scala - 回调中的 future 在同一线程中运行

Question

当我运行这段代码时:

val intList = List(1, 6, 8)

val listOfFutures = intList.map{
  i => future {
    println("future " +i)
    Thread.sleep(i* 1000)
    if (i == 12)
      throw new Exception("6 is not legal.")
    i
  }

}
val futureOfList = Future.sequence(listOfFutures)

futureOfList onSuccess{
  case int => {
    logInfo("onSuccess")
    int.foreach(i => {
      Future{
      logInfo("h : " + i)
      }
    })
  }
}

futureOfList onFailure{
  case int => {
    logInfo("onFailure")
  }
}

logInfo("after")

嵌套的 futures 都在同一个线程中运行:

future 1
2013/11/24 11:55:00.002 [ModelUtilsTest] [main]: INFO: [] after
future 6
future 8
2013/11/24 11:55:14.878 [ModelUtilsTest] [ForkJoinPool-1-worker-1]: INFO: [] onSuccess 
2013/11/24 11:55:15.378 [ModelUtilsTest] [ForkJoinPool-1-worker-1]: INFO: [] h : 1 
2013/11/24 11:55:15.378 [ModelUtilsTest] [ForkJoinPool-1-worker-1]: INFO: [] h : 6 
2013/11/24 11:55:15.378 [ModelUtilsTest] [ForkJoinPool-1-worker-1]: INFO: [] h : 8

为什么会这样？我需要内在的 future 并行运行。

Answer 1

Future 实现检查它是否要在拥有当前线程的 ForkJoinPool 上运行某些东西。在这种情况下，它将 ForkJoinTask.fork()，将新的工作单元添加到当前工作线程的工作队列中。任何其他线程都必须窃取该工作才能执行它。您不会期望在可用线程上看到任务的均匀分布，尤其是琐碎的任务。

在接下来的运行中，worker-9首当其冲:

scala> val f = Future sequence (1 to 20 map (i => Future(i)))
f: scala.concurrent.Future[scala.collection.immutable.IndexedSeq[Int]] = scala.concurrent.impl.Promise$DefaultPromise@47fe4ff8

scala> f onSuccess { case is => is foreach (i => Future(println(s"$i on ${Thread.currentThread}"))) }

scala> 2 on Thread[ForkJoinPool-1-worker-9,5,main]
5 on Thread[ForkJoinPool-1-worker-15,5,main]
4 on Thread[ForkJoinPool-1-worker-3,5,main]
6 on Thread[ForkJoinPool-1-worker-9,5,main]
1 on Thread[ForkJoinPool-1-worker-5,5,main]
12 on Thread[ForkJoinPool-1-worker-7,5,main]
11 on Thread[ForkJoinPool-1-worker-9,5,main]
15 on Thread[ForkJoinPool-1-worker-9,5,main]
16 on Thread[ForkJoinPool-1-worker-9,5,main]
17 on Thread[ForkJoinPool-1-worker-9,5,main]
18 on Thread[ForkJoinPool-1-worker-9,5,main]
19 on Thread[ForkJoinPool-1-worker-9,5,main]
20 on Thread[ForkJoinPool-1-worker-9,5,main]
3 on Thread[ForkJoinPool-1-worker-1,5,main]
10 on Thread[ForkJoinPool-1-worker-13,5,main]
9 on Thread[ForkJoinPool-1-worker-11,5,main]
8 on Thread[ForkJoinPool-1-worker-3,5,main]
7 on Thread[ForkJoinPool-1-worker-15,5,main]
14 on Thread[ForkJoinPool-1-worker-7,5,main]
13 on Thread[ForkJoinPool-1-worker-5,5,main]