Why does this simple Akka Streams program never terminate?

[Matthew Adams]

NB:  posted at http://stackoverflow.com/questions/35583739/why-does-this-simple-akka-streams-program-never-terminate before I thought to post here.

Should be a simple question. I'm using Akka 2.4.2 (contains Akka Streams & HTTP). I expected this Source to complete & the program to terminate because the Source is finite, but it never does. Why doesn't this program terminate?

import scala.concurrent._

import scala.collection.immutable._

import akka._

import akka.actor._

import akka.stream._

import akka.stream.scaladsl._

import akka.util._

object Test extends App {

  implicit val system = ActorSystem("TestSystem")

  implicit val materializer = ActorMaterializer()

  val s = Source.single(1)

  s.runForeach(println)

}

Output:

$ sbt run

...

[info] Running Test

[DEBUG] [02/23/2016 10:59:19.904] [run-main-0] [EventStream(akka://TestSystem)] logger log1-Logging$DefaultLogger started

[DEBUG] [02/23/2016 10:59:19.904] [run-main-0] [EventStream(akka://TestSystem)] Default Loggers started

1

Relevant portion of my build.sbt file:

scalaVersion  := "2.11.7"

scalacOptions := Seq("-unchecked", "-deprecation", "-encoding", "utf8")

libraryDependencies ++= {

  val akkaVersion = "2.4.2"

  Seq(

    "com.typesafe.akka" %%  "akka-stream" % akkaVersion

  )

}

-matthew

[Konrad Malawski]

Because Akka uses non-daemonic threads, so it'll keep the app running until you shut-down the ActorSystem.

You can do so via:

import system.dispatcher

s.runForeach(println).onComplete { _ =>  system.terminate() }

-- 

Cheers,

Konrad 'ktoso’ Malawski
Akka @ Lightbend

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[Matthew Adams]

That was it.  Good to know.  Thanks.

-matthew

[Endre Varga]

Also, the stream itself runs in a separate thread than main(), so runForeach returns before the stream has been processed.

-Endre