未来不完整？

object Executor extends App {
  implicit val system = ActorSystem()
  implicit val materializer = ActorMaterializer()
  implicit val ec = system.dispatcher
  import akka.stream.io._
  val file = new File("res/AdviceAnimals.tsv")
  import akka.stream.io.Implicits._
  val foreach: Future[Long] = SynchronousFileSource(file)
    .to( Sink.outputStream(()=>System.out))
    .run()

  foreach onComplete { v =>
    println(s"the foreach is ${v.get}")  // the will not be print
  }
}

但如果我改变Sink.outputStream(()=>System.out) to Sink.ignore, the println(s"the foreach is ${v.get}")将打印。

有人可以解释为什么吗？

您不会等待流完成，相反，您的 main 方法（Executor 的主体）将完成，并且由于 main 方法完成退出，JVM 将关闭。

您想要做的是阻止该线程，并且在未来完成之前不退出应用程序。

object Executor extends App {
  // ...your stuff with streams...
  val yourFuture: Future[Long] = ???

  val result = Await.result(yourFuture, 5 seconds)
  println(s"the foreach is ${result}")

  // stop the actor system (or it will keep the app alive)
  system.terminate()
}