What is the difference between Behaviors.setup and Behaviors.receive in Akka Typed?

Question

In the Akka tutorials i sometimes see

Behaviors.setup { ctx =>
 ...

 Behaviors.receiveMessage[String] { msg =>
  ... // some operations with ctx and message  
 }

}

and sometimes just

Behaviors.receive[String] { (ctx , msg) =>
  ... // some operations with ctx and message  
}

What is the difference?

Answer 1

The function (ActorContext[String] => Behavior[String]) passed to Behaviors.setup is executed when the actor is spawned, regardless of whether there's a message to process.

The function ((ActorContext[String], String) => Behavior[String]) passed to Behaviors.receive is not executed until there's a message to process.

Note that if you had

Behaviors.setup { ctx =>
  // code block A

  Behaviors.receiveMessage[String] { msg =>
    // code block B
  }
}

Behaviors.receive[String] { (ctx, msg) =>
  // code block A
  // code block B
}

code block A in the Behaviors.receive would execute with every message (unless and until a new Behavior was installed by code block B).