Scala future 超时奇怪的行为

Question

这是示例代码 scala 2.13.3:

import scala.concurrent.{ExecutionContext}

val scheduller: ScheduledExecutorService = Executors.newSingleThreadScheduledExecutor()

val promise = Promise[A]()
scheduller.schedule(
  new Runnable {
    override def run(): Unit = {
      promise.failure(new TimeoutException)
      scheduller.shutdown()
    }
  },
  500,
  TimeUnit.MILLISECONDS
)
Future.firstCompletedOf(Seq(INF_FUTURE, promise.future))

如果 INF_FUTURE = Future.never，超时将起作用但如果 INF_FUTURE = Future {while (true) {}} 执行不会停止
你能解释一下这里发生了什么吗？

Answer 1

那是因为你用了singleThreadScheduledExecutor .我假设您创建了 ExecutionContext来自 scheduller :

val scheduller: ScheduledExecutorService = Executors.newSingleThreadScheduledExecutor()
implicit val ec: ExecutionContextExecutor = scala.concurrent.ExecutionContext.fromExecutorService(scheduller)

val promise = Promise[Int]()
scheduller.schedule(
  new Runnable {
    override def run(): Unit = {
      promise.failure(new TimeoutException)
      scheduller.shutdown()
    }
  },
  500,
  TimeUnit.MILLISECONDS
)

如果您仅使用一个执行程序创建执行上下文，您的程序将在 while (true) 内评估同步和卡住。环形。
如果您将切换到全局执行上下文，问题将被忽略(如果您的机器有多个内核):

implicit val ec: ExecutionContextExecutor = scala.concurrent.ExecutionContext.global

在 Future.never 的情况下如果你会像这样等待结果:

Await.result(Future.firstCompletedOf(Seq(INF_FUTURE, promise.future)).map {
  x =>
    println(x)
}, 10.seconds)

你会得到异常:

Exception in thread "main"java.util.concurrent.RejectedExecutionException: Taskjava.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask@eb88a9dfrejected fromjava.util.concurrent.ScheduledThreadPoolExecutor@e24c7ced[Shuttingdown, pool size = 1, active threads = 1, queued tasks = 0, completedtasks = 1]

这会告诉您线程池大小的问题。
所以，您对 Future.never 没有任何问题因为你没有等待它的结果。