Akka Actor life cycle

Akka provides life cycle methods for Actor. There are following methods to which you can override and provide specific implementation accordingly.

Akka Actor life cycle methods

preStart()
postStop()
preRestart(reason: Throwable, message: Option[Any])
postRestart(reason: Throwable)

1) preStart()

This is overridable method, so you can override preStart() method to provide specific implementation for an Actor. It is invoked right after the starting of Actor and when an Actor is first created. In case of restart, it is called by postRestart() method.

Akka Actor preStart() Method Example

import akka.actor._
import akka.actor.TypedActor.PreStart

class LifeCyleMethodsExample extends Actor{
  def receive = {
    case msg:String => println(msg+" "+self.path.name);  // Getting name of Actor
  }
  override def preStart(){    // overriding preStart method
    println("preStart method is called");
  }
}

object ActorMain{
  def main(args:Array[String]){
    val actorSystem = ActorSystem("ActorSystem");
    val actor = actorSystem.actorOf(Props[LifeCyleMethodsExample],"RootActor");
    actor ! "Hello"
  }
}

Output:

preStart method is called
Hello RootActor

2) postStop()

You can override this method also. After stopping an actor, postStop() method is called. It is an asynchronous method. This method is used to release resources after stopping the Actor. It may be used for deregistering this Actor. Messages sent to a stopped actor will be redirected to the deadLetters of the ActorSystem.

Akka Actor PostStop() Method Example

import akka.actor._
import akka.actor.TypedActor.PreStart

class LifeCyleMethodsExample extends Actor{
  def receive = {
    case msg:String => println(msg+" "+self.path.name);  // Getting name of Actor
  }
  override def postStop(){    // Overriding postStop method
    println("postStop method is called");
  }
}

object ActorMain{
  def main(args:Array[String]){
    val actorSystem = ActorSystem("ActorSystem");
    val actor = actorSystem.actorOf(Props[LifeCyleMethodsExample],"RootActor");
    actor ! "Hello"
    
    println("stopping Actor");
    actorSystem.stop(actor);      // Stopping Actor by passing actor reference. 
  }
}

Output:

Hello RootActor
stopping Actor
postStop method is called

3) preRestart()

Actor may be restarted in case an exception is thrown. When an actor is restarted, preRestart() method is invoked. The preRestart() method is called with the exception that caused the restart. By default it disposes of all children Actors and then calls postStop() method.

Akka Actor preRestart() Method Example

 println(msg+" "+self.path.name);  // Getting name of Actor
    var a:Int =  10/0;      // ArithmethicException occurred
  }
  override def preRestart(reason:Throwable, message: Option[Any]){    // Overriding preRestart method
    println("preRestart method is called");
    println("Reason: "+reason)
  }
}

object ActorMain{
  def main(args:Array[String]){
    val actorSystem = ActorSystem("ActorSystem");
    val actor = actorSystem.actorOf(Props[LifeCyleMethodsExample],"RootActor");
    actor ! "Hello"
  }
}

Output:

Hello RootActor
preRestart method is called
Reason: java.lang.ArithmeticException: / by zero

4) postRestart()

This method is invoked right after restarting of newly created Actor. It is used to allow reinitialization after an Actor crash due to exception.

Akka Actor postRestart() Method Example

import akka.actor._

class LifeCyleMethodsExample extends Actor{
  def receive = {
    case msg:String => println(msg+" "+self.path.name);  // Getting name of Actor
    var a:Int =  10/0;      // ArithmethicException occurred
  }
  override def postRestart(reason:Throwable){    // Overriding preRestart method
    println("postRestart method is called");
    println("Reason: "+reason)
  }
}

object ActorMain{
  def main(args:Array[String]){
    val actorSystem = ActorSystem("ActorSystem");
    val actor = actorSystem.actorOf(Props[LifeCyleMethodsExample],"RootActor");
    actor ! "Hello"
  }
}

Output

Hello RootActor
postRestart method is called
Reason: java.lang.ArithmeticException: / by zero

Akka Complete LifeCyle Example

import akka.actor.{Actor,ActorSystem, Props}

class RootActor extends Actor{
  def receive = {
    case msg => println("Message received: "+msg);
    10/0;
  }
  override def preStart(){
    super.preStart();
    println("preStart method is called");
  }
  override def postStop(){
    super.postStop();
    println("postStop method is called");
  }
  override def preRestart(reason:Throwable, message: Option[Any]){
    super.preRestart(reason, message);
    println("preRestart method is called");
    println("Reason: "+reason);
  }
  override def postRestart(reason:Throwable){
    super.postRestart(reason);
    println("postRestart is called");
    println("Reason: "+reason);
  }
}

Output:

preStart method is called
Message received: Hello
Stoping actor
postStop method is called
preRestart method is called
Reason: java.lang.ArithmeticException: / by zero
preStart method is called
postRestart is called
Reason: java.lang.ArithmeticException: / by zero
[ERROR] [01/12/2017 15:22:18.475] [ActorSystem-akka.actor.default-dispatcher-5] 
[akka://ActorSystem/user/RootActor] / by zero
java.lang.ArithmeticException: / by zero
	at RootActor$$anonfun$receive$1.applyOrElse(ActorLifeCyle.scala:6)
	at akka.actor.Actor$class.aroundReceive(Actor.scala:496)
	at RootActor.aroundReceive(ActorLifeCyle.scala:3)
	at akka.actor.ActorCell.receiveMessage(ActorCell.scala:526)
	at akka.actor.ActorCell.invoke(ActorCell.scala:495)
	at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:257)
	at akka.dispatch.Mailbox.run(Mailbox.scala:224)
	at akka.dispatch.Mailbox.exec(Mailbox.scala:234)
	at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
	at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
	at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
	at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)
	postStop method is called

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