在scala中定义时自引用val

Question

这是我在scala中发现的东西,它有效,我无法弄清楚为什么,任何人都可以解释为什么这有效吗？

基本上我可以使用val的引用,而我仍在定义它(因为我的其他对象/ actor将其作为参数接受)

val backbone: ActorRef = context.actorOf(
  F_BackBone.props(
    context.actorOf(F_PictureHandler.props(backbone)), 
    context.actorOf(F_UserHandler.props(backbone)), 
    context.actorOf(F_PageProfileHandler.props(backbone))
  )
)

如果我没有明确定义类型,我将得到编译器错误,这是有道理的.

Answer 1

请注意，在这种特定情况下，尽管代码可以编译，但它的行为不会正确，因为正如其他答案的注释中所建议的那样，backbone传递给其他参与者的值为“null”。

这个例子证明了这一点：

import akka.actor.{Props, Actor, ActorRef, ActorSystem}

class SenderReceiver(sendTo:ActorRef) extends Actor{

  override def preStart(): Unit = {
    self ! "Start"
  }

  def receive = {
    case "Start" => sendTo ! "Hello"
    case "Hello" => println("Received Hello")
  }
}

object SenderReceiver {
  def props(sendTo:ActorRef):Props = Props(new SenderReceiver(sendTo))
}

object Example extends App {

  val system = ActorSystem()

  val actor: ActorRef = system.actorOf(SenderReceiver.props(actor))
  system.awaitTermination()
}

这会产生以下结果（重复，因为主管策略尝试重新启动参与者）：

[info] [ERROR] [12/01/2015 09:47:04.543] [default-akka.actor.default-dispatcher-9] [akka://default/user/$a] null
[info] java.lang.NullPointerException
[info]  at SenderReceiver$$anonfun$receive$1.applyOrElse(example.scala:10)
[info]  at akka.actor.Actor$class.aroundReceive(Actor.scala:467)
[info]  at SenderReceiver.aroundReceive(example.scala:3)
[info]  at akka.actor.ActorCell.receiveMessage(ActorCell.scala:516)
[info]  at akka.actor.ActorCell.invoke(ActorCell.scala:487)
[info]  at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:238)
[info]  at akka.dispatch.Mailbox.run(Mailbox.scala:220)
[info]  at akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:397)
[info]  at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
[info]  at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
[info]  at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
[info]  at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)