并行运行多个 future，超时返回默认值

我必须并行运行多个 future，并且程序不应崩溃或挂起。

现在，我一一等待 future，如果出现 TimeoutException，则使用后备值。

val future1 = // start future1
val future2 = // start future2
val future3 = // start future3

// <- at this point all 3 futures are running

// waits for maximum of timeout1 seconds
val res1 = toFallback(future1, timeout1, Map[String, Int]())
// .. timeout2 seconds 
val res2 = toFallback(future2, timeout2, List[Int]())
// ... timeout3 seconds
val res3 = toFallback(future3, timeout3, Map[String, BigInt]()) 

def toFallback[T](f: Future[T], to: Int, default: T) = {
  Try(Await.result(f, to seconds))
    .recover { case to: TimeoutException => default }
}

正如我所看到的，该片段的最大等待时间是timeout1 + timeout2 + timeout3

我的问题是：我怎样才能同时等待所有这些期货，这样我就可以减少等待时间max(timeout1, timeout2, timeout3)?

编辑：最后我使用了 @Jatin 和 @senia 答案的修改：

private def composeWaitingFuture[T](fut: Future[T], 
                                    timeout: Int, default: T) =
  future { Await.result(fut, timeout seconds) } recover {
    case e: Exception => default
  }

后来它的使用如下：

// starts futures immediately and waits for maximum of timeoutX seconds
val res1 = composeWaitingFuture(future1, timeout1, Map[String, Int]())
val res2 = composeWaitingFuture(future2, timeout2, List[Int]())
val res3 = composeWaitingFuture(future3, timeout3, Map[String, BigInt]()) 

// takes the maximum of max(timeout1, timeout2, timeout3) to complete
val combinedFuture =
  for {
    r1 <- res1
    r2 <- res2
    r3 <- res3
  } yield (r1, r2, r3)

后来我用combinedFuture我认为合适。

你可以创建future返回所有 3 个 future 的结果flatMap或用于理解：

val combinedFuture =
  for {
    r1 <- future1
    r2 <- future2
    r3 <- future3
  } yield (r1, r2, r3)

val (r1, r2, r3) = Await.result(combinedFuture , Seq(timeout1, timeout2, timeout3).max)

如果您正在使用akka您可以在超时后使用默认值完成您的未来：

implicit class FutureHelper[T](f: Future[T]) extends AnyVal{
  import akka.pattern.after
  def orDefault(t: Timeout, default: => T)(implicit system: ActorSystem): Future[T] = {
    val delayed = after(t.duration, system.scheduler)(Future.successful(default))
    Future firstCompletedOf Seq(f, delayed)
  }
}

val combinedFuture =
  for {
    r1 <- future1.orDefault(timeout1, Map())
    r2 <- future2.orDefault(timeout2, List())
    r3 <- future3.orDefault(timeout3, Map())
  } yield (r1, r2, r3)

val (r1, r2, r3) = Await.result(combinedFuture , allowance + Seq(timeout1, timeout2, timeout3).max)