Why doesn't onComplete wait for Promise.success in the code in Scala?

Question

I'm reading about Futures and Promises in Scala and wrote the following code:

def printSomething(): Future[String] = {
  val p = Promise[String]
  val sayHello = future {
    Thread.sleep(1000)
    p.success("hello")
  }
  p.future
}

def main(args: Array[String]) {
  val something: Future[String] = printSomething()
  something onComplete {
    case Success(p) => println(p)
  }
}

The problem is the onComplete callback doesn't print anything (unless I debug it).

Wouldn't the onComplete have to wait for the p.success("hello") in the printSomething ?

I had to write `val sayHello = Future { ... }` to get this to compile. — Jean-Philippe Pellet, Nov 26 '13 at 15:59

score 6 · Accepted Answer · edited May 23 '17 at 11:51

My guess is that this has to do with the ExecutionContext you are using daemon threads and thus terminating when your main gets past the onComplete. If you add a sleep after the onComplete, you should get what you want. A slightly modified version of your code showing this:

import concurrent._
import ExecutionContext.Implicits._

object PromTest {
  def printSomething(): Future[String] = {
    val p = Promise[String]
    val sayHello = future {
      Thread.sleep(1000)
      p.success("hello")
    }
    p.future
  }

  def main(args: Array[String]) {
    val something: Future[String] = printSomething()
    something onComplete {
      case result => println(result)
    }
    Thread.sleep(2000)
  }
}

score 2 · Answer 2 · answered Nov 27 '13 at 09:57

As other people have stated, your execution terminates before your future has had a chance to run. I feel, however, that adding a hard-coded sleep call isn't the cleanest solution, and that waiting for the future to complete (with a timeout, to prevent your code from getting stuck) is preferable:

Await.ready(something, Duration(2000, MILLISECONDS))

This will wait at most 2000 milliseconds for something to be complete, or fail otherwise.

score 1 · Answer 3 · answered Nov 26 '13 at 15:58

1

Your app exits before your promise is completed. At the end of your main method, just add something like Thread.sleep(2000) and you'll see your result.

answered Nov 26 '13 at 15:58

Jean-Philippe Pellet

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Why doesn't onComplete wait for Promise.success in the code in Scala?

3 Answers3