Got the following error report on IRC. The code is almost exactly the same as we have in our docs but shows a NPE. It looks like an error due to receive being called before the class is fully initialised. Apparently this could be a bug in 2.0.x. We should write a test and backport it. If it breaks 2.0.x then we will want to update our docs to either fix the problem or remove it as an example.

mikeX: hi, I'm trying to implement the composable actor described in the Akka 2.0.4 documentation, but I'm getting the following error: https://gist.github.com/4202268 The example I'm using is https://gist.github.com/4202263 Any idea what I'm doing wrong?

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scala> Act.main(Array())

scala> [ERROR] [12/04/2012 09:47:39.60] [MySystem-akka.actor.default-dispatcher-2] [akka://MySystem/user/myactor] error while creating actor
java.lang.NullPointerException
	at ComposableActor$class.receive(Act.scala:9)
	at MyComposableActor.receive(Act.scala:12)
	at akka.actor.Actor$class.$init$(Actor.scala:343)
	at MyComposableActor.<init>(Act.scala:12)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
	at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
	at java.lang.Class.newInstance0(Class.java:355)
	at java.lang.Class.newInstance(Class.java:308)
	at akka.actor.Props$$anonfun$withCreator$2.apply(Props.scala:159)
	at akka.actor.Props$$anonfun$withCreator$2.apply(Props.scala:159)
	at akka.actor.ActorCell.newActor(ActorCell.scala:488)
	at akka.actor.ActorCell.create$1(ActorCell.scala:506)
	at akka.actor.ActorCell.systemInvoke(ActorCell.scala:600)
	at akka.dispatch.Mailbox.processAllSystemMessages(Mailbox.scala:209)
	at akka.dispatch.Mailbox.run(Mailbox.scala:178)
	at akka.dispatch.ForkJoinExecutorConfigurator$MailboxExecutionTask.exec(AbstractDispatcher.scala:516)
	at akka.jsr166y.ForkJoinTask.doExec(ForkJoinTask.java:259)
	at akka.jsr166y.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:975)
	at akka.jsr166y.ForkJoinPool.runWorker(ForkJoinPool.java:1479)
	at akka.jsr166y.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:104)

import akka.actor._

trait ComposableActor extends Actor {
  private var receives: List[Receive] = List()
  protected def registerReceive(rec: Receive) {
      receives = rec :: receives
  }
  def receive: Receive = receives reduce { _ orElse _ }
}

class MyComposableActor extends ComposableActor {
  override def preStart() {
    registerReceive({
        case "foo" => println("foo")
      })
    registerReceive({
        case "bar" => println("bar")
      })
  }
}

object Act {
  def main(args: Array[String]) = {
     val system = ActorSystem("MySystem")
     val myActor = system.actorOf(Props[MyComposableActor], name = "myactor")
     myActor ! "foo"
  }
}