Java API: compatible with lambda expressions

Actor base class that should be extended to create Java actors that use lambdas.

Example:

public class MyActorForJavaDoc extends AbstractActor{
   @Override
   public Receive createReceive() {
       return receiveBuilder()
               .match(Double.class, d -> {
                   sender().tell(d.isNaN() ? 0 : d, self());
               })
               .match(Integer.class, i -> {
                   sender().tell(i * 10, self());
               })
               .match(String.class, s -> s.startsWith("foo"), s -> {
                   sender().tell(s.toUpperCase(), self());
               })
               .build();
   }
}

Java API: compatible with lambda expressions

Actor base class that should be extended to create an actor with a stash.

The stash enables an actor to temporarily stash away messages that can not or should not be handled using the actor's current behavior.

Example:

public class MyActorWithStash extends AbstractActorWithStash {
  int count = 0;

  public MyActorWithStash() {
    receive(ReceiveBuilder. match(String.class, s -> {
      if (count < 0) {
        sender().tell(new Integer(s.length()), self());
      } else if (count == 2) {
        count = -1;
        unstashAll();
      } else {
        count += 1;
        stash();
      }}).build()
    );
  }
}

  akka.actor.mailbox.requirements {
    "akka.dispatch.BoundedDequeBasedMessageQueueSemantics" = your-custom-mailbox
  }

For a Stash based actor that enforces unbounded deques see akka.actor.AbstractActorWithUnboundedStash. There is also an unrestricted version akka.actor.AbstractActorWithUnrestrictedStash that does not enforce the mailbox type.

Java API: Support for scheduled self messages via TimerScheduler.

Timers are bound to the lifecycle of the actor that owns it, and thus are cancelled automatically when it is restarted or stopped.

Java API: compatible with lambda expressions

Actor base class with Stash that enforces an unbounded deque for the actor. The proper mailbox has to be configured manually, and the mailbox should extend the akka.dispatch.DequeBasedMessageQueueSemantics marker trait. See akka.actor.AbstractActorWithStash for details on how Stash works.

Java API: compatible with lambda expressions

Actor base class with Stash that does not enforce any mailbox type. The mailbox of the actor has to be configured manually. See akka.actor.AbstractActorWithStash for details on how Stash works.

Java API: compatible with lambda expressions

Finite State Machine actor abstract base class.

Java API: compatible with lambda expressions

Finite State Machine actor abstract base class with Stash support.

Java API: compatible with lambda expressions

Actor base class that mixes in logging into the Actor.

Java API: compatible with lambda expressions

Finite State Machine actor abstract base class.

An Akka scheduler service. This one needs one special behavior: if Closeable, it MUST execute all outstanding tasks upon .close() in order to properly shutdown all dispatchers.

Furthermore, this timer service MUST throw IllegalStateException if it cannot schedule a task. Once scheduled, the task MUST be executed. If executed upon close(), the task may execute before its timeout.

Scheduler implementation are loaded reflectively at ActorSystem start-up with the following constructor arguments: 1) the system’s com.typesafe.config.Config (from system.settings.config) 2) a akka.event.LoggingAdapter 3) a java.util.concurrent.ThreadFactory

Actor base trait that should be extended by or mixed to create an Actor with the semantics of the 'Actor Model': https://en.wikipedia.org/wiki/Actor_model

An actor has a well-defined (non-cyclic) life-cycle.

• RUNNING (created and started actor) - can receive messages
• SHUTDOWN (when 'stop' is invoked) - can't do anything

The Actor's own akka.actor.ActorRef is available as self, the current message’s sender as sender() and the akka.actor.ActorContext as context. The only abstract method is receive which shall return the initial behavior of the actor as a partial function (behavior can be changed using context.become and context.unbecome).

This is the Scala API (hence the Scala code below), for the Java API see akka.actor.AbstractActor.

class ExampleActor extends Actor {

  override val supervisorStrategy = OneForOneStrategy(maxNrOfRetries = 10, withinTimeRange = 1 minute) {
    case _: ArithmeticException      => Resume
    case _: NullPointerException     => Restart
    case _: IllegalArgumentException => Stop
    case _: Exception                => Escalate
  }

  def receive = {
                                     // directly calculated reply
    case Request(r)               => sender() ! calculate(r)

                                     // just to demonstrate how to stop yourself
    case Shutdown                 => context.stop(self)

                                     // error kernel with child replying directly to 'sender()'
    case Dangerous(r)             => context.actorOf(Props[ReplyToOriginWorker]).tell(PerformWork(r), sender())

                                     // error kernel with reply going through us
    case OtherJob(r)              => context.actorOf(Props[ReplyToMeWorker]) ! JobRequest(r, sender())
    case JobReply(result, orig_s) => orig_s ! result
  }
}

The last line demonstrates the essence of the error kernel design: spawn one-off actors which terminate after doing their job, pass on sender() to allow direct reply if that is what makes sense, or round-trip the sender as shown with the fictitious JobRequest/JobReply message pair.

If you don’t like writing context you can always import context._ to get direct access to actorOf, stop etc. This is not default in order to keep the name-space clean.

The actor context - the view of the actor cell from the actor. Exposes contextual information for the actor and the current message.

There are several possibilities for creating actors (see akka.actor.Props for details on props):

// Java or Scala
context.actorOf(props, "name")
context.actorOf(props)

// Scala
context.actorOf(Props[MyActor])
context.actorOf(Props(classOf[MyActor], arg1, arg2), "name")

// Java
getContext().actorOf(Props.create(MyActor.class));
getContext().actorOf(Props.create(MyActor.class, arg1, arg2), "name");

Where no name is given explicitly, one will be automatically generated.

Reply to akka.actor.Identify. Contains Some(ref) with the ActorRef of the actor replying to the request or None if no actor matched the request. The correlationId is taken from the messageId in the Identify message.

An ActorInitializationException is thrown when the initialization logic for an Actor fails.

There is an extractor which works for ActorInitializationException and its subtypes:

ex match {
  case ActorInitializationException(actor, message, cause) => ...
}

When an InterruptedException is thrown inside an Actor, it is wrapped as an ActorInterruptedException as to avoid cascading interrupts to other threads than the originally interrupted one.

ActorKilledException is thrown when an Actor receives the akka.actor.Kill message

Scala API: Mix in ActorLogging into your Actor to easily obtain a reference to a logger, which is available under the name "log".

class MyActor extends Actor with ActorLogging {
  def receive = {
    case "pigdog" => log.info("We've got yet another pigdog on our hands")
  }
}

When ActorSelection#resolveOne can't identify the actor the Future is completed with this failure.

Actor path is a unique path to an actor that shows the creation path up through the actor tree to the root actor.

ActorPath defines a natural ordering (so that ActorRefs can be put into collections with this requirement); this ordering is intended to be as fast as possible, which owing to the bottom-up recursive nature of ActorPath is sorted by path elements FROM RIGHT TO LEFT, where RootActorPath > ChildActorPath in case the number of elements is different.

Two actor paths are compared equal when they have the same name and parent elements, including the root address information. That does not necessarily mean that they point to the same incarnation of the actor if the actor is re-created with the same path. In other words, in contrast to how actor references are compared the unique id of the actor is not taken into account when comparing actor paths.

Immutable and serializable handle to an actor, which may or may not reside on the local host or inside the same akka.actor.ActorSystem. An ActorRef can be obtained from an akka.actor.ActorRefFactory, an interface which is implemented by ActorSystem and akka.actor.ActorContext. This means actors can be created top-level in the ActorSystem or as children of an existing actor, but only from within that actor.

ActorRefs can be freely shared among actors by message passing. Message passing conversely is their only purpose, as demonstrated in the following examples:

Scala:

import akka.pattern.ask
import scala.concurrent.Await

class ExampleActor extends Actor {
  val other = context.actorOf(Props[OtherActor], "childName") // will be destroyed and re-created upon restart by default

  def receive {
    case Request1(msg) => other ! refine(msg)     // uses this actor as sender reference, reply goes to us
    case Request2(msg) => other.tell(msg, sender()) // forward sender reference, enabling direct reply
    case Request3(msg) =>
      implicit val timeout = Timeout(5.seconds)
      (other ? msg) pipeTo sender()
      // the ask call will get a future from other's reply
      // when the future is complete, send its value to the original sender
  }
}

Java:

import static akka.pattern.Patterns.ask;
import static akka.pattern.Patterns.pipe;

public class ExampleActor extends AbstractActor {
  // this child will be destroyed and re-created upon restart by default
  final ActorRef other = getContext().actorOf(Props.create(OtherActor.class), "childName");
  @Override
  public Receive createReceive() {
    return receiveBuilder()
      .match(Request1.class, msg ->
        // uses this actor as sender reference, reply goes to us
        other.tell(msg, getSelf()))
      .match(Request2.class, msg ->
        // forward sender reference, enabling direct reply
        other.tell(msg, getSender()))
      .match(Request3.class, msg ->
        // the ask call will get a future from other's reply
        // when the future is complete, send its value to the original sender
        pipe(ask(other, msg, 5000), context().dispatcher()).to(getSender()))
      .build();
  }
}

ActorRef does not have a method for terminating the actor it points to, use akka.actor.ActorRefFactory.stop(ref), or send a akka.actor.PoisonPill, for this purpose.

Two actor references are compared equal when they have the same path and point to the same actor incarnation. A reference pointing to a terminated actor doesn't compare equal to a reference pointing to another (re-created) actor with the same path.

If you need to keep track of actor references in a collection and do not care about the exact actor incarnation you can use the ActorPath as key because the unique id of the actor is not taken into account when comparing actor paths.

Interface implemented by ActorSystem and ActorContext, the only two places from which you can get fresh actors.

Interface for all ActorRef providers to implement. Not intended for extension outside of Akka.

An ActorSelection is a logical view of a section of an ActorSystem's tree of Actors, allowing for broadcasting of messages to that section.

An actor system is a hierarchical group of actors which share common configuration, e.g. dispatchers, deployments, remote capabilities and addresses. It is also the entry point for creating or looking up actors.

There are several possibilities for creating actors (see akka.actor.Props for details on props):

// Java or Scala
system.actorOf(props, "name")
system.actorOf(props)

// Scala
system.actorOf(Props[MyActor], "name")
system.actorOf(Props(classOf[MyActor], arg1, arg2), "name")

// Java
system.actorOf(Props.create(MyActor.class), "name");
system.actorOf(Props.create(MyActor.class, arg1, arg2), "name");

Where no name is given explicitly, one will be automatically generated.

Important Notice:

This class is not meant to be extended by user code. If you want to actually roll your own Akka, it will probably be better to look into extending akka.actor.ExtendedActorSystem instead, but beware that you are completely on your own in that case!

The address specifies the physical location under which an Actor can be reached. Examples are local addresses, identified by the ActorSystem’s name, and remote addresses, identified by protocol, host and port.

This class is final to allow use as a case class (copy method etc.); if for example a remote transport would want to associate additional information with an address, then this must be done externally.

Subscribe to this class to be notified about all DeadLetter (also the suppressed ones) and Dropped.

Not for user extension

Applies the fault handling Directive (Resume, Restart, Stop) specified in the Decider to all children when one fails, as opposed to akka.actor.OneForOneStrategy that applies it only to the child actor that failed.

ChildRestartStats is the statistics kept by every parent Actor for every child Actor and is used for SupervisorStrategies to know how to deal with problems that occur for the children.

Glue API introduced to allow minimal user effort integration between classic and typed for example for streams.

Not for user extension.

Glue API introduced to allow minimal user effort integration between classic and typed for example for streams.

Not for user extension.

When a message is sent to an Actor that is terminated before receiving the message, it will be sent as a DeadLetter to the ActorSystem's EventStream.

When this message was sent without a sender ActorRef, sender will be system.deadLetters.

Use with caution: Messages extending this trait will not be logged by the default dead-letters listener. Instead they will be wrapped as SuppressedDeadLetter and may be subscribed for explicitly.

A DeathPactException is thrown by an Actor that receives a Terminated(someActor) message that it doesn't handle itself, effectively crashing the Actor and escalating to the supervisor.

This class represents deployment configuration for a given actor path. It is marked final in order to guarantee stable merge semantics (i.e. what overrides what in case multiple configuration sources are available) and is fully extensible via its Scope argument, and by the fact that an arbitrary Config section can be passed along with it (which will be merged when merging two Deploys).

The path field is used only when inserting the Deploy into a deployer and not needed when just doing deploy-as-you-go:

val remoteProps = someProps.withDeploy(Deploy(scope = RemoteScope("someOtherNodeName")))

Scala API: Mix in DiagnosticActorLogging into your Actor to easily obtain a reference to a logger with MDC support, which is available under the name "log". In the example bellow "the one who knocks" will be available under the key "iam" for using it in the logback pattern.

class MyActor extends Actor with DiagnosticActorLogging {

  override def mdc(currentMessage: Any): MDC = {
    Map("iam", "the one who knocks")
  }

  def receive = {
    case "pigdog" => log.info("We've got yet another pigdog on our hands")
  }
}

Envelope that is published on the eventStream wrapped in akka.actor.DeadLetter for every message that is dropped due to overfull queues or routers with no routees.

When this message was sent without a sender ActorRef, sender will be ActorRef.noSender, i.e. null.

The DynamicAccess implementation is the class which is used for loading all configurable parts of an actor system (the akka.actor.ReflectiveDynamicAccess is the default implementation).

This is an internal facility and users are not expected to encounter it unless they are extending Akka in ways which go beyond simple Extensions.

Not for user extension

More powerful interface to the actor system’s implementation which is presented to extensions (see akka.actor.Extension).

Important Notice:

This class is not meant to be extended by user code. If you want to actually roll your own Akka, beware that you are completely on your own in that case!

The basic ActorSystem covers all that is needed for locally running actors, using futures and so on. In addition, more features can hook into it and thus become visible to actors et al by registering themselves as extensions. This is accomplished by providing an extension—which is an object implementing this trait—to ActorSystem.registerExtension(...) or by specifying the corresponding option in the configuration passed to ActorSystem, which will then instantiate (without arguments) each FQCN and register the result.

The extension itself can be created in any way desired and has full access to the ActorSystem implementation.

This trait is only a marker interface to signify an Akka Extension. This is how an extension is normally constructed.

Scala API:

object MyExt extends ExtensionId[Ext] with ExtensionIdProvider {

  override def lookup = MyExt

  override def createExtension(system: ExtendedActorSystem): Ext = new Ext(system)

  // Java API: retrieve the extension for the given system.
  override def get(system: ActorSystem): UdpExt = super.get(system)
  override def get(system: ClassicActorSystemProvider): UdpExt = super.get(system)
}

class Ext(system: ExtendedActorSystem) extends Extension {
  ...
}

Java API:

public class MyExt extends AbstractExtensionId<MyExtImpl>
  implements ExtensionIdProvider {
  public final static MyExt MyExtProvider = new MyExt();

  private MyExt() {}

  public MyExt lookup() {
    return MyExt.MyExtProvider;
  }

  public MyExtImpl createExtension(ExtendedActorSystem system) {
    return new MyExtImpl();
  }
}

public class MyExtImpl implements Extension {
   ...
}

To be able to load an ExtensionId from the configuration, a class that implements ExtensionIdProvider must be specified. The lookup method should return the canonical reference to the extension.

Finite State Machine actor trait. Use as follows:

  object A {
    trait State
    case class One extends State
    case class Two extends State

    case class Data(i : Int)
  }

  class A extends Actor with FSM[A.State, A.Data] {
    import A._

    startWith(One, Data(42))
    when(One) {
        case Event(SomeMsg, Data(x)) => ...
        case Event(SomeOtherMsg, _) => ... // when data not needed
    }
    when(Two, stateTimeout = 5 seconds) { ... }
    initialize()
  }

Within the partial function the following values are returned for effecting state transitions:

• stay for staying in the same state
• stay using Data(...) for staying in the same state, but with different data
• stay forMax 5.millis for staying with a state timeout; can be combined with using
• goto(...) for changing into a different state; also supports using and forMax
• stop for terminating this FSM actor

Each of the above also supports the method replying(AnyRef) for sending a reply before changing state.

While changing state, custom handlers may be invoked which are registered using onTransition. This is meant to enable concentrating different concerns in different places; you may choose to use when for describing the properties of a state, including of course initiating transitions, but you can describe the transitions using onTransition to avoid having to duplicate that code among multiple paths which lead to a transition:

onTransition {
  case Active -> _ => cancelTimer("activeTimer")
}

Multiple such blocks are supported and all of them will be called, not only the first matching one.

Another feature is that other actors may subscribe for transition events by sending a SubscribeTransitionCallback message to this actor. Stopping a listener without unregistering will not remove the listener from the subscription list; use UnsubscribeTransitionCallback before stopping the listener.

State timeouts set an upper bound to the time which may pass before another message is received in the current state. If no external message is available, then upon expiry of the timeout a StateTimeout message is sent. Note that this message will only be received in the state for which the timeout was set and that any message received will cancel the timeout (possibly to be started again by the next transition).

Another feature is the ability to install and cancel single-shot as well as repeated timers which arrange for the sending of a user-specified message:

  setTimer("tock", TockMsg, 1 second, true) // repeating
  setTimer("lifetime", TerminateMsg, 1 hour, false) // single-shot
  cancelTimer("tock")
  isTimerActive("tock")

A message all Actors will understand, that when processed will reply with akka.actor.ActorIdentity containing the ActorRef. The messageId is returned in the ActorIdentity message as correlationId.

IllegalActorStateException is thrown when a core invariant in the Actor implementation has been violated. For instance, if you try to create an Actor that doesn't extend Actor.

This interface defines a class of actor creation strategies deviating from the usual default of just reflectively instantiating the Actor subclass. It can be used to allow a dependency injection framework to determine the actual actor class and how it shall be instantiated.

An InvalidActorNameException is thrown when you try to convert something, usually a String, to an Actor name which doesn't validate.

InvalidMessageException is thrown when an invalid message is sent to an Actor; Currently only null is an invalid message.

This scheduler implementation is based on a revolving wheel of buckets, like Netty’s HashedWheelTimer, which it advances at a fixed tick rate and dispatches tasks it finds in the current bucket to their respective ExecutionContexts. The tasks are held in TaskHolders, which upon cancellation null out their reference to the actual task, leaving only this shell to be cleaned up when the wheel reaches that bucket next time. This enables the use of a simple linked list to chain the TaskHolders off the wheel.

Also noteworthy is that this scheduler does not obtain a current time stamp when scheduling single-shot tasks, instead it always rounds up the task delay to a full multiple of the TickDuration. This means that tasks are scheduled possibly one tick later than they could be (if checking that “now() + delay <= nextTick” were done).

Stackable trait for akka.actor.FSM which adds a rolling event log and debug logging capabilities (analogous to akka.event.LoggingReceive).

This is the default value and as such allows overrides.

Applies the fault handling Directive (Resume, Restart, Stop) specified in the Decider to the child actor that failed, as opposed to akka.actor.AllForOneStrategy that applies it to all children.

A PostRestartException is thrown when constructor or postRestart() method fails during a restart attempt.

A PreRestartException is thrown when the preRestart() method failed; this exception is not propagated to the supervisor, as it originates from the already failed instance, hence it is only visible as log entry on the event stream.

Props is a configuration object using in creating an Actor; it is immutable, so it is thread-safe and fully shareable.

Examples on Scala API:

val props = Props.empty
val props = Props[MyActor]
val props = Props(classOf[MyActor], arg1, arg2)

val otherProps = props.withDispatcher("dispatcher-id")
val otherProps = props.withDeploy(<deployment info>)

Examples on Java API:

final Props props = Props.empty();
final Props props = Props.create(MyActor.class, arg1, arg2);

final Props otherProps = props.withDispatcher("dispatcher-id");
final Props otherProps = props.withDeploy(<deployment info>);

This is the default akka.actor.DynamicAccess implementation used by akka.actor.ExtendedActorSystem unless overridden. It uses reflection to turn fully-qualified class names into Class[_] objects and creates instances from there using getDeclaredConstructor() and invoking that. The class loader to be used for all this is determined by the actor system’s class loader by default.

Not for user extension or construction

Root of the hierarchy of ActorPaths. There is exactly root per ActorSystem and node (for remote-enabled or clustered systems).

An Akka scheduler service.

For scheduling within actors with Timers should be preferred.

Please note that this scheduler implementation is highly optimised for high-throughput and high-frequency events. It is not to be confused with long-term schedulers such as Quartz. The scheduler will throw an exception if attempts are made to schedule too far into the future (which by default is around 8 months (Int.MaxValue seconds).

It's possible to implement a custom Scheduler, although that should rarely be needed.

A Scheduler implementation needs one special behavior: if Closeable, it MUST execute all outstanding tasks that implement Scheduler.TaskRunOnClose upon .close() in order to properly shutdown all dispatchers.

Furthermore, this timer service MUST throw IllegalStateException if it cannot schedule a task. Once scheduled, the task MUST be executed. If executed upon close(), the task may execute before its timeout.

Scheduler implementation are loaded reflectively at ActorSystem start-up with the following constructor arguments: 1) the system’s com.typesafe.config.Config (from system.settings.config) 2) a akka.event.LoggingAdapter 3) a java.util.concurrent.ThreadFactory

The scope of a akka.actor.Deploy serves two purposes: as a marker for pattern matching the “scope” (i.e. local/remote/cluster) as well as for extending the information carried by the final Deploy class. Scopes can be used in conjunction with a custom akka.actor.ActorRefProvider, making Akka actors fully extensible.

The Stash trait enables an actor to temporarily stash away messages that can not or should not be handled using the actor's current behavior.

Example:

   class ActorWithProtocol extends Actor with Stash {
     def receive = {
       case "open" =>
         unstashAll()
         context.become({
           case "write" => // do writing...
           case "close" =>
             unstashAll()
             context.unbecome()
           case msg => stash()
         }, discardOld = false)
       case "done" => // done
       case msg    => stash()
     }
   }

Note that the Stash trait can only be used together with actors that have a deque-based mailbox. By default Stash based actors request a Deque based mailbox since the stash trait extends RequiresMessageQueue[DequeBasedMessageQueueSemantics]. You can override the default mailbox provided when DequeBasedMessageQueueSemantics are requested via config:

   akka.actor.mailbox.requirements {
     "akka.dispatch.BoundedDequeBasedMessageQueueSemantics" = your-custom-mailbox
   }

For a Stash that also enforces unboundedness of the deque see akka.actor.UnboundedStash. For a Stash that does not enforce any mailbox type see akka.actor.UnrestrictedStash.

Note that the Stash trait must be mixed into (a subclass of) the Actor trait before any trait/class that overrides the preRestart callback. This means it's not possible to write Actor with MyActor with Stash if MyActor overrides preRestart.

An Akka SupervisorStrategy is the policy to apply for crashing children.

IMPORTANT:

You should not normally need to create new subclasses, instead use the existing akka.actor.OneForOneStrategy or akka.actor.AllForOneStrategy, but if you do, please read the docs of the methods below carefully, as incorrect implementations may lead to “blocked” actor systems (i.e. permanently suspended actors).

Implement this interface in order to configure the supervisorStrategy for the top-level guardian actor (/user). An instance of this class must be instantiable using a no-arg constructor.

Similar to DeadLetter with the slight twist of NOT being logged by the default dead letters listener. Messages which end up being suppressed dead letters are internal messages for which ending up as dead-letter is both expected and harmless.

It is possible to subscribe to suppressed dead letters on the ActorSystem's EventStream explicitly.

When Death Watch is used, the watcher will receive a Terminated(watched) message when watched is terminated. Terminated message can't be forwarded to another actor, since that actor might not be watching the subject. Instead, if you need to forward Terminated to another actor you should send the information in your own message.

Support for scheduled self messages in an actor. It is used by mixing in trait Timers in Scala or extending AbstractActorWithTimers in Java.

Timers are bound to the lifecycle of the actor that owns it, and thus are cancelled automatically when it is restarted or stopped.

TimerScheduler is not thread-safe, i.e. it must only be used within the actor that owns it.

Scala API: Mix in Timers into your Actor to get support for scheduled self messages via TimerScheduler.

Timers are bound to the lifecycle of the actor that owns it, and thus are cancelled automatically when it is restarted or stopped.

This message is published to the EventStream whenever an Actor receives a message it doesn't understand

A version of akka.actor.Stash that does not enforce any mailbox type. The proper mailbox has to be configured manually, and the mailbox should extend the akka.dispatch.DequeBasedMessageQueueSemantics marker trait.

If the validation of the ReceiveBuilder match logic turns out to be a bottleneck for some of your actors you can consider to implement it at lower level by extending UntypedAbstractActor instead of AbstractActor. The partial functions created by the ReceiveBuilder consist of multiple lambda expressions for every match statement, where each lambda is referencing the code to be run. This is something that the JVM can have problems optimizing and the resulting code might not be as performant as the untyped version. When extending UntypedAbstractActor each message is received as an untyped Object and you have to inspect and cast it to the actual message type in other ways (instanceof checks).

This trait represents the Scala Actor API There are implicit conversions in package.scala from ActorRef -> ScalaActorRef and back