java - 使用 hasNext() 和 next() 遍历异步生成的元素流

Question

我必须使用 hasNext() 和 next() 方法实现一个 Iterator 接口(interface)(由 Java API 定义)，它应该返回源自异步处理的 HTTP 响应(使用 Akka actor 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，因为生成大型结果集可能需要一段时间(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)
• Iterator.hasNext() 应该返回 true，只要还有更多元素(即使下一个元素尚不可用)
• 结果总数事先未知。结果生产者将在完成时发送特定的“结束消息”。
• 尽量避免使用 InterruptedException，例如当迭代器正在等待一个空队列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)，所以我怀疑是否有任何合适的解决方案。

目前，我的 Scala 实现 stub 使用 java.util.concurrent.BlockingQueue 并且看起来像这样:

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。我尝试过查看下一个元素并使用“完成”标志，但我希望有更简单的解决方案。

奖励问题:

• 单元测试如何涵盖同步/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的？

Answer 1

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的域，而应该在Actor的域。

// 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
}