build.sbt

scalaVersion := "2.11.2"

libraryDependencies += "com.typesafe.akka" %% "akka-actor" % "2.3.6"

log.txt

[info] Running sample.Main 
(class akka.dispatch.sysmsg.Terminate,DeadLetter(Terminate(),Actor[akka://aaa/user/parent/child#727572459],Actor[akka://aaa/user/parent/child#727572459]))
[INFO] [09/18/2014 11:50:47.573] [aaa-akka.actor.default-dispatcher-3] [akka://aaa/user/parent/child] Message [akka.dispatch.sysmsg.Terminate] from Actor[akka://aaa/user/parent/child#727572459] to Actor[akka://aaa/user/parent/child#727572459] 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'.

Main.scala

package sample

import akka.actor._

object Main{
  def main (args: Array[String]) {
    val system = ActorSystem("aaa")
    system.eventStream.subscribe(system.actorOf(Props[DeadLetterLogger]), classOf[DeadLetter])
    val a = system.actorOf(Props[Parent], "parent")
    Thread.sleep(3000)
    a ! Stop
    Thread.sleep(3000)
    system.shutdown()
  }
}

object Stop

class DeadLetterLogger extends Actor{
  def receive = {
    case l: DeadLetter =>
      println((l.message.getClass, l))
  }
}

class Parent extends Actor{

  override def preStart(): Unit ={
    val child = context.actorOf(Props[Child], "child")
    context.watch(child)
  }

  def receive = {
    case Stop =>
      context.children.foreach(context.unwatch)
      self ! PoisonPill
      context.children.foreach(_ ! PoisonPill) // ここをコメントアウトすると、DeadLetter発生しない
  }
}

class Child extends Actor{
  def receive = {
    case message =>
  }
}