Testing Akka actors that mixin Stash with TestActorRef

Question

I'm running into a problem with an actor that extends Stash and which works perfectly fine when instantiating it with actorOf in a simple ActorSystem. Now I would actually like to write some tests for my stashing actors before using them in my program. But I cannot figure out a way to use an TestActorRef with this actor in my test suite.

The code that works looks like this:

import akka.actor.{Stash, Actor, ActorSystem, Props}
import com.typesafe.config.ConfigFactory

object StashTest {
  val config = ConfigFactory.parseString(
    """
      |akka.actor.default-mailbox {
      | mailbox-type = "akka.dispatch.UnboundedDequeBasedMailbox"
      |}
    """.stripMargin)
}

class StashTestActor extends Stash {

  def receive: Actor.Receive = {
    case "unstash" =>
      unstashAll()
      context become print
    case msg => stash()
  }

  def print: Actor.Receive = {
    case msg => println(s"Unstashed message: $msg")
  }
}

val system = ActorSystem("stashSystem", StashTest.config)
val ref = system.actorOf(Props[StashTestActor])

ref ! "stash me"
ref ! "blah"
ref ! "unstash"

Which prints

Unstashed message: stash me

Unstashed message: blah

But if I try to write a WordSpec test for this actor, it leaves me with some nasty exceptions I can't figure out what they would like me to change in my code.

The test class looks like this

import akka.testkit.{TestActorRef, TestKit}
import akka.actor.{Stash, Actor, ActorSystem}
import org.scalatest.{WordSpecLike, MustMatchers}
import com.typesafe.config.ConfigFactory


class StashTestActor extends Stash {

  def receive: Actor.Receive = {
    case "unstash" =>
      unstashAll()
      context become print
    case msg => stash()
  }

  def print: Actor.Receive = {
    case msg => println(s"Unstashed message: $msg")
  }
}


class StashTest extends TestKit(ActorSystem("testSystem", StashTest.config))
with WordSpecLike
with MustMatchers {

  "A simple stashing actor" must {
    val actorRef = TestActorRef[StashTestActor]

    "stash messages" in {
      actorRef ! "stash me!"
    }

    "unstash all messages" in {
      actorRef ! "unstash"
    }
  }
}

object StashTest {
  val config = ConfigFactory.parseString(
    """
      |akka.actor.default-mailbox {
      | mailbox-type = "akka.dispatch.UnboundedDequeBasedMailbox"
      |}
    """.stripMargin)
}

When running the test, I get following exceptions that are thrown during the instantiation of the TestActorRef.

[ERROR] [08/20/2013 14:19:40.765] [testSystem-akka.actor.default-dispatcher-3] [akka://testSystem/user/$$a] Could not instantiate Actor
Make sure Actor is NOT defined inside a class/trait,
if so put it outside the class/trait, f.e. in a companion object,
OR try to change: 'actorOf(Props[MyActor]' to 'actorOf(Props(new MyActor)'.
akka.actor.ActorInitializationException: exception during creation
    at akka.actor.ActorInitializationException$.apply(Actor.scala:218)
    at akka.actor.ActorCell.create(ActorCell.scala:578)
    at akka.actor.ActorCell.invokeAll$1(ActorCell.scala:425)
    at akka.actor.ActorCell.systemInvoke(ActorCell.scala:447)
    at akka.dispatch.Mailbox.processAllSystemMessages(Mailbox.scala:262)
    at akka.testkit.CallingThreadDispatcher.process$1(CallingThreadDispatcher.scala:244)
    at akka.testkit.CallingThreadDispatcher.runQueue(CallingThreadDispatcher.scala:284)
    at akka.testkit.CallingThreadDispatcher.register(CallingThreadDispatcher.scala:153)
    at akka.dispatch.MessageDispatcher.attach(AbstractDispatcher.scala:133)
    at akka.actor.dungeon.Dispatch$class.start(Dispatch.scala:84)
    at akka.actor.ActorCell.start(ActorCell.scala:338)
    at akka.testkit.TestActorRef.<init>(TestActorRef.scala:50)
    at akka.testkit.TestActorRef$.apply(TestActorRef.scala:141)
    at akka.testkit.TestActorRef$.apply(TestActorRef.scala:137)
    at akka.testkit.TestActorRef$.apply(TestActorRef.scala:146)
    at akka.testkit.TestActorRef$.apply(TestActorRef.scala:144)
    at stashActorTest.StashTest$$anonfun$1.apply$mcV$sp(StashTestActor.scala:29)
    at stashActorTest.StashTest$$anonfun$1.apply(StashTestActor.scala:28)
    at stashActorTest.StashTest$$anonfun$1.apply(StashTestActor.scala:28)
    at org.scalatest.SuperEngine.registerNestedBranch(Engine.scala:613)
    at org.scalatest.WordSpecLike$class.org$scalatest$WordSpecLike$$registerBranch(WordSpecLike.scala:120)
    at org.scalatest.WordSpecLike$$anon$2.apply(WordSpecLike.scala:851)
    at org.scalatest.words.MustVerb$StringMustWrapperForVerb$class.must(MustVerb.scala:189)
    at org.scalatest.matchers.MustMatchers$StringMustWrapper.must(MustMatchers.scala:6167)
    at stashActorTest.StashTest.<init>(StashTestActor.scala:28)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
    at java.lang.Class.newInstance(Class.java:374)
    at org.scalatest.tools.Framework$ScalaTestTask.execute(Framework.scala:444)
    at sbt.TestRunner.runTest$1(TestFramework.scala:84)
    at sbt.TestRunner.run(TestFramework.scala:94)
    at sbt.TestFramework$$anon$2$$anonfun$$init$$1$$anonfun$apply$8.apply(TestFramework.scala:224)
    at sbt.TestFramework$$anon$2$$anonfun$$init$$1$$anonfun$apply$8.apply(TestFramework.scala:224)
    at sbt.TestFramework$.sbt$TestFramework$$withContextLoader(TestFramework.scala:212)
    at sbt.TestFramework$$anon$2$$anonfun$$init$$1.apply(TestFramework.scala:224)
    at sbt.TestFramework$$anon$2$$anonfun$$init$$1.apply(TestFramework.scala:224)
    at sbt.TestFunction.apply(TestFramework.scala:229)
    at sbt.Tests$$anonfun$7.apply(Tests.scala:196)
    at sbt.Tests$$anonfun$7.apply(Tests.scala:196)
    at sbt.std.Transform$$anon$3$$anonfun$apply$2.apply(System.scala:45)
    at sbt.std.Transform$$anon$3$$anonfun$apply$2.apply(System.scala:45)
    at sbt.std.Transform$$anon$4.work(System.scala:64)
    at sbt.Execute$$anonfun$submit$1$$anonfun$apply$1.apply(Execute.scala:237)
    at sbt.Execute$$anonfun$submit$1$$anonfun$apply$1.apply(Execute.scala:237)
    at sbt.ErrorHandling$.wideConvert(ErrorHandling.scala:18)
    at sbt.Execute.work(Execute.scala:244)
    at sbt.Execute$$anonfun$submit$1.apply(Execute.scala:237)
    at sbt.Execute$$anonfun$submit$1.apply(Execute.scala:237)
    at sbt.ConcurrentRestrictions$$anon$4$$anonfun$1.apply(ConcurrentRestrictions.scala:160)
    at sbt.CompletionService$$anon$2.call(CompletionService.scala:30)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
    at java.util.concurrent.FutureTask.run(FutureTask.java:166)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
    at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
    at java.util.concurrent.FutureTask.run(FutureTask.java:166)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:724)
Caused by: akka.actor.ActorInitializationException: Could not instantiate Actor
Make sure Actor is NOT defined inside a class/trait,
if so put it outside the class/trait, f.e. in a companion object,
OR try to change: 'actorOf(Props[MyActor]' to 'actorOf(Props(new MyActor)'.
    at akka.actor.ActorInitializationException$.apply(Actor.scala:218)
    at akka.testkit.TestActorRef$$anonfun$apply$2$$anonfun$apply$1.applyOrElse(TestActorRef.scala:148)
    at akka.testkit.TestActorRef$$anonfun$apply$2$$anonfun$apply$1.applyOrElse(TestActorRef.scala:147)
    at scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:33)
    at scala.util.Failure$$anonfun$recover$1.apply(Try.scala:185)
    at scala.util.Try$.apply(Try.scala:161)
    at scala.util.Failure.recover(Try.scala:185)
    at akka.testkit.TestActorRef$$anonfun$apply$2.apply(TestActorRef.scala:147)
    at akka.testkit.TestActorRef$$anonfun$apply$2.apply(TestActorRef.scala:153)
    at akka.actor.CreatorFunctionConsumer.produce(Props.scala:369)
    at akka.actor.Props.newActor(Props.scala:323)
    at akka.actor.ActorCell.newActor(ActorCell.scala:534)
    at akka.actor.ActorCell.create(ActorCell.scala:560)
    ... 58 more
Caused by: java.lang.NullPointerException
    at akka.actor.UnrestrictedStash$class.$init$(Stash.scala:82)
    at stashActorTest.StashTestActor.<init>(StashTestActor.scala:9)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
    at akka.actor.ReflectiveDynamicAccess$$anonfun$createInstanceFor$2.apply(DynamicAccess.scala:78)
    at scala.util.Try$.apply(Try.scala:161)
    at akka.actor.ReflectiveDynamicAccess.createInstanceFor(DynamicAccess.scala:73)
    ... 64 more

I don't have any problems with using TestActorRefs with actors that don't extend Stash. So I don't know if it is a configuration error or something else I'm missing.

Answer 1

TestActorRef can't be used together with Stash. TestActorRef requires a CallingThreadMailbox and Stash requires DequeBasedMessageQueueSemantics. The documentation should include this limitation and the error messages should be improved.

Answer 2

Using the default akka dispatcher allowed me to use Stash and TestActorRef:

val myActor = TestActorRef[MyActor](Props(classOf[MyActor]).withDispatcher("akka.actor.default-dispatcher"))

Note that this means your tests will no longer use the default CallingThreadDispatcher and will lose the benefits highlighted in the akka docs