Scala 为什么Future.firstCompletedOf在超时时不调用回调？

我正在从学校做练习

有关代码注释中的练习问题

程序输出正确的HTML内容以获得正确的URL和足够的超时

程序为正确的URL和低超时打印“出错”

但是，对于无效URL，不会打印“出错”。下面的代码有什么问题

/*
 * Implement a command-line program that asks the user to input a URL of some website, 
 * and displays the HTML of that website. Between the time that the user hits ENTER and 
 * the time that the HTML is retrieved, the program should repetitively print a . to the 
 * standard output every 50 milliseconds, with a two seconds timeout. Use only futures 
 * and promises, and avoid the synchronization primitives from the previous chapters. 
 * You may reuse the timeout method defined in this chapter.
 */
object Excersices extends App {

  val timer = new Timer()

  def timeout(t: Long = 1000): Future[Unit] = {
    val p = Promise[Unit]
    val timer = new Timer(true)
    timer.schedule(new TimerTask() {
      override def run() = {
        p success ()
        timer cancel()
      }
    }, t)
    p future
  }

  def printDot = println(".")

  val taskOfPrintingDot = new TimerTask {
    override def run() = printDot
  }

  println("Enter a URL")
  val lines = io.Source.stdin.getLines()
  val url = if (lines hasNext) Some(lines next) else None

  timer.schedule(taskOfPrintingDot, 0L, 50.millisecond.toMillis)
  val timeOut2Sec = timeout(2.second.toMillis)

  val htmlContents = Future {
    url map { x =>
      blocking {
        Source fromURL (x) mkString
      }
    }
  }

  Future.firstCompletedOf(Seq(timeOut2Sec, htmlContents)) map { x =>
    timer cancel ()
    x match {
      case Some(x) =>
        println(x)
      case _ =>
        println("Error occured")
    }

  }

  Thread sleep 5000
}

---

正如@Gábor Bakos所说，异常会产生

故障

，而map不会处理该故障：

val fut = Future { Some(Source fromURL ("hhhttp://google.com")) }

scala> fut  map { x => println(x) } //nothing printed
res12: scala.concurrent.Future[Unit] = scala.concurrent.impl.Promise$DefaultPromise@5e025724

处理故障-使用

恢复
方法：

scala> fut recover { case failure => None } map { x => println(x) } 
None
res13: scala.concurrent.Future[Unit] = scala.concurrent.impl.Promise$DefaultPromise@578afc83

在您的上下文中，它类似于：

Future.firstCompletedOf(Seq(timeOut2Sec, htmlContents)) recover {case x => println("Error:" + x); None} map { x => ...}

使用@dk14建议的recover后的完整代码：

object Exercises extends App {

  val timer = new Timer()
  def timeout(t: Long = 1000): Future[Unit] = {
    val p = Promise[Unit]
    val timer = new Timer(true)
    timer.schedule(new TimerTask() {
      override def run() = {
        p success ()
        timer cancel ()
      }
    }, t)
    p future
  }

  def printDot = println(".")

  val taskOfPrintingDot = new TimerTask {
    override def run() = {
      printDot
    }
  }

  println("Enter a URL")
  val lines = io.Source.stdin.getLines()
  val url = if (lines hasNext) Some(lines next) else None

  timer.schedule(taskOfPrintingDot, 0L, 50.millisecond.toMillis)
  val timeOut2Sec = timeout(2.second.toMillis)

  val htmlContents = Future {
    url map { x =>
      blocking {
        Source fromURL (x) mkString
      }
    }
  }

  Future.firstCompletedOf(Seq(timeOut2Sec, htmlContents))
    .recover { case x => println("Error:" + x); None }
    .map { x =>
      timer cancel ()
      x match {
        case Some(x) =>
          println(x)
        case _ =>
          println("Timeout occurred")
      }
    }

  Thread sleep 5000
}

对于无效URL，最有可能的是
fromURL
终止时出现
异常
，因此在该分支上没有成功（但失败），
map
将不会被调用，
case=>
将不匹配。@Gábor Bakos oh yeah得到它。使用Try和it worked将htmlContents更改为此worked
val htmlContents=Future{url map{x=>val html=Try（来源于url（x）mkString）html匹配{case Success（x）=>x case Failure（error）=>“发生错误：可能是无效的url”}
为什么不直接尝试/捕获而不是尝试+匹配？您也破坏了您的类型安全，因为现在您无法区分错误和正常结果。因此，使用recover（它使用Try-inside）而不是显式Try是错误的：
Future.firstCompletedOf（Seq（timeOut2Sec，htmlContents））recover{case x=>println（“Error:+x”）；None}map{x=>…}
？@dk14。是的，这好多了。以前没有检查recover返回的内容。因此，我已使用此特定解决方案更新了答案-因此，如果您愿意，可以将其标记为right（通过单击复选框）。