NPE when sending an ask in a closure

[Simon Schäfer]

I have the following code:

import scala.concurrent.Future
import scala.concurrent.Promise
import scala.concurrent.duration.DurationInt
import scala.concurrent.duration.FiniteDuration

import akka.actor.Actor
import akka.actor.ActorRef
import akka.pattern.ask
import akka.util.Timeout.durationToTimeout

trait ActorHelpers {

  self: Actor =>

  import context.dispatcher

  def keepAsking(recipient: ActorRef, msg: Any, interval: FiniteDuration): Future[Any] = {
    val p = Promise[Any]()
    val cancel = context.system.scheduler.schedule(0.millis, interval) {
      if (context != null)
        (recipient ? msg)(1.hour) foreach p.trySuccess
    }
    val f = p.future
    f.onComplete(_ => cancel.cancel())
    f
  }

}

The idea of this code snippet is that a message is resend as long as an answer is retrieved. The problem is that I sometimes get a NPE on the line that contains the ask.

Without the check if context is null I get even more NPEs. I don't understand why I get NPEs here, because I can't see where context or some of its state is accessed. Can someone explain it?

[√iktor Ҡlang]

Hint: remove import context.dispatcher

 

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--

Cheers,

√

Viktor Klang
Director of Engineering

Typesafe

Twitter: @viktorklang

[√iktor Ҡlang]

As for implementation, I'd probably do something like this: (WARNING: NOT COMPILED)

 import akka.pattern.after

 def keepAsking(recipient: ActorRef, msg: Any, every: FiniteDuration)(implicit ec: ExecutionContext, scheduler: Scheduler): Future[Any] = {

    def retry(): Future[Any] = after(every, scheduler)((recipient ? msg)(every)) recoverWith { case _ => retry() }

    retry()

  }

--

Cheers,

√

Viktor Klang
Concurrent, Distributed

Twitter: @viktorklang

[Simon Schäfer]

On Wednesday, January 15, 2014 6:22:35 PM UTC+1, √ wrote:

Hint: remove import context.dispatcher

Awesome, removing the import and adding it the ExecutionContext as implicit argument solved the problem.

Nevertheless, I do not understand what the difference is. I had to add the import in the class that calls my helper function, which gets available through trait mixin. Thus, why is it not the same reference?

[√iktor Ҡlang]

Hint: context is a method. What gets closed over?

--

[Simon Schäfer]

On 01/18/2014 09:17 AM, √iktor Ҡlang wrote:

Hint: context is a method. What gets closed over?

Ok, got it. Thanks for help.

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[√iktor Ҡlang]

Happy hAkking! :)