在异步生成的元素流上使用 hasNext() 和 next() 进行迭代

Question

我必须使用 hasNext() 和 next() 方法实现一个 Iterator 接口（由 Java API 定义），它应该返回源自异步处理的 HTTP 响应（由 Akka 演员处理）的结果元素。

必须满足以下要求：

• 不要阻塞并等待异步操作完成，因为大型结果集的生成可能需要一段时间（迭代器应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（或者如果没有更多元素出现，则抛出异常）
• 只要还有更多元素，Iterator.hasNext() 应该返回 true（即使下一个尚不可用）
• 结果的总数事先是未知的。生成结果的 Actor 将在完成后发送特定的“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遍历队列（有限流），我怀疑是否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，并使用 None。我已经尝试查看下一个元素并使用“完成”标志，但我希望有一个更简单的解决方案。

额外问题：

• 如何使用单元测试涵盖同步/异步实现，尤其是测试延迟的结果生成？
• 如何使迭代器成为线程安全的？

Answer 1

我听从了jiro的建议，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。这样可以确保任何时候只有一个线程修改队列。

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()
}

Answer 2

您可以使用变量存储下一个元素，然后在两种方法的开头等待它：

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

以下代码将满足要求。 Actor 的字段可以在 Actor 的接收器中安全地修改。 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
}