Scala Futures -内置超时？

Question

有一个方面的未来，我不完全理解从官方教程参考。http://docs.scala-lang.org/overviews/core/futures.html

scala中的期货是否有某种内置的超时机制？假设下面的示例是一个5GB的文本文件...“Implicits.global”的隐含范围最终会导致onFailure以非阻塞方式触发还是可以定义？如果没有某种默认的超时，这是否意味着成功或失败都不会触发？

import scala.concurrent._
import ExecutionContext.Implicits.global

val firstOccurence: Future[Int] = future {
  val source = scala.io.Source.fromFile("myText.txt")
  source.toSeq.indexOfSlice("myKeyword")
}
firstOccurence onSuccess {
  case idx => println("The keyword first appears at position: " + idx)
}
firstOccurence onFailure {
  case t => println("Could not process file: " + t.getMessage)
}

Answer 1

我刚刚为一位同事创建了一个 TimeoutFuture 类：

package model

import scala.concurrent._
import scala.concurrent.duration._
import play.libs.Akka
import play.api.libs.concurrent.Execution.Implicits._

object TimeoutFuture {
  def apply[A](timeout: FiniteDuration)(block: => A): Future[A] = {

    val prom = promise[A]

    // timeout logic
    Akka.system.scheduler.scheduleOnce(timeout) {
      prom tryFailure new java.util.concurrent.TimeoutException
    }

    // business logic
    Future { 
      prom success block
    }

    prom.future
  } 
}

val future = TimeoutFuture(10 seconds) { 
  // do stuff here
}

future onComplete {
  case Success(stuff) => // use "stuff"
  case Failure(exception) => // catch exception (either TimeoutException or an exception inside the given block)
}

• 假设 Play！ 框架（但适应起来很容易）
• 每段代码都在同一个执行文本中运行，这可能并不理想。

Answer 2

所有这些答案都需要额外的相关性。我决定使用java.util编写一个版本。定时器，这是将来运行函数的有效方式，在这种情况下触发超时。

更多详情请点击此处

利用这一点和Scala的promise，我们可以做出如下带有超时的未来:

package justinhj.concurrency

import java.util.concurrent.TimeoutException
import java.util.{Timer, TimerTask}

import scala.concurrent.duration.FiniteDuration
import scala.concurrent.{ExecutionContext, Future, Promise}
import scala.language.postfixOps

object FutureUtil {

  // All Future's that use futureWithTimeout will use the same Timer object
  // it is thread safe and scales to thousands of active timers
  // The true parameter ensures that timeout timers are daemon threads and do not stop
  // the program from shutting down

  val timer: Timer = new Timer(true)

  /**
    * Returns the result of the provided future within the given time or a timeout exception, whichever is first
    * This uses Java Timer which runs a single thread to handle all futureWithTimeouts and does not block like a
    * Thread.sleep would
    * @param future Caller passes a future to execute
    * @param timeout Time before we return a Timeout exception instead of future's outcome
    * @return Future[T]
    */
  def futureWithTimeout[T](future : Future[T], timeout : FiniteDuration)(implicit ec: ExecutionContext): Future[T] = {

    // Promise will be fulfilled with either the callers Future or the timer task if it times out
    val p = Promise[T]

    // and a Timer task to handle timing out

    val timerTask = new TimerTask() {
      def run() : Unit = {
            p.tryFailure(new TimeoutException())
        }
      }

    // Set the timeout to check in the future
    timer.schedule(timerTask, timeout.toMillis)

    future.map {
      a =>
        if(p.trySuccess(a)) {
          timerTask.cancel()
        }
    }
    .recover {
      case e: Exception =>
        if(p.tryFailure(e)) {
          timerTask.cancel()
        }
    }

    p.future
  }

}

Answer 3

只有当您使用阻塞来获取Future的结果时，您才会出现超时行为。如果您想使用非阻塞回调on完成、on成功或onFailure，那么您必须滚动自己的超时处理。Akka为参与者之间的请求/响应（？）消息传递内置了超时处理，但不确定您是否要开始使用Akka。FWIW，在Akka中，为了超时处理，它们通过Future.firstCompletedOf将两个Futures组合在一起，一个表示实际的异步任务，另一个表示超时。如果超时计时器（通过HashedWheelTimer）首先弹出，则异步回调失败。

一个非常简单的滚动自己的示例可能会像这样。首先，一个用于调度超时的对象：

import org.jboss.netty.util.{HashedWheelTimer, TimerTask, Timeout}
import java.util.concurrent.TimeUnit
import scala.concurrent.duration.Duration
import scala.concurrent.Promise
import java.util.concurrent.TimeoutException

object TimeoutScheduler{
  val timer = new HashedWheelTimer(10, TimeUnit.MILLISECONDS)
  def scheduleTimeout(promise:Promise[_], after:Duration) = {
    timer.newTimeout(new TimerTask{
      def run(timeout:Timeout){              
        promise.failure(new TimeoutException("Operation timed out after " + after.toMillis + " millis"))        
      }
    }, after.toNanos, TimeUnit.NANOSECONDS)
  }
}

然后一个函数来获取Future并向其添加超时行为：

import scala.concurrent.{Future, ExecutionContext, Promise}
import scala.concurrent.duration.Duration

def withTimeout[T](fut:Future[T])(implicit ec:ExecutionContext, after:Duration) = {
  val prom = Promise[T]()
  val timeout = TimeoutScheduler.scheduleTimeout(prom, after)
  val combinedFut = Future.firstCompletedOf(List(fut, prom.future))
  fut onComplete{case result => timeout.cancel()}
  combinedFut
}

请注意，我在这里使用的HashedWheelTimer来自Netty。