CODEWITHSUNDEEP
scala
user404345
user404345

Reputation:

Why is this Future not executed?

Given this code:

object Test {

    import scala.concurrent.ExecutionContext.Implicits.global

    def main(args: Array[String]): Unit = {
        val f: Future[String] = Future { "Test" }
        f.onComplete {
            case Success(name) => println(name)
            case Failure(t) => t.printStackTrace()
        }
    }
}

I see no output, however changing println to System.out.println works:

object Test {

    import scala.concurrent.ExecutionContext.Implicits.global

    def main(args: Array[String]): Unit = {
        val f: Future[String] = Future { "Test" }
        f.onComplete {
            case Success(name) => System.out.println(name)
            case Failure(t) => t.printStackTrace()
        }
    }
}

Also using println with a standard Thread also works:

object Test {

    def main(args: Array[String]): Unit = {
        val t = new Thread() {
            override def run(): Unit = println("Test")
        }
        t.start()
        t.join()
    }
}

Why is this?

Upvotes: 1

Views: 905

Answers (4)

user404345
user404345

Reputation:

So it seems that the answer lies in this answer. Scala's println is not an alias for System.out.println but Console.println which handles threads differently

Upvotes: 0

yanana
yanana

Reputation: 2331

You have to wait for the Future's completion. So your System.out.println version also might output or not.

object Test {

    import scala.concurrent.ExecutionContext.Implicits.global

    def main(args: Array[String]): Unit = {
        val f: Future[String] = Future { "Test" }
        f.onComplete {
            case Success(name) => println(name)
            case Failure(t) => t.printStackTrace()
        }
        Await.result(f)
    }
}

Upvotes: 4

Zernike
Zernike

Reputation: 1766

Future and onComplete execute in separated daemon threads. When your main thread terminates (after last instruction) daemon threads also terminate. In your case they have no time to execute code. Make your main code live long, for example, Thread.sleep(100).

Upvotes: 4

chengpohi
chengpohi

Reputation: 14217

Because your program has exited:

def main(args: Array[String]): Unit = {
    val f: Future[String] = Future { "Test" }
    f.onComplete {
      case Success(name) => println(name)
      case Failure(t) => t.printStackTrace()
    }
    Thread.sleep(100)
  }

Upvotes: 0

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