在异步生成的元素流中用hasnext()和next()迭代

Question

我必须实现迭代界面（由java api定义），带有hasnext（）和next（）方法，这些效果应返回来自异步处理的http响应的结果元素（与Akka Actors进行处理）。

必须满足以下要求：

• 不要阻止和等待异步操作完成，因为大型结果的产生可能需要一段时间（迭代器应在可用的结果元素返回结果元素）
）
• iterator.next（）应阻止直到下一个元素可用（如果没有更多的元素，请进行异常）
）
• iterator.hasnext（）应该返回true，只要有更多元素要发生（即使下一个元素尚未可用）
）
• 结果的总数未知。结果制作者将在完成后发送特定的"结束消息"。
• 尽量避免使用中断的使用，例如当迭代器在排空队列时等待但不会生成更多元素时。

我尚未研究Java 8流或Akka流。但是，由于我基本上必须在队列上迭代（有限的流），所以我怀疑有任何合适的解决方案。

当前，我的Scala实现stub使用Java.util.concurrent.blockingquequequequequequequequequequequeququeququequeque：

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)
        }
    }
}

我使用选项[结果]指示结果流的结束。我已经尝试了窥视下一个元素并使用"完成"标志，但我希望有一个更容易的解决方案。

奖励问题：

• 如何通过单位测试涵盖同步/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？

Answer 1

以下代码将有能力满足要求。演员的领域可以在Actor的接收器中安全修改。因此，Resultsquelue不应在迭代器的领域中，而应在Actor的领域中。

// 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，然后在两种方法开始时等待它：

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jiro的建议，并根据需要进行一些改编。通常，我喜欢将getNext()和next()作为发送给Actor的ask消息实现的方法。这样可以确保任何时候只有一个线程可以修改队列。

但是，我不确定此实现的性能，因为ask和Await.result将为hasNext()和next()的每个调用创建两个线程。

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()
}