Akka-http - write to response output stream

import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.http.scaladsl.model._
import akka.stream.ActorMaterializer

import scala.io.StdIn

object Main extends App {
  implicit val system = ActorSystem()
  implicit val materializer = ActorMaterializer()
  implicit val executionContext = system.dispatcher

  val route =
    path("hello") {
      get {
        val responseEntity = HttpEntity(ContentTypes.`text/plain(UTF-8)`, "Hello, World!")
        complete(HttpResponse(entity = responseEntity))
      }
    }

  val bindingFuture = Http().bindAndHandle(route, "localhost", 8080)

  println("Server online at http://localhost:8080/\nPress RETURN to stop...")
  StdIn.readLine()
  bindingFuture
    .flatMap(_.unbind())
    .onComplete(_ => system.terminate())
}