Sample Scala code for creating a list of futures then executing them in parallel then shutting down when they are complete.

conc1.scala

import java.util.concurrent.Executors

import scala.concurrent.{ExecutionContext, Future, Await}
import scala.concurrent.duration._


object conc1 {

  val numThreads = 4

  implicit val ec = new ExecutionContext {

    val threadPool = Executors.newFixedThreadPool(numThreads)

    def execute(runnable: Runnable) {
      threadPool.submit(runnable)
    }

    def reportFailure(t: Throwable) {}
  }

  val r = scala.util.Random

  def getThreadID = Thread.currentThread().getName()

  def slowRandom(n: Int)(implicit ec: ExecutionContext): Future[Unit] = {

    Future {
      Thread.sleep(2000)
      val a = r.nextInt(n)
      println(s"produced random number $a on thread ${getThreadID}")
    }

  }


  def main(args: Array[String]) : Unit  ={

    println("start")

    val f = (1 to 20) map {
      _ => slowRandom(1000)
    }

    val fs = Future.sequence(f)

    println("waiting for futures")

    Await.result(fs, 20 seconds)

    ec.threadPool.shutdown()

    println("end")



  }

}