(foo: String)

object SchedKey
case class ScheduledMessage(foo: String)

class TestTimerActor extends Actor with Timers {
  override def receive = {
    case TriggerScheduling(foo) => triggerScheduling(ScheduledMessage(foo))
  }

  def triggerScheduling(msg: ScheduledMessage) =
    timers.startSingleTimer(SchedKey, msg, 500.millis)
}

Java

copysourcestatic class TestTimerActor extends AbstractActorWithTimers {
  private static Object SCHED_KEY = "SchedKey";

  static final class TriggerScheduling {}

  static final class ScheduledMessage {}

  @Override
  public Receive createReceive() {
    return receiveBuilder().match(TriggerScheduling.class, msg -> triggerScheduling()).build();
  }

  void triggerScheduling() {
    getTimers().startSingleTimer(SCHED_KEY, new ScheduledMessage(), Duration.ofMillis(500));
  }
}

You can override the method that does the scheduling in your test:

Scala

copysourceimport akka.testkit.TestProbe
import akka.actor.Props

val probe = TestProbe()
val actor = system.actorOf(Props(new TestTimerActor() {
  override def triggerScheduling(msg: ScheduledMessage) =
    probe.ref ! msg
}))

actor ! TriggerScheduling("abc")
probe.expectMsg(ScheduledMessage("abc"))

Java

copysourcenew TestKit(system) {
  {
    final TestKit probe = new TestKit(system);
    final ActorRef target =
        system.actorOf(
            Props.create(
                TestTimerActor.class,
                () ->
                    new TestTimerActor() {
                      @Override
                      void triggerScheduling() {
                        probe.getRef().tell(new ScheduledMessage(), getSelf());
                      }
                    }));
    target.tell(new TestTimerActor.TriggerScheduling(), ActorRef.noSender());
    probe.expectMsgClass(TestTimerActor.ScheduledMessage.class);
  }
};

Timing Assertions

Another important part of functional testing concerns timing: certain events must not happen immediately (like a timer), others need to happen before a deadline. Therefore, all examination methods accept an upper time limit within the positive or negative result must be obtained. Lower time limits need to be checked external to the examination, which is facilitated by a new construct for managing time constraints:

Scala

copysourceimport akka.actor.Props
import scala.concurrent.duration._

val worker = system.actorOf(Props[Worker])
within(200 millis) {
  worker ! "some work"
  expectMsg("some result")
  expectNoMessage // will block for the rest of the 200ms
  Thread.sleep(300) // will NOT make this block fail
}

Java

copysourcenew TestKit(system) {
  {
    getRef().tell(42, ActorRef.noSender());
    within(
        Duration.ZERO,
        Duration.ofSeconds(1),
        () -> {
          assertEquals((Integer) 42, expectMsgClass(Integer.class));
          return null;
        });
  }
};

The block given toin within must complete after a Duration which is between min and max, where the former defaults to zero. The deadline calculated by adding the max parameter to the block’s start time is implicitly available within the block to all examination methods, if you do not specify it, it is inherited from the innermost enclosing within block.

It should be noted that if the last message-receiving assertion of the block is expectNoMessage or receiveWhile, the final check of the within is skipped in order to avoid false positives due to wake-up latencies. This means that while individual contained assertions still use the maximum time bound, the overall block may take arbitrarily longer in this case.

Accounting for Slow Test Systems

The tight timeouts you use during testing on your lightning-fast notebook will invariably lead to spurious test failures on the heavily loaded Jenkins server (or similar). To account for this situation, all maximum durations are internally scaled by a factor taken from the Configuration, akka.test.timefactor, which defaults to 1.

You can scale other durations with the same factor by using the implicit conversion in akka.testkit package object to add dilated function to Durationdilated method in TestKit.

Scala

copysourceimport scala.concurrent.duration._
import akka.testkit._
10.milliseconds.dilated

Java

copysourcenew TestKit(system) {
  {
    final Duration original = Duration.ofSeconds(1);
    final Duration stretched = dilated(original);
    assertTrue("dilated", stretched.compareTo(original) >= 0);
  }
};

copysourceclass MySpec()
    extends TestKit(ActorSystem("MySpec"))
    with ImplicitSender
    with WordSpecLike
    with Matchers
    with BeforeAndAfterAll {

Using Multiple Probe Actors

When the actors under test are supposed to send various messages to different destinations, it may be difficult distinguishing the message streams arriving at the testActor when using the TestKit as a mixinshown until now. Another approach is to use it for creation of simple probe actors to be inserted in the message flows. To make this more powerful and convenient, there is a concrete implementation called TestProbe. The functionality is best explained using a small example:

Scala

copysourceimport scala.concurrent.duration._
import akka.actor._
import akka.testkit.TestProbe

copysourceclass MyDoubleEcho extends Actor {
  var dest1: ActorRef = _
  var dest2: ActorRef = _
  def receive = {
    case (d1: ActorRef, d2: ActorRef) =>
      dest1 = d1
      dest2 = d2
    case x =>
      dest1 ! x
      dest2 ! x
  }
}

copysourceval probe1 = TestProbe()
val probe2 = TestProbe()
val actor = system.actorOf(Props[MyDoubleEcho])
actor ! ((probe1.ref, probe2.ref))
actor ! "hello"
probe1.expectMsg(500 millis, "hello")
probe2.expectMsg(500 millis, "hello")

Java

copysourcenew TestKit(system) {
  {
    // simple actor which only forwards messages
    class Forwarder extends AbstractActor {
      final ActorRef target;

      @SuppressWarnings("unused")
      public Forwarder(ActorRef target) {
        this.target = target;
      }

      @Override
      public Receive createReceive() {
        return receiveBuilder()
            .matchAny(message -> target.forward(message, getContext()))
            .build();
      }
    }

    // create a test probe
    final TestKit probe = new TestKit(system);

    // create a forwarder, injecting the probe’s testActor
    final Props props = Props.create(Forwarder.class, this, probe.getRef());
    final ActorRef forwarder = system.actorOf(props, "forwarder");

    // verify correct forwarding
    forwarder.tell(42, getRef());
    probe.expectMsgEquals(42);
    assertEquals(getRef(), probe.getLastSender());
  }
};

Here the system under test is simulated by MyDoubleEcho, which is supposed to mirror its input to two outputs. Attaching two test probes enables verification of the (simplistic) behaviorThis simple test verifies an equally simple Forwarder actor by injecting a probe as the forwarder’s target. Another example would be two actors A and B which collaborate by A sending messages to B. In order to verify this message flow, a TestProbe could be inserted as target of A, using the forwarding capabilities or auto-pilot described below to include a real B in the test setup.

If you have many test probes, you can name them to get meaningful actor names in test logs and assertions:

Scala

copysourceval worker = TestProbe("worker")
val aggregator = TestProbe("aggregator")

worker.ref.path.name should startWith("worker")
aggregator.ref.path.name should startWith("aggregator")

Java

copysourcenew TestKit(system) {
  {
    final TestProbe worker = new TestProbe(system, "worker");
    final TestProbe aggregator = new TestProbe(system, "aggregator");

    assertTrue(worker.ref().path().name().startsWith("worker"));
    assertTrue(aggregator.ref().path().name().startsWith("aggregator"));
  }
};

Probes may also be equipped with custom assertions to make your test code even more concise and clear:

Scala

copysourcefinal case class Update(id: Int, value: String)

val probe = new TestProbe(system) {
  def expectUpdate(x: Int) = {
    expectMsgPF() {
      case Update(id, _) if id == x => ()
    }
    sender() ! "ACK"
  }
}

Java

copysourcenew TestKit(system) {
  {
    class MyProbe extends TestKit {
      public MyProbe() {
        super(system);
      }

      public void assertHello() {
        expectMsgEquals("hello");
      }
    }

    final MyProbe probe = new MyProbe();
    probe.getRef().tell("hello", ActorRef.noSender());
    probe.assertHello();
  }
};

You have complete flexibility here in mixing and matching the TestKit facilities with your own checks and choosing an intuitive name for it. In real life your code will probably be a bit more complicated than the example given above; just use the power!

Watching Other Actors from Probes

A TestProbeTestKit can register itself for DeathWatch of any other actor:

Scala

copysourceval probe = TestProbe()
probe.watch(target)
target ! PoisonPill
probe.expectTerminated(target)

Java

copysourcenew TestKit(system) {
  {
    final TestKit probe = new TestKit(system);
    probe.watch(target);
    target.tell(PoisonPill.getInstance(), ActorRef.noSender());
    final Terminated msg = probe.expectMsgClass(Terminated.class);
    assertEquals(msg.getActor(), target);
  }
};

Replying to Messages Received by Probes

The probes keep track of the communications channel for replies, if possible, so they can also replyThe probe stores the sender of the last dequeued message (i.e. after its expectMsg* reception), which may be retrieved using the getLastSender() method. This information can also implicitly be used for having the probe reply to the last received message:

Scala

copysourceval probe = TestProbe()
val future = probe.ref ? "hello"
probe.expectMsg(0 millis, "hello") // TestActor runs on CallingThreadDispatcher
probe.reply("world")
assert(future.isCompleted && future.value.contains(Success("world")))

Java

copysourcenew TestKit(system) {
  {
    final TestKit probe = new TestKit(system);
    probe.getRef().tell("hello", getRef());
    probe.expectMsgEquals("hello");
    probe.reply("world");
    expectMsgEquals("world");
    assertEquals(probe.getRef(), getLastSender());
  }
};

Forwarding Messages Received by Probes

Given a destination actor dest which in the nominal actor network would receive a message from actor source. If you arrange for the message to be sent to a TestProbe probe instead, you can make assertions concerning volume and timing of the message flow while still keeping the network functioningThe probe can also forward a received message (i.e. after its expectMsg* reception), retaining the original sender:

Scala

copysourceclass Source(target: ActorRef) extends Actor {
  def receive = {
    case "start" => target ! "work"
  }
}

class Destination extends Actor {
  def receive = {
    case x => // Do something..
  }
}

copysourceval probe = TestProbe()
val source = system.actorOf(Props(classOf[Source], probe.ref))
val dest = system.actorOf(Props[Destination])
source ! "start"
probe.expectMsg("work")
probe.forward(dest)

Java

copysourcenew TestKit(system) {
  {
    final TestKit probe = new TestKit(system);
    probe.getRef().tell("hello", getRef());
    probe.expectMsgEquals("hello");
    probe.forward(getRef());
    expectMsgEquals("hello");
    assertEquals(getRef(), getLastSender());
  }
};

The dest actor will receive the same message invocation as if no test probe had intervened.

Auto-Pilot

Receiving messages in a queue for later inspection is nice, but in order to keep a test running and verify traces later you can also install an AutoPilot in the participating test probes (actually in any TestKit) which is invoked before enqueueing to the inspection queue. This code can be used to forward messages, e.g. in a chain A --> Probe --> B, as long as a certain protocol is obeyed.

Scala

copysourceval probe = TestProbe()
probe.setAutoPilot(new TestActor.AutoPilot {
  def run(sender: ActorRef, msg: Any): TestActor.AutoPilot =
    msg match {
      case "stop" => TestActor.NoAutoPilot
      case x      => testActor.tell(x, sender); TestActor.KeepRunning
    }
})

Java

copysourcenew TestKit(system) {
  {
    final TestKit probe = new TestKit(system);
    // install auto-pilot
    probe.setAutoPilot(
        new TestActor.AutoPilot() {
          public AutoPilot run(ActorRef sender, Object msg) {
            sender.tell(msg, ActorRef.noSender());
            return noAutoPilot();
          }
        });
    // first one is replied to directly ...
    probe.getRef().tell("hello", getRef());
    expectMsgEquals("hello");
    // ... but then the auto-pilot switched itself off
    probe.getRef().tell("world", getRef());
    expectNoMessage();
  }
};

The run method must return the auto-pilot for the next message, which may be KeepRunning to retain the current one or NoAutoPilot to switch it offwrapped in an Option; setting it to None terminates the auto-pilot.

Caution about Timing Assertions

The behavior of within blocks when using test probes might be perceived as counter-intuitive: you need to remember that the nicely scoped deadline as described above is local to each probe. Hence, probes do not react to each other’s deadlines or to the deadline set in an enclosing TestKit instance:

Scala

copysourceval probe = TestProbe()
within(1 second) {
  probe.expectMsg("hello")
}

Java

copysourcenew TestKit(system) {
  {
    final TestKit probe = new TestKit(system);
    within(Duration.ofSeconds(1), () -> probe.expectMsgEquals("hello"));
  }
};

Here, the expectMsgexpectMsgEquals call will use the default timeout.

Testing parent-child relationships

The parent of an actor is always the actor that created it. At times this leads to a coupling between the two that may not be straightforward to test. There are several approaches to improve testability of a child actor that needs to refer to its parent:

1. when creating a child, pass an explicit reference to its parent
2. create the child with a TestProbe as parent
3. create a fabricated parent when testing

Conversely, a parent’s binding to its child can be lessened as follows:

1. when creating a parent, tell the parent how to create its child

For example, the structure of the code you want to test may follow this pattern:

Scala

copysourceclass Parent extends Actor {
  val child = context.actorOf(Props[Child], "child")
  var ponged = false

  def receive = {
    case "pingit" => child ! "ping"
    case "pong"   => ponged = true
  }
}

class Child extends Actor {
  def receive = {
    case "ping" => context.parent ! "pong"
  }
}

Java

copysourcestatic class Parent extends AbstractActor {
  final ActorRef child = getContext().actorOf(Props.create(Child.class), "child");
  boolean ponged = false;

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals("pingit", message -> child.tell("ping", getSelf()))
        .matchEquals("pong", message -> ponged = true)
        .build();
  }
}

static class Child extends AbstractActor {
  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals(
            "ping",
            message -> {
              getContext().getParent().tell("pong", getSelf());
            })
        .build();
  }
}

Introduce child to its parent

The first option is to avoid use of the context.parent function and create a child with a custom parent by passing an explicit reference to its parent instead.

Scala

copysourceclass DependentChild(parent: ActorRef) extends Actor {
  def receive = {
    case "ping" => parent ! "pong"
  }
}

Java

copysourceclass DependentChild extends AbstractActor {
  private final ActorRef parent;

  public DependentChild(ActorRef parent) {
    this.parent = parent;
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals("ping", message -> parent.tell("pong", getSelf()))
        .build();
  }
}

Create the child using TestProbeTestKit

The TestProbeTestKit class can in fact create actors that will run with the test probe as parent. This will cause any messages the child actor sends to context.parentgetContext().getParent() to end up in the test probe.

Scala

copysource"A TestProbe serving as parent" should {
  "test its child responses" in {
    val parent = TestProbe()
    val child = parent.childActorOf(Props(new Child))
    parent.send(child, "ping")
    parent.expectMsg("pong")
  }
}

Java

copysourceTestKit parent = new TestKit(system);
ActorRef child = parent.childActorOf(Props.create(Child.class));

parent.send(child, "ping");
parent.expectMsgEquals("pong");

Using a fabricated parent

If you prefer to avoid modifying the parent or child constructor you can create a fabricated parent in your test. This, however, does not enable you to test the parent actor in isolation.

Scala

copysource"A fabricated parent" should {
  "test its child responses" in {
    val proxy = TestProbe()
    val parent = system.actorOf(Props(new Actor {
      val child = context.actorOf(Props(new Child), "child")
      def receive = {
        case x if sender == child => proxy.ref.forward(x)
        case x                    => child.forward(x)
      }
    }))

    proxy.send(parent, "ping")
    proxy.expectMsg("pong")
  }
}

Java

copysourceclass FabricatedParentCreator implements Creator<Actor> {
  private final TestProbe proxy;

  public FabricatedParentCreator(TestProbe proxy) {
    this.proxy = proxy;
  }

  @Override
  public Actor create() throws Exception {
    return new AbstractActor() {
      final ActorRef child = getContext().actorOf(Props.create(Child.class), "child");

      @Override
      public Receive createReceive() {
        return receiveBuilder()
            .matchAny(
                message -> {
                  if (getSender().equals(child)) {
                    proxy.ref().forward(message, getContext());
                  } else {
                    child.forward(message, getContext());
                  }
                })
            .build();
      }
    };
  }
}

copysourceTestProbe proxy = new TestProbe(system);
ActorRef parent = system.actorOf(Props.create(Actor.class, new FabricatedParentCreator(proxy)));

proxy.send(parent, "ping");
proxy.expectMsg("pong");

Externalize child making from the parent

Alternatively, you can tell the parent how to create its child. There are two ways to do this: by giving it a Props object or by giving it a function which takes care of creating the child actor:

Scala

copysourceclass DependentParent(childProps: Props, probe: ActorRef) extends Actor {
  val child = context.actorOf(childProps, "child")

  def receive = {
    case "pingit" => child ! "ping"
    case "pong"   => probe ! "ponged"
  }
}

class GenericDependentParent(childMaker: ActorRefFactory => ActorRef) extends Actor {
  val child = childMaker(context)
  var ponged = false

  def receive = {
    case "pingit" => child ! "ping"
    case "pong"   => ponged = true
  }
}

Java

copysourceclass DependentParent extends AbstractActor {
  final ActorRef child;
  final ActorRef probe;

  public DependentParent(Props childProps, ActorRef probe) {
    child = getContext().actorOf(childProps, "child");
    this.probe = probe;
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals("pingit", message -> child.tell("ping", getSelf()))
        .matchEquals("pong", message -> probe.tell("ponged", getSelf()))
        .build();
  }
}

copysourceclass GenericDependentParent extends AbstractActor {
  final ActorRef child;
  boolean ponged = false;

  public GenericDependentParent(Function<ActorRefFactory, ActorRef> childMaker) throws Exception {
    child = childMaker.apply(getContext());
  }

  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals("pingit", message -> child.tell("ping", getSelf()))
        .matchEquals("pong", message -> ponged = true)
        .build();
  }
}

Creating the PropsActor is straightforward and the function may look like this in your test code:

Scala

copysourceval maker = (_: ActorRefFactory) => probe.ref
val parent = system.actorOf(Props(new GenericDependentParent(maker)))

Java

copysourceFunction<ActorRefFactory, ActorRef> maker = param -> probe.ref();
ActorRef parent = system.actorOf(Props.create(GenericDependentParent.class, maker));

And like this in your application code:

Scala

copysourceval maker = (f: ActorRefFactory) => f.actorOf(Props(new Child))
val parent = system.actorOf(Props(new GenericDependentParent(maker)))

Java

copysourceFunction<ActorRefFactory, ActorRef> maker = f -> f.actorOf(Props.create(Child.class));
ActorRef parent = system.actorOf(Props.create(GenericDependentParent.class, maker));

Which of these methods is the best depends on what is most important to test. The most generic option is to create the parent actor by passing it a function that is responsible for the Actor creation, but theusing TestProbe or having a fabricated parent is often sufficient.

CallingThreadDispatcher

The CallingThreadDispatcher serves good purposes in unit testing, as described above, but originally it was conceived in order to allow contiguous stack traces to be generated in case of an error. As this special dispatcher runs everything which would normally be queued directly on the current thread, the full history of a message’s processing chain is recorded on the call stack, so long as all intervening actors run on this dispatcher.

How to use it

Just set the dispatcher as you normally would:

Scala

copysourceimport akka.testkit.CallingThreadDispatcher
val ref = system.actorOf(Props[MyActor].withDispatcher(CallingThreadDispatcher.Id))

Java

copysourcesystem.actorOf(Props.create(MyActor.class).withDispatcher(CallingThreadDispatcher.Id()));

How it works

When receiving an invocation, the CallingThreadDispatcher checks whether the receiving actor is already active on the current thread. The simplest example for this situation is an actor which sends a message to itself. In this case, processing cannot continue immediately as that would violate the actor model, so the invocation is queued and will be processed when the active invocation on that actor finishes its processing; thus, it will be processed on the calling thread, but after the actor finishes its previous work. In the other case, the invocation is processed immediately on the current thread. Futures scheduled via this dispatcher are also executed immediately.

This scheme makes the CallingThreadDispatcher work like a general purpose dispatcher for any actors which never block on external events.

In the presence of multiple threads it may happen that two invocations of an actor running on this dispatcher happen on two different threads at the same time. In this case, both will be processed directly on their respective threads, where both compete for the actor’s lock and the loser has to wait. Thus, the actor model is left intact, but the price is loss of concurrency due to limited scheduling. In a sense this is equivalent to traditional mutex style concurrency.

The other remaining difficulty is correct handling of suspend and resume: when an actor is suspended, subsequent invocations will be queued in thread-local queues (the same ones used for queuing in the normal case). The call to resume, however, is done by one specific thread, and all other threads in the system will probably not be executing this specific actor, which leads to the problem that the thread-local queues cannot be emptied by their native threads. Hence, the thread calling resume will collect all currently queued invocations from all threads into its own queue and process them.

Limitations

If an actor’s behavior blocks on a something which would normally be affected by the calling actor after having sent the message, this will obviously dead-lock when using this dispatcher. This is a common scenario in actor tests based on CountDownLatch for synchronization:

val latch = new CountDownLatch(1)
actor ! startWorkAfter(latch)   // actor will call latch.await() before proceeding
doSomeSetupStuff()
latch.countDown()

The example would hang indefinitely within the message processing initiated on the second line and never reach the fourth line, which would unblock it on a normal dispatcher.

Thus, keep in mind that the CallingThreadDispatcher is not a general-purpose replacement for the normal dispatchers. On the other hand it may be quite useful to run your actor network on it for testing, because if it runs without dead-locking chances are very high that it will not dead-lock in production.

Thread Interruptions

If the CallingThreadDispatcher sees that the current thread has its isInterrupted() flag set when message processing returns, it will throw an InterruptedException after finishing all its processing (i.e. all messages which need processing as described above are processed before this happens). As tell cannot throw exceptions due to its contract, this exception will then be caught and logged, and the thread’s interrupted status will be set again.

If during message processing an InterruptedException is thrown then it will be caught inside the CallingThreadDispatcher’s message handling loop, the thread’s interrupted flag will be set and processing continues normally.

Benefits

To summarize, these are the features with the CallingThreadDispatcher has to offer:

• Deterministic execution of single-threaded tests while retaining nearly full actor semantics
• Full message processing history leading up to the point of failure in exception stack traces
• Exclusion of certain classes of dead-lock scenarios

Tracing Actor Invocations

The testing facilities described up to this point were aiming at formulating assertions about a system’s behavior. If a test fails, it is usually your job to find the cause, fix it and verify the test again. This process is supported by debuggers as well as logging, where the Akka toolkit offers the following options:

• Logging of exceptions thrown within Actor instances This is always on; in contrast to the other logging mechanisms, this logs at ERROR level.

copysourceimport akka.event.LoggingReceive
def receive = LoggingReceive {
  case msg => // Do something ...
}
def otherState: Receive = LoggingReceive.withLabel("other") {
  case msg => // Do something else ...
}

• Logging of special messages Actors handle certain special messages automatically, e.g. Kill, PoisonPill, etc. Tracing of these message invocations is enabled by the setting akka.actor.debug.autoreceive, which enables this on all actors.
• Logging of the actor lifecycle Actor creation, start, restart, monitor start, monitor stop and stop may be traced by enabling the setting akka.actor.debug.lifecycle; this, too, is enabled uniformly on all actors.

All these messages are logged at DEBUG level. To summarize, you can enable full logging of actor activities using this configuration fragment:

akka {
  loglevel = "DEBUG"
  actor {
    debug {
      receive = on
      autoreceive = on
      lifecycle = on
    }
  }
}

copysourceimport akka.testkit.TestKitBase

class MyTest extends TestKitBase {
  implicit lazy val system = ActorSystem()

  val probe = TestProbe()
  probe.send(testActor, "hello")
  try expectMsg("hello")
  catch { case NonFatal(e) => system.terminate(); throw e }

  shutdown(system)
}

Configuration

There are several configuration properties for the TestKit module, please refer to the reference configuration.

copysourceimport scala.util.Random

import org.scalatest.BeforeAndAfterAll
import org.scalatest.WordSpecLike
import org.scalatest.Matchers

import com.typesafe.config.ConfigFactory

import akka.actor.Actor
import akka.actor.ActorRef
import akka.actor.ActorSystem
import akka.actor.Props
import akka.testkit.{ DefaultTimeout, ImplicitSender, TestActors, TestKit }
import scala.concurrent.duration._
import scala.collection.immutable

/**
 * a Test to show some TestKit examples
 */
class TestKitUsageSpec
    extends TestKit(ActorSystem("TestKitUsageSpec", ConfigFactory.parseString(TestKitUsageSpec.config)))
    with DefaultTimeout
    with ImplicitSender
    with WordSpecLike
    with Matchers
    with BeforeAndAfterAll {
  import TestKitUsageSpec._

  val echoRef = system.actorOf(TestActors.echoActorProps)
  val forwardRef = system.actorOf(Props(classOf[ForwardingActor], testActor))
  val filterRef = system.actorOf(Props(classOf[FilteringActor], testActor))
  val randomHead = Random.nextInt(6)
  val randomTail = Random.nextInt(10)
  val headList = immutable.Seq().padTo(randomHead, "0")
  val tailList = immutable.Seq().padTo(randomTail, "1")
  val seqRef =
    system.actorOf(Props(classOf[SequencingActor], testActor, headList, tailList))

  override def afterAll: Unit = {
    shutdown()
  }

  "An EchoActor" should {
    "Respond with the same message it receives" in {
      within(500 millis) {
        echoRef ! "test"
        expectMsg("test")
      }
    }
  }
  "A ForwardingActor" should {
    "Forward a message it receives" in {
      within(500 millis) {
        forwardRef ! "test"
        expectMsg("test")
      }
    }
  }
  "A FilteringActor" should {
    "Filter all messages, except expected messagetypes it receives" in {
      var messages = Seq[String]()
      within(500 millis) {
        filterRef ! "test"
        expectMsg("test")
        filterRef ! 1
        expectNoMsg
        filterRef ! "some"
        filterRef ! "more"
        filterRef ! 1
        filterRef ! "text"
        filterRef ! 1

        receiveWhile(500 millis) {
          case msg: String => messages = msg +: messages
        }
      }
      messages.length should be(3)
      messages.reverse should be(Seq("some", "more", "text"))
    }
  }
  "A SequencingActor" should {
    "receive an interesting message at some point " in {
      within(500 millis) {
        ignoreMsg {
          case msg: String => msg != "something"
        }
        seqRef ! "something"
        expectMsg("something")
        ignoreMsg {
          case msg: String => msg == "1"
        }
        expectNoMsg
        ignoreNoMsg
      }
    }
  }
}

object TestKitUsageSpec {
  // Define your test specific configuration here
  val config = """
    akka {
      loglevel = "WARNING"
    }
    """

  /**
   * An Actor that forwards every message to a next Actor
   */
  class ForwardingActor(next: ActorRef) extends Actor {
    def receive = {
      case msg => next ! msg
    }
  }

  /**
   * An Actor that only forwards certain messages to a next Actor
   */
  class FilteringActor(next: ActorRef) extends Actor {
    def receive = {
      case msg: String => next ! msg
      case _           => None
    }
  }

  /**
   * An actor that sends a sequence of messages with a random head list, an
   * interesting value and a random tail list. The idea is that you would
   * like to test that the interesting value is received and that you cant
   * be bothered with the rest
   */
  class SequencingActor(next: ActorRef, head: immutable.Seq[String], tail: immutable.Seq[String]) extends Actor {
    def receive = {
      case msg => {
        head.foreach { next ! _ }
        next ! msg
        tail.foreach { next ! _ }
      }
    }
  }
}

Synchronous Testing: TestActorRef

Testing the business logic inside Actor classes can be divided into two parts: first, each atomic operation must work in isolation, then sequences of incoming events must be processed correctly, even in the presence of some possible variability in the ordering of events. The former is the primary use case for single-threaded unit testing, while the latter can only be verified in integration tests.

Normally, the ActorRef shields the underlying Actor instance from the outside, the only communications channel is the actor’s mailbox. This restriction is an impediment to unit testing, which led to the inception of the TestActorRef. This special type of reference is designed specifically for test purposes and allows access to the actor in two ways: either by obtaining a reference to the underlying actor instance, or by invoking or querying the actor’s behavior (receive). Each one warrants its own section below.

Obtaining a Reference to an Actor

Having access to the actual Actor object allows application of all traditional unit testing techniques on the contained methods. Obtaining a reference is done like this:

Scala

copysourceimport akka.testkit.TestActorRef

val actorRef = TestActorRef[MyActor]
val actor = actorRef.underlyingActor

Java

copysourcestatic class MyActor extends AbstractActor {
  @Override
  public Receive createReceive() {
    return receiveBuilder()
        .matchEquals(
            "say42",
            message -> {
              getSender().tell(42, getSelf());
            })
        .match(
            Exception.class,
            (Exception ex) -> {
              throw ex;
            })
        .build();
  }

  public boolean testMe() {
    return true;
  }
}

@Test
public void demonstrateTestActorRef() {
  final Props props = Props.create(MyActor.class);
  final TestActorRef<MyActor> ref = TestActorRef.create(system, props, "testA");
  final MyActor actor = ref.underlyingActor();
  assertTrue(actor.testMe());
}

Since TestActorRef is generic in the actor type it returns the underlying actor with its proper static type. From this point on you may bring any unit testing tool to bear on your actor as usual.

copysourceimport akka.testkit.TestFSMRef
import scala.concurrent.duration._

val fsm = TestFSMRef(new TestFsmActor)

val mustBeTypedProperly: TestActorRef[TestFsmActor] = fsm

assert(fsm.stateName == 1)
assert(fsm.stateData == "")
fsm ! "go" // being a TestActorRef, this runs also on the CallingThreadDispatcher
assert(fsm.stateName == 2)
assert(fsm.stateData == "go")

fsm.setState(stateName = 1)
assert(fsm.stateName == 1)

assert(fsm.isTimerActive("test") == false)
fsm.setTimer("test", 12, 10 millis, true)
assert(fsm.isTimerActive("test") == true)
fsm.cancelTimer("test")
assert(fsm.isTimerActive("test") == false)

Testing the Actor’s Behavior

When the dispatcher invokes the processing behavior of an actor on a message, it actually calls apply on the current behavior registered for the actor. This starts out with the return value of the declared receive method, but it may also be changed using become and unbecome in response to external messages. All of this contributes to the overall actor behavior and it does not lend itself to easy testing on the Actor itself. Therefore the TestActorRef offers a different mode of operation to complement the Actor testing: it supports all operations also valid on normal ActorRef. Messages sent to the actor are processed synchronously on the current thread and answers may be sent back as usual. This trick is made possible by the CallingThreadDispatcher described below (see CallingThreadDispatcher); this dispatcher is set implicitly for any actor instantiated into a TestActorRef.

Scala

copysourceimport akka.testkit.TestActorRef
import akka.pattern.ask

val actorRef = TestActorRef(new MyActor)
// hypothetical message stimulating a '42' answer
val future = actorRef ? Say42
val Success(result: Int) = future.value.get
result should be(42)

Java

copysourcefinal Props props = Props.create(MyActor.class);
final TestActorRef<MyActor> ref = TestActorRef.create(system, props, "testB");
final CompletableFuture<Object> future =
    Patterns.ask(ref, "say42", Duration.ofMillis(3000)).toCompletableFuture();
assertTrue(future.isDone());
assertEquals(42, future.get());

As the TestActorRef is a subclass of LocalActorRef with a few special extras, also aspects like supervision and restarting work properly, but beware that execution is only strictly synchronous as long as all actors involved use the CallingThreadDispatcher. As soon as you add elements which include more sophisticated scheduling you leave the realm of unit testing as you then need to think about asynchronicity again (in most cases the problem will be to wait until the desired effect had a chance to happen).

One more special aspect which is overridden for single-threaded tests is the receiveTimeout, as including that would entail asynchronous queuing of ReceiveTimeout messages, violating the synchronous contract.

The Way In-Between: Expecting Exceptions

If you want to test the actor behavior, including hotswapping, but without involving a dispatcher and without having the TestActorRef swallow any thrown exceptions, then there is another mode available for you: use the receive method on TestActorRef, which will be forwarded to the underlying actor:

Scala

copysourceimport akka.testkit.TestActorRef

val actorRef = TestActorRef(new Actor {
  def receive = {
    case "hello" => throw new IllegalArgumentException("boom")
  }
})
intercept[IllegalArgumentException] { actorRef.receive("hello") }

Java

copysourcefinal Props props = Props.create(MyActor.class);
final TestActorRef<MyActor> ref = TestActorRef.create(system, props, "myActor");
try {
  ref.receive(new Exception("expected"));
  Assert.fail("expected an exception to be thrown");
} catch (Exception e) {
  assertEquals("expected", e.getMessage());
}

Use Cases

You may mix and match both modi operandi of TestActorRef as suits your test needs:

• one common use case is setting up the actor into a specific internal state before sending the test message
• another is to verify correct internal state transitions after having sent the test message

Feel free to experiment with the possibilities, and if you find useful patterns, don’t hesitate to let the Akka forums know about them! Who knows, common operations might even be worked into nice DSLs.