Scala的未来firstcompletedof超时

Question

I think I have a problem understanding firstCompletedOf. Given a completing future and a non completing future, I get a timeout when running the following for four times.

Given this method f:

def f = {
  val completingFuture = Future {
    1
  }
  val nonCompletingFuture = Future {
    while (true) {}
  }
  val combinedFuture = Future.firstCompletedOf(List(
   completingFuture,
    nonCompletingFuture
  ))
  val result = Await.result(combinedFuture, 10.seconds)
  println(result)
}


f
f
f
f

When I run it four times as shown above, I get a timed out exception. So I thought there cannot be such an exception because the completing future always returns.

The exception is

Exception in thread "main" java.util.concurrent.TimeoutException: Futures timed out after [10 seconds]

Where am I mistaken?

Answer 1

Your task while (true) {} never completes. It blocks a thread in an execution context forever. So every time you run f , you lose one thread in the execution context until it runs out of threads and you start receiving timeouts.