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Akka/Scala example source code file (PeekMailbox.scala)

This example Akka source code file (PeekMailbox.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Akka and Scala source code examples by using tags.

All credit for the original source code belongs to akka.io; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Akka tags/keywords

actor, actorcontext, actorref, actorsystem, akka, dispatch, extendedactorsystem, mail, mailbox, marker, option, peekmailbox, peekmailboxextension, some, unit

The PeekMailbox.scala Akka example source code

/**
 * Copyright (C) 2009-2014 Typesafe Inc. <http://www.typesafe.com>
 */
package akka.contrib.mailbox

import java.util.concurrent.{ ConcurrentHashMap, ConcurrentLinkedQueue }

import com.typesafe.config.Config

import akka.actor.{ ActorContext, ActorRef, ActorSystem, ExtendedActorSystem, Extension, ExtensionId, ExtensionIdProvider }
import akka.dispatch.{ Envelope, MailboxType, MessageQueue, UnboundedQueueBasedMessageQueue }

object PeekMailboxExtension extends ExtensionId[PeekMailboxExtension] with ExtensionIdProvider {
  def lookup = this
  def createExtension(s: ExtendedActorSystem) = new PeekMailboxExtension(s)

  def ack()(implicit context: ActorContext): Unit = PeekMailboxExtension(context.system).ack()
}

class PeekMailboxExtension(val system: ExtendedActorSystem) extends Extension {
  private val mailboxes = new ConcurrentHashMap[ActorRef, PeekMailbox]

  def register(actorRef: ActorRef, mailbox: PeekMailbox): Unit =
    mailboxes.put(actorRef, mailbox)

  def unregister(actorRef: ActorRef): Unit = mailboxes.remove(actorRef)

  def ack()(implicit context: ActorContext): Unit =
    mailboxes.get(context.self) match {
      case null    ⇒ throw new IllegalArgumentException("Mailbox not registered for: " + context.self)
      case mailbox ⇒ mailbox.ack()
    }
}

/**
 * configure the mailbox via dispatcher configuration:
 * {{{
 *   peek-dispatcher {
 *      mailbox-type = "example.PeekMailboxType"
 *   }
 * }}}
 */
class PeekMailboxType(settings: ActorSystem.Settings, config: Config) extends MailboxType {
  override def create(owner: Option[ActorRef], system: Option[ActorSystem]) = (owner, system) match {
    case (Some(o), Some(s)) ⇒
      val retries = config.getInt("max-retries")
      if (retries < 1) throw new akka.ConfigurationException("max-retries must be at least 1")
      val mailbox = new PeekMailbox(o, s, retries)
      PeekMailboxExtension(s).register(o, mailbox)
      mailbox
    case _ ⇒ throw new Exception("no mailbox owner or system given")
  }
}

class PeekMailbox(owner: ActorRef, system: ActorSystem, maxRetries: Int)
  extends UnboundedQueueBasedMessageQueue {
  final val queue = new ConcurrentLinkedQueue[Envelope]()

  /*
   * Since the queue itself is used to determine when to schedule the actor
   * (see Mailbox.hasMessages), we cannot poll() on the first try and then
   * continue handing back out that same message until ACKed, peek() must be
   * used. The retry limit logic is then formulated in terms of the `tries`
   * field, which holds
   *  0             if clean slate (i.e. last dequeue was ack()ed)
   *  1..maxRetries if not yet ack()ed
   *  Marker        if last try was done (at which point we had to poll())
   *  -1            during cleanUp (in order to disable the ack() requirement)
   */
  // the mutable state is only ever accessed by the actor (i.e. dequeue() side)
  var tries = 0
  val Marker = maxRetries + 1

  // this logic does not work if maxRetries==0, but then you could also use a normal mailbox
  override def dequeue(): Envelope = tries match {
    case -1 ⇒
      queue.poll()
    case 0 | Marker ⇒
      val e = queue.peek()
      tries = if (e eq null) 0 else 1
      e
    case `maxRetries` ⇒
      tries = Marker
      queue.poll()
    case n ⇒
      tries = n + 1
      queue.peek()
  }

  def ack(): Unit = {
    // do not dequeue for real if double-ack() or ack() on last try
    if (tries != 0 && tries != Marker) queue.poll()
    tries = 0
  }

  override def cleanUp(owner: ActorRef, deadLetters: MessageQueue): Unit = {
    tries = -1 // put the queue into auto-ack mode
    super.cleanUp(owner, deadLetters)
    PeekMailboxExtension(system).unregister(owner)
  }
}

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