Scala特征的隐式Akka系统

Question

How can I make my ActorSystem available in ServiceA and ServiceB , this is the code im trying to get get working. It compile but i get a null pointer exception for the implicit system in Tcp .

Why do I get a nullpointer exception and what can i change to get my system into my trait services? Why does it compile?

trait ServiceA {
  implicit val system: ActorSystem
}

trait ServiceB {
  implicit val system: ActorSystem
  Tcp().outgo... // Code compile but i get nullpointer exception
}

object Main extends App with ServiceA with ServiceB {
  override implicit val system: ActorSystem = ActorSystem("MySys")
}

Answer 1

Constructors of SystemA and SystemB (which uses system ) are executed before constructor of Main which includes initialization of system . Use implicit lazy val system = ... in Main (it is allowed to implement an abstract val in this way) and this problem should be fixed.

Answer 2

As an addition to answer by @alexei_romanov . What do you want to achieve by placing Tcp... call in a trait body without assigning it to val or def ? It is now just an action which is executed at some moment during initialization of Main . It can leat to undefined behavior since it is an action on not yet initialized object.

Answer 3

This is caused by the trait initialization order, and it is indeed unfortunate that the type system cannot defend against this.

One option to resolve this is to put the ActorSystem initialization into its own trait and make sure that one gets mixed in first, so:

trait ServiceA {
  implicit val system: ActorSystem
}

trait ServiceB {
  implicit val system: ActorSystem
  Tcp().outgo... // Code compile but i get nullpointer exception
}

trait ActorSystemInitialization {
  val system: ActorSystem = ActorSystem("MySys")
}

object Main extends App with ActorSystemInitialization with ServiceA with ServiceB {

}