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scala - Future map 和 foreach 的行为

问题描述

我有以下代码在地图和未来打印当前线程名称

object ConcurrencyTest1 {
  def main(args: Array[String]) {
    import scala.concurrent.ExecutionContext.Implicits.global
    import scala.concurrent.Future
    println("main thread: " + Thread.currentThread().getName)


    Future {
      println("future job: " + Thread.currentThread().getName)
      Thread.sleep(1000)
      10
    }.map { x => {
      println("map: " + Thread.currentThread().getName)
      Thread.sleep(1000)
      x * x
    }
    }.foreach { x => {
      Thread.sleep(1000)
      println("foreach: " + Thread.currentThread().getName)
      println(x)
    }
    }

    Thread.sleep(5000)
  }
}

输出是：

main thread: main
future job: ForkJoinPool-1-worker-5
map: ForkJoinPool-1-worker-5
foreach: ForkJoinPool-1-worker-5

从输出来看，future job、map 和 foreach 都在同一个线程中运行。我想问这个结果是否是确定性的，即它总是会输出相同的结果。或者，它们可能在不同的线程中运行

标签： scala

解决方案

我不认为这是保证...

但是，在您的情况下，我认为这是来自Thread.sleep调用（当它们完成时，等待处理的下一个 Future 是后续的 map/foreach；其他 Futures 已经在其他可用线程上启动）。

您可以通过删除Thread.sleep调用来看到这一点：

注意：为了获得更多数据，这会遍历许多 args。

object ConcurrencyTestNoSleep {
  def main(args: Array[String]) {
    import scala.concurrent.ExecutionContext.Implicits.global
    import scala.concurrent.Future
    println("main thread: " + Thread.currentThread().getName)

    args.map { a =>
      Future {
        println(s"future job $a: " + Thread.currentThread().getName)
        // Thread.sleep(1)
        10
      }.map { x => {
        println(s"map $a: " + Thread.currentThread().getName)
        // Thread.sleep(1)
        x * x
      }
      }.foreach { x => {
        // Thread.sleep(1)
        println(s"foreach $a:" + Thread.currentThread().getName)
        // println(x)
      }
    }
  }
  Thread.sleep(5000)
  }
}

运行时：

@ ConcurrencyTestNoSleep.main(Array[String]("a", "b", "c", "d", "e", "f", "g") )
main thread: main
future job a: scala-execution-context-global-954
map a: scala-execution-context-global-954
foreach a:scala-execution-context-global-955
future job c: scala-execution-context-global-955
map c: scala-execution-context-global-955
foreach c:scala-execution-context-global-955
future job d: scala-execution-context-global-955
map d: scala-execution-context-global-955
foreach d:scala-execution-context-global-955
future job e: scala-execution-context-global-955
map e: scala-execution-context-global-955
foreach e:scala-execution-context-global-955
future job f: scala-execution-context-global-955
map f: scala-execution-context-global-955
foreach f:scala-execution-context-global-955
future job g: scala-execution-context-global-955
map g: scala-execution-context-global-955
foreach g:scala-execution-context-global-955
future job b: scala-execution-context-global-954
map b: scala-execution-context-global-954
foreach b:scala-execution-context-global-954

您可以在第三行中看到，这次"a"它跨越了两个不同的线程。

但是，随着睡眠，它似乎确实重用了相同的线程：

object ConcurrencyTestSleep {
  def main(args: Array[String]) {
    import scala.concurrent.ExecutionContext.Implicits.global
    import scala.concurrent.Future
    println("main thread: " + Thread.currentThread().getName)

    args.map { a =>
      Future {
        println(s"future job $a: " + Thread.currentThread().getName)
        Thread.sleep(1)
        10
      }.map { x => {
        println(s"map $a: " + Thread.currentThread().getName)
        Thread.sleep(1)
        x * x
      }
      }.foreach { x => {
        Thread.sleep(1)
        println(s"foreach $a:" + Thread.currentThread().getName)
        // println(x)
      }
    }
  }
  Thread.sleep(5000)
  }
}

这里每个 arg 使用相同的线程（在 map 和 foreach 中）：

@ ConcurrencyTestSleep.main(Array[String]("a", "b", "c", "d", "e", "f", "g") )
main thread: main
future job a: scala-execution-context-global-1162
map a: scala-execution-context-global-1162
future job c: scala-execution-context-global-1164
future job b: scala-execution-context-global-1163
future job d: scala-execution-context-global-1165
map b: scala-execution-context-global-1163
map c: scala-execution-context-global-1164
map d: scala-execution-context-global-1165
foreach a:scala-execution-context-global-1162
future job e: scala-execution-context-global-1162
foreach b:scala-execution-context-global-1163
foreach d:scala-execution-context-global-1165
future job g: scala-execution-context-global-1165
foreach c:scala-execution-context-global-1164
map e: scala-execution-context-global-1162
future job f: scala-execution-context-global-1163
map g: scala-execution-context-global-1165
map f: scala-execution-context-global-1163
foreach e:scala-execution-context-global-1162
foreach f:scala-execution-context-global-1162
foreach g:scala-execution-context-global-1165

也就是说，我不会依赖这个！

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