How to throw an exception in akka stream?

Question

I would like to throw an exception as the following:

  Source.empty
      .map {
        throw new RuntimeException("Failed")
      }
      .runWith(Sink.foreach(println))
      .onComplete {
        case Success(_) =>
          println()
        case Failure(e) =>
          println(s"Thrown ${e.getMessage}")
      }  

But the exception does not appears in the onComplete method. It prints

Exception in thread "main" java.lang.RuntimeException: Failed
    at com.sweetsoft.App$.main(App.scala:30)
    at com.sweetsoft.App.main(App.scala) 

How to throw an exception, that will stop the stream and appears at the end?

Answer 1

Akka has build in Error handling : Akka Supervision Strategies

val testSupervisionDecider: Supervision.Decider = {
        case ex: java.lang.RuntimeException =>
          println(s"some run time exception ${ex.getMessage}")
          Supervision.Stop
        case ex: Exception =>
          println("Exception occurred and stopping stream",
            ex)
          Supervision.Stop
      }

and you can use the supervision decider as

val list = List.range(1, 100)

  Source(list).map { item =>
    if ((item % 2) == 0) {
      throw new RuntimeException(s"$item")
    } else {
      item
    }
  }.withAttributes(ActorAttributes.supervisionStrategy(testSupervisionDecider))
    .runWith(Sink.foreach(println)).onComplete {
    case Success(_) =>
      println()
    case Failure(e) =>
      println(s"Thrown ${e.getMessage}")
  }