There's more... as context is also null in the Future's callback, anything
using context won't work, see below for an updated example using the Akka's
scheduler and dispatcher:
import akka.actor._
import scala.concurrent._
import scala.concurrent.duration._
case class Done(s: String)
case object Greet
case class Complete(p: Promise[Unit])
class Greeter extends Actor {
import context.dispatcher
def receive = {
case Done(s) => println(s"Done($s) (system still running)!")
case Complete(p) => p.success(())
case Greet =>
val p = Promise[Unit]()
val saveMyself = self
val sched = context.system.scheduler
val disp = context.dispatcher
p.future onComplete {
case _ =>
println(s"self is $self")
println(s"self is expexted to be $saveMyself")
// cannot use self to send a message, need to close on self
self ! Done("Send using self")
saveMyself ! Done("Send using saveMyself")
// are we supposed to avoid using context here?
println(s"context is $context") // outputs: context is null
// is this meant to work?
// context.system.scheduler.scheduleOnce(1.millisecond,
saveMyself, Done)
// will fail because we can't get the scheduler as context is null
try { context.system.scheduler.scheduleOnce(1.millisecond,
saveMyself, Done("scheduler 1")) } catch { case e => println(e) }
// The following will still fail because implicit dispatcher is
context.dispatcher and context is null
try { sched.scheduleOnce(1.millisecond, saveMyself,
Done("scheduler 2")) } catch { case e => println(e) }
// that will work...
sched.scheduleOnce(1.millisecond, saveMyself, Done("scheduler
3"))(disp, saveMyself)
}
self ! Complete(p)
throw new Exception("Messing with onComplete callback, self will be
dead letters")
}
}
object HelloAkkaScala extends App {
val system = ActorSystem("helloakka")
val greeter = system.actorOf(Props[Greeter], "greeter")
greeter ! Greet
}
Program output :
Running HelloAkkaScala
self is Actor[akka://helloakka/deadLetters]
[ERROR] [06/18/2015 16:47:50.464]
[helloakka-akka.actor.default-dispatcher-3] [akka://helloakka/user/greeter]
Messing with onComplete callback, self will be dead letters
java.lang.Exception: Messing with onComplete callback, self will be dead
letters
at Greeter$anonfun$receive$1.applyOrElse(HelloAkkaScala.scala:40)
at akka.actor.Actor$class.aroundReceive(Actor.scala:467)
at Greeter.aroundReceive(HelloAkkaScala.scala:9)
at akka.actor.ActorCell.receiveMessage(ActorCell.scala:516)
at akka.actor.ActorCell.invoke(ActorCell.scala:487)
at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:238)
at akka.dispatch.Mailbox.run(Mailbox.scala:220)
at
akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:397)
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
at
scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
at
scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
self is expexted to be Actor[akka://helloakka/user/greeter#-1327535750]
context is null
java.lang.NullPointerException
java.lang.NullPointerException
Done(Send using saveMyself) (system still running)!
[INFO] [06/18/2015 16:47:50.480]
[helloakka-akka.actor.default-dispatcher-4] [akka://helloakka/deadLetters]
Message [Done] from Actor[akka://helloakka/deadLetters] to
Actor[akka://helloakka/deadLetters] was not delivered. [1] dead letters
encountered. This logging can be turned off or adjusted with configuration
settings 'akka.log-dead-letters' and
'akka.log-dead-letters-during-shutdown'.
Done(scheduler 3) (system still running)!
On Thursday, June 18, 2015 at 2:37:35 PM UTC-7, Frederic wrote:
>
> Hello,
>
> I found a quite specific case involving Actors, Future callbacks and
> exceptions where self becomes deadLetters.
>
> I have the feeling I'm using self the way it is meant to be used, as
> described here:
> http://doc.akka.io/docs/akka/2.3.11/general/jmm.html#jmm-shared-state
>
> Am I doing something wrong or is it a bug?
>
> Please see code and output below to reproduce/illustrate the issue.
>
> Thanks, Fred
>
> import akka.actor._
> import scala.concurrent._
>
> case object Done
> case object Greet
> case class Complete(p: Promise[Unit])
>
> class Greeter extends Actor {
> import context.dispatcher
> def receive = {
> case Done => println("Done (system still running)!")
> case Complete(p) => p.success(())
> case Greet =>
> val p = Promise[Unit]()
> val saveMyself = self
> p.future onComplete {
> case _ =>
> println(s"self is $self")
> println(s"self is expexted to be $saveMyself")
> // cannot use self to send a message, bug?
> self ! Done
> saveMyself ! Done
> }
> self ! Complete(p)
> throw new Exception("Messing with onComplete callback, self will be
> dead letters")
> }
> }
>
> object HelloAkkaScala extends App {
> val system = ActorSystem("helloakka")
> val greeter = system.actorOf(Props[Greeter], "greeter")
> greeter ! Greet
> }
>
> Program output :
>
> Running HelloAkkaScala
> self is Actor[akka://helloakka/deadLetters]
> [ERROR] [06/18/2015 14:25:41.510]
> [helloakka-akka.actor.default-dispatcher-4] [akka://helloakka/user/greeter]
> Messing with onComplete callback, self will be dead letters
> java.lang.Exception: Messing with onComplete callback, self will be dead
> letters
> at Greeter$anonfun$receive$1.applyOrElse(HelloAkkaScala.scala:27)
> at akka.actor.Actor$class.aroundReceive(Actor.scala:467)
> at Greeter.aroundReceive(HelloAkkaScala.scala:8)
> at akka.actor.ActorCell.receiveMessage(ActorCell.scala:516)
> at akka.actor.ActorCell.invoke(ActorCell.scala:487)
> at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:238)
> at akka.dispatch.Mailbox.run(Mailbox.scala:220)
> at
> akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:397)
> at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
> at
> scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
> at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
> at
> scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
>
> self is expexted to be Actor[akka://helloakka/user/greeter#990239115]
> Done (system still running)!
> [INFO] [06/18/2015 14:25:41.516]
> [helloakka-akka.actor.default-dispatcher-3] [akka://helloakka/deadLetters]
> Message [Done$] from Actor[akka://helloakka/deadLetters] to
> Actor[akka://helloakka/deadLetters] was not delivered. [1] dead letters
> encountered. This logging can be turned off or adjusted with configuration
> settings 'akka.log-dead-letters' and
> 'akka.log-dead-letters-during-shutdown'.
>
--
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>>>>>>>>>> Check the FAQ:
>>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html
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