ActorSystem singleton injection

Question

In my project, I have to use actors and schedulers. All my need can be accomplished by using one actor system only. But I need the schedulers and actors in more than one class. So my question, if I should inject the actor system in each class like below, will it create only one actor System or more than one? I don't want to create more than one actor system since it is not a recommended practice.

import akka.actor.ActorSystem

@Singleton
class someClass @Inject()(actorSystem: ActorSystem){....} // abstract class ActorSystem extends ActorRefFactory

@Singleton
class anotherClass @Inject()(actorSystem: ActorSystem){....}

Or I should create another object and declare an actor system within it and use it everywhere like this:

import akka.actor._

object actorSystemObject {
    val system: ActorSystem = ActorSystem()
}

Which method will be better and standard?

Answer 1

Assuming you are using guice, try providing a singleton like so

@Provides
@Singleton
def getActorSystem: ActorSystem = ActorSystem()

for example

import akka.actor.ActorSystem
import com.google.inject.{AbstractModule, Guice, Inject, Injector, Provides, Singleton}
import scala.jdk.CollectionConverters._

class MyModule extends AbstractModule {
  @Provides
  @Singleton
  def getActorSystem: ActorSystem = ActorSystem()
}

@Singleton
class SomeClass @Inject()(actorSystem: ActorSystem) {
  println(actorSystem.hashCode())
}

@Singleton
class SomeOtherClass @Inject()(actorSystem: ActorSystem) {
  println(actorSystem.hashCode())
}

object Hello extends App {
  val injector: Injector = Guice.createInjector(List(new MyModule).asJava)
  injector.getInstance(classOf[SomeClass])
  injector.getInstance(classOf[SomeOtherClass])
}

which outputs something like

1731656333
1731656333

where we see the same ActorSystem is injected as evident by the same hashCode.

Say we remove @Singleton provider like so

@Provides
def getActorSystem: ActorSystem = ActorSystem()

then hashCodes differ, for example,

2050462663
1117871068