Question

我必须使用hasNext（）和next（）方法实现Iterator接口（由Java API定义），该接口应返回源自异步处理的HTTP响应的结果元素（使用Akka actor处理）。 / p>

必须满足以下要求：

不要阻塞并等待异步操作完成，因为生成大型结果集可能需要一段时间（迭代器应该在结果元素可用时立即返回）
Iterator.next（）应该阻塞，直到下一个元素可用（如果没有更多元素可以抛出异常）
Iterator.hasNext（）应该返回true，只要有更多的元素要来（即使下一个元素还没有）
事先知道结果总数。生成actor的结果将在完成时发送特定的“结束消息”。
尽量避免使用InterruptedException，例如：当迭代器在空队列上等待但不会生成更多元素时。

我还没有研究过Java 8流或Akka流。但由于我基本上必须迭代一个队列（一个有限的流），我怀疑还有任何合适的解决方案。

目前，我的Scala实现存根使用java.util.concurrent.BlockingQueue，如下所示：

class ResultStreamIterator extends Iterator[Result] {
    val resultQueue = new ArrayBlockingQueue[Option[Result]](100)

    def hasNext(): Boolean = ???  // return true if not done yet
    def next(): Result = ???      // take() next element if not done yet

    case class Result(value: Any) // sent by result producing actor
    case object Done              // sent by result producing actor when finished

    class ResultCollector extends Actor {
        def receive = {
           case Result(value) => resultQueue.put(Some(value))
           case Done          => resultQueue.put(None)
        }
    }
}

我使用Option [Result]来指示结果流的结尾为None。我已经尝试过窥视下一个元素并使用“完成”标志，但我希望有一个更简单的解决方案。

奖金问题：

如何通过单元测试覆盖同步/异步实现，尤其是测试延迟结果生成？
如何使迭代器成为线程安全的？

Answer 1

以下代码可满足要求。可以在Actor的接收器中安全地修改Actor的字段。所以resultQueue不应该在Iterator的字段中，而应该在Actor的字段中。

// ResultCollector should be initialized.
// Initilize code is like...
// resultCollector ! Initialize(100)
class ResultStreamIterator(resultCollector: ActorRef) extends Iterator[Result] {

  implicit val timeout: Timeout = ???

  override def hasNext(): Boolean = Await.result(resultCollector ? HasNext, Duration.Inf) match {
    case ResponseHasNext(hasNext) => hasNext
  }

  @scala.annotation.tailrec
  final override def next(): Result = Await.result(resultCollector ? RequestResult, Duration.Inf) match {
    case ResponseResult(result) => result
    case Finished => throw new NoSuchElementException("There is not result.")
    case WaitingResult => next()// should be wait for a moment.
  }

}

case object RequestResult
case object HasNext

case class ResponseResult(result: Result)
case class ResponseHasNext(hasNext: Boolean)
case object Finished
case object WaitingResult

case class Initialize(expects: Int)

// This code may be more ellegant if using Actor FSM
// Acotr's State is (beforeInitialized)->(collecting)->(allCollected)
class ResultCollector extends Actor with Stash {

  val results = scala.collection.mutable.Queue.empty[Result]

  var expects = 0

  var counts = 0

  var isAllCollected = false

  def beforeInitialized: Actor.Receive = {
    case Initialize(n) =>
      expects = n
      if (expects != 0) context become collecting
      else context become allCollected
      unstashAll
    case _ => stash()
  }

  def collecting: Actor.Receive = {
    case RequestResult =>
      if (results.isEmpty) sender ! WaitingResult
      else sender ! ResponseResult(results.dequeue())
    case HasNext => ResponseHasNext(true)
    case result: Result =>
      results += result
      counts += 1
      isAllCollected = counts >= expects
      if (isAllCollected) context become allCollected
  }

  def allCollected: Actor.Receive = {
    case RequestResult =>
      if (results.isEmpty) sender ! Finished
      else sender ! ResponseResult(results.dequeue())
    case HasNext => ResponseHasNext(!results.isEmpty)
  }

  def receive = beforeInitialized
}

Answer 2

您可以使用变量存储下一个元素，并在两个方法的开头等待它：

private var nextNext: Option[Result] = null

def hasNext(): Boolean = {
  if (nextNext == null) nextNext = resultQueue.take()
  return !nextNext.isEmpty
}

def next(): Result = {
  if (nextNext == null) nextNext = resultQueue.take()
  if (nextNext.isEmpty) throw new NoSuchElementException()
  val result = nextNext.get
  nextNext = null
  return result
}

Answer 3

我遵循了次郎的建议，并根据需要做了一些调整。一般来说，我喜欢将getNext()和next()实现为发送给actor的ask消息的方法。这可确保在任何时候只有一个线程可以修改队列。

但是，我不确定此实现的效果，因为ask和Await.result会为hasNext()和next()的每次调用创建两个线程。

import scala.concurrent.{Await, Future}
import scala.concurrent.duration._
import scala.language.postfixOps

import akka.actor.{ActorRef, ActorSystem, Props, Stash}
import akka.pattern.ask
import akka.util.Timeout

case object HasNext
case object GetNext

case class Result(value: Any)
case object Done

class ResultCollector extends Actor with Stash {

  val queue = scala.collection.mutable.Queue.empty[Result]

  def collecting: Actor.Receive = {
    case HasNext       => if (queue.isEmpty) stash else sender ! true
    case GetNext       => if (queue.isEmpty) stash else sender ! queue.dequeue
    case value: Result => unstashAll; queue += value
    case Done          => unstashAll; context become serving
  }

  def serving: Actor.Receive = {
    case HasNext => sender ! queue.nonEmpty
    case GetNext => sender ! { if (queue.nonEmpty) queue.dequeue else new NoSuchElementException }
  }

  def receive = collecting
}

class ResultStreamIteration(resultCollector: ActorRef) extends Iterator {

  implicit val timeout: Timeout = Timeout(30 seconds)

  override def hasNext(): Boolean = Await.result(resultCollector ? HasNext, Duration.Inf) match {
    case b: Boolean => b
  }

  override def next(): Any = Await.result(resultCollector ? GetNext, Duration.Inf) match {
    case Result(value: Any) => value
    case e: Throwable       => throw e
  }
}

object Test extends App {
  implicit val exec = scala.concurrent.ExecutionContext.global
  val system = ActorSystem.create("Test")
  val actorRef = system.actorOf(Props[ResultCollector])
  Future {
    for (i <- 1 to 10000) actorRef ! Result(s"Result $i"); actorRef ! Done
  }
  val iterator = new ResultStreamIteration(actorRef)
  while (iterator.hasNext()) println(iterator.next)
  system.shutdown()
}

在异步生成的元素流

3 个答案: