.. _untyped-actors-java:

################
 Actors (Java)
################


The `Actor Model`_ provides a higher level of abstraction for writing concurrent
and distributed systems. It alleviates the developer from having to deal with
explicit locking and thread management, making it easier to write correct
concurrent and parallel systems. Actors were defined in the 1973 paper by Carl
Hewitt but have been popularized by the Erlang language, and used for example at
Ericsson with great success to build highly concurrent and reliable telecom
systems.

The API of Akka’s Actors is similar to Scala Actors which has borrowed some of
its syntax from Erlang.

.. _Actor Model: http://en.wikipedia.org/wiki/Actor_model


Creating Actors
===============

.. note::

  Since Akka enforces parental supervision every actor is supervised and
  (potentially) the supervisor of its children, it is advisable that you
  familiarize yourself with :ref:`actor-systems` and :ref:`supervision` and it
  may also help to read :ref:`actorOf-vs-actorFor` (the whole of
  :ref:`addressing` is recommended reading in any case).

Defining an Actor class
-----------------------

Actor in Java are implemented by extending the ``UntypedActor`` class and implementing the
:meth:`onReceive` method. This method takes the message as a parameter.

Here is an example:

.. includecode:: code/docs/actor/MyUntypedActor.java#my-untyped-actor

Props
-----

``Props`` is a configuration class to specify options for the creation
of actors. Here are some examples on how to create a ``Props`` instance.

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java#creating-props-config


Creating Actors with Props
--------------------------

Actors are created by passing in a ``Props`` instance into the ``actorOf`` factory method.

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java#creating-props


Creating Actors with default constructor
----------------------------------------

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java
   :include: imports

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java
   :include: system-actorOf

The call to :meth:`actorOf` returns an instance of ``ActorRef``. This is a handle to
the ``UntypedActor`` instance which you can use to interact with the ``UntypedActor``. The
``ActorRef`` is immutable and has a one to one relationship with the Actor it
represents. The ``ActorRef`` is also serializable and network-aware. This means
that you can serialize it, send it over the wire and use it on a remote host and
it will still be representing the same Actor on the original node, across the
network.

In the above example the actor was created from the system. It is also possible
to create actors from other actors with the actor ``context``. The difference is
how the supervisor hierarchy is arranged. When using the context the current actor
will be supervisor of the created child actor. When using the system it will be
a top level actor, that is supervised by the system (internal guardian actor).

.. includecode:: code/docs/actor/FirstUntypedActor.java#context-actorOf

The name parameter is optional, but you should preferably name your actors, since
that is used in log messages and for identifying actors. The name must not be empty
or start with ``$``, but it may contain URL encoded characters (eg. ``%20`` for a blank space).
If the given name is already in use by another child to the
same parent actor an `InvalidActorNameException` is thrown.

Actors are automatically started asynchronously when created.
When you create the ``UntypedActor`` then it will automatically call the ``preStart``
callback method on the ``UntypedActor`` class. This is an excellent place to
add initialization code for the actor.

.. code-block:: java

  @Override
  public void preStart() {
    ... // initialization code
  }

Creating Actors with non-default constructor
--------------------------------------------

If your UntypedActor has a constructor that takes parameters then you can't create it using
'actorOf(new Props(clazz))'. Then you can instead pass in 'new Props(new UntypedActorFactory() {..})'
in which you can create the Actor in any way you like.

Here is an example:

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java#creating-constructor

This way of creating the Actor is also great for integrating with Dependency Injection
(DI) frameworks like Guice or Spring.

.. warning::

  You might be tempted at times to offer an ``UntypedActor`` factory which
  always returns the same instance, e.g. by using a static field. This is not
  supported, as it goes against the meaning of an actor restart, which is
  described here: :ref:`supervision-restart`.

UntypedActor API
================

The :class:`UntypedActor` class defines only one abstract method, the above mentioned
:meth:`onReceive(Object message)`, which implements the behavior of the actor.

If the current actor behavior does not match a received message, it's recommended that
you call the :meth:`unhandled` method, which by default publishes a ``new
akka.actor.UnhandledMessage(message, sender, recipient)`` on the actor system’s
event stream (set configuration item ``akka.actor.debug.unhandled`` to ``on`` 
to have them converted into actual Debug messages).

In addition, it offers:

* :obj:`getSelf()` reference to the :class:`ActorRef` of the actor

* :obj:`getSender()` reference sender Actor of the last received message, typically used as described in :ref:`UntypedActor.Reply`

* :obj:`supervisorStrategy()` user overridable definition the strategy to use for supervising child actors

  This strategy is typically declared inside the actor in order to have access
  to the actor’s internal state within the decider function: since failure is
  communicated as a message sent to the supervisor and processed like other
  messages (albeit outside of the normal behavior), all values and variables
  within the actor are available, as is the ``getSender()`` reference (which will
  be the immediate child reporting the failure; if the original failure
  occurred within a distant descendant it is still reported one level up at a
  time).

* :obj:`getContext()` exposes contextual information for the actor and the current message, such as:

  * factory methods to create child actors (:meth:`actorOf`)
  * system that the actor belongs to
  * parent supervisor
  * supervised children
  * lifecycle monitoring
  * hotswap behavior stack as described in :ref:`UntypedActor.HotSwap`

The remaining visible methods are user-overridable life-cycle hooks which are
described in the following:

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java#lifecycle-callbacks

The implementations shown above are the defaults provided by the :class:`UntypedActor`
class.

.. _deathwatch-java:

Lifecycle Monitoring aka DeathWatch
-----------------------------------

In order to be notified when another actor terminates (i.e. stops permanently,
not temporary failure and restart), an actor may register itself for reception
of the :class:`Terminated` message dispatched by the other actor upon
termination (see `Stopping Actors`_). This service is provided by the
:class:`DeathWatch` component of the actor system.

Registering a monitor is easy (see fourth line, the rest is for demonstrating
the whole functionality):

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java#watch

It should be noted that the :class:`Terminated` message is generated
independent of the order in which registration and termination occur.
Registering multiple times does not necessarily lead to multiple messages being
generated, but there is no guarantee that only exactly one such message is
received: if termination of the watched actor has generated and queued the
message, and another registration is done before this message has been
processed, then a second message will be queued, because registering for
monitoring of an already terminated actor leads to the immediate generation of
the :class:`Terminated` message.

It is also possible to deregister from watching another actor’s liveliness
using ``context.unwatch(target)``, but obviously this cannot guarantee
non-reception of the :class:`Terminated` message because that may already have
been queued.

Start Hook
----------

Right after starting the actor, its :meth:`preStart` method is invoked.

::

  @Override
  public void preStart() {
    // registering with other actors
    someService.tell(Register(getSelf());
  }


Restart Hooks
-------------

All actors are supervised, i.e. linked to another actor with a fault
handling strategy. Actors may be restarted in case an exception is thrown while
processing a message (see :ref:`supervision`). This restart involves the hooks
mentioned above:

1. The old actor is informed by calling :meth:`preRestart` with the exception
   which caused the restart and the message which triggered that exception; the
   latter may be ``None`` if the restart was not caused by processing a
   message, e.g. when a supervisor does not trap the exception and is restarted
   in turn by its supervisor, or if an actor is restarted due to a sibling’s
   failure. If the message is available, then that message’s sender is also
   accessible in the usual way (i.e. by calling ``getSender()``).
   
   This method is the best place for cleaning up, preparing hand-over to the
   fresh actor instance, etc.  By default it stops all children and calls
   :meth:`postStop`.

2. The initial factory from the ``actorOf`` call is used
   to produce the fresh instance.

3. The new actor’s :meth:`postRestart` method is invoked with the exception
   which caused the restart. By default the :meth:`preStart`
   is called, just as in the normal start-up case.

An actor restart replaces only the actual actor object; the contents of the
mailbox is unaffected by the restart, so processing of messages will resume
after the :meth:`postRestart` hook returns. The message
that triggered the exception will not be received again. Any message
sent to an actor while it is being restarted will be queued to its mailbox as
usual.

.. warning::

  Be aware that the ordering of failure notifications relative to user messages
  is not deterministic. In particular, a parent might restart its child before
  it has processed the last messages sent by the child before the failure.
  See :ref:`message-ordering` for details.


Stop Hook
---------

After stopping an actor, its :meth:`postStop` hook is called, which may be used
e.g. for deregistering this actor from other services. This hook is guaranteed
to run after message queuing has been disabled for this actor, i.e. messages
sent to a stopped actor will be redirected to the :obj:`deadLetters` of the
:obj:`ActorSystem`.


Identifying Actors
==================

As described in :ref:`addressing`, each actor has a unique logical path, which
is obtained by following the chain of actors from child to parent until
reaching the root of the actor system, and it has a physical path, which may
differ if the supervision chain includes any remote supervisors. These paths
are used by the system to look up actors, e.g. when a remote message is
received and the recipient is searched, but they are also useful more directly:
actors may look up other actors by specifying absolute or relative
paths—logical or physical—and receive back an :class:`ActorRef` with the
result::

  getContext().actorFor("/user/serviceA/actor") // will look up this absolute path
  getContext().actorFor("../joe") // will look up sibling beneath same supervisor

The supplied path is parsed as a :class:`java.net.URI`, which basically means
that it is split on ``/`` into path elements. If the path starts with ``/``, it
is absolute and the look-up starts at the root guardian (which is the parent of
``"/user"``); otherwise it starts at the current actor. If a path element equals
``..``, the look-up will take a step “up” towards the supervisor of the
currently traversed actor, otherwise it will step “down” to the named child.
It should be noted that the ``..`` in actor paths here always means the logical
structure, i.e. the supervisor.

If the path being looked up does not exist, a special actor reference is
returned which behaves like the actor system’s dead letter queue but retains
its identity (i.e. the path which was looked up).

Remote actor addresses may also be looked up, if remoting is enabled::

  getContext().actorFor("akka.tcp://app@otherhost:1234/user/serviceB")

These look-ups return a (possibly remote) actor reference immediately, so you
will have to send to it and await a reply in order to verify that ``serviceB``
is actually reachable and running. An example demonstrating actor look-up is
given in :ref:`remote-lookup-sample-java`.

Messages and immutability
=========================

**IMPORTANT**: Messages can be any kind of object but have to be
immutable. Akka can’t enforce immutability (yet) so this has to be by
convention.

Here is an example of an immutable message:

.. includecode:: code/docs/actor/ImmutableMessage.java#immutable-message


Send messages
=============

Messages are sent to an Actor through one of the following methods.

* ``tell`` means “fire-and-forget”, e.g. send a message asynchronously and return
  immediately.
* ``ask`` sends a message asynchronously and returns a :class:`Future`
  representing a possible reply.

Message ordering is guaranteed on a per-sender basis.

.. note::

    There are performance implications of using ``ask`` since something needs to
    keep track of when it times out, there needs to be something that bridges
    a ``Promise`` into an ``ActorRef`` and it also needs to be reachable through
    remoting. So always prefer ``tell`` for performance, and only ``ask`` if you must.

In all these methods you have the option of passing along your own ``ActorRef``.
Make it a practice of doing so because it will allow the receiver actors to be able to respond
to your message, since the sender reference is sent along with the message.

Tell: Fire-forget
-----------------

This is the preferred way of sending messages. No blocking waiting for a
message. This gives the best concurrency and scalability characteristics.

.. code-block:: java

  actor.tell("Hello");

.. _actors-tell-sender-java:

Or with the sender reference passed along with the message and available to the receiving Actor
in its ``getSender: ActorRef`` member field. The target actor can use this
to reply to the original sender, by using ``getSender().tell(replyMsg)``.

.. code-block:: java

  actor.tell("Hello", getSelf());

If invoked without the sender parameter the sender will be
:obj:`deadLetters` actor reference in the target actor.

Ask: Send-And-Receive-Future
----------------------------

The ``ask`` pattern involves actors as well as futures, hence it is offered as
a use pattern rather than a method on :class:`ActorRef`:

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java#import-askPipe

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java#ask-pipe

This example demonstrates ``ask`` together with the ``pipe`` pattern on
futures, because this is likely to be a common combination. Please note that
all of the above is completely non-blocking and asynchronous: ``ask`` produces
a :class:`Future`, two of which are composed into a new future using the
:meth:`Futures.sequence` and :meth:`map` methods and then ``pipe`` installs
an ``onComplete``-handler on the future to effect the submission of the
aggregated :class:`Result` to another actor.

Using ``ask`` will send a message to the receiving Actor as with ``tell``, and
the receiving actor must reply with ``getSender().tell(reply)`` in order to
complete the returned :class:`Future` with a value. The ``ask`` operation
involves creating an internal actor for handling this reply, which needs to
have a timeout after which it is destroyed in order not to leak resources; see
more below.

.. warning::

    To complete the future with an exception you need send a Failure message to the sender.
    This is *not done automatically* when an actor throws an exception while processing a message.

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java#reply-exception

If the actor does not complete the future, it will expire after the timeout period,
specified as parameter to the ``ask`` method; this will complete the
:class:`Future` with an :class:`AskTimeoutException`.

See :ref:`futures-java` for more information on how to await or query a
future.

The ``onComplete``, ``onSuccess``, or ``onFailure`` methods of the ``Future`` can be
used to register a callback to get a notification when the Future completes.
Gives you a way to avoid blocking.

.. warning::

  When using future callbacks, inside actors you need to carefully avoid closing over
  the containing actor’s reference, i.e. do not call methods or access mutable state
  on the enclosing actor from within the callback. This would break the actor
  encapsulation and may introduce synchronization bugs and race conditions because
  the callback will be scheduled concurrently to the enclosing actor. Unfortunately
  there is not yet a way to detect these illegal accesses at compile time. See also:
  :ref:`jmm-shared-state`

Forward message
---------------

You can forward a message from one actor to another. This means that the
original sender address/reference is maintained even though the message is going
through a 'mediator'. This can be useful when writing actors that work as
routers, load-balancers, replicators etc.
You need to pass along your context variable as well.

.. code-block:: java

  myActor.forward(message, getContext());

Receive messages
================

When an actor receives a message it is passed into the ``onReceive`` method, this is
an abstract method on the ``UntypedActor`` base class that needs to be defined.

Here is an example:

.. includecode:: code/docs/actor/MyUntypedActor.java#my-untyped-actor

An alternative to using if-instanceof checks is to use `Apache Commons MethodUtils
<http://commons.apache.org/beanutils/api/org/apache/commons/beanutils/MethodUtils.html#invokeMethod(java.lang.Object,%20java.lang.String,%20java.lang.Object)>`_
to invoke a named method whose parameter type matches the message type.

.. _UntypedActor.Reply:

Reply to messages
=================

If you want to have a handle for replying to a message, you can use
``getSender()``, which gives you an ActorRef. You can reply by sending to
that ActorRef with ``getSender().tell(replyMsg)``. You can also store the ActorRef
for replying later, or passing on to other actors. If there is no sender (a
message was sent without an actor or future context) then the sender
defaults to a 'dead-letter' actor ref.

.. code-block:: java

  public void onReceive(Object request) {
    String result = process(request);
    getSender().tell(result);       // will have dead-letter actor as default
  }

Receive timeout
===============

The `UntypedActorContext` :meth:`setReceiveTimeout` defines the inactivity timeout after which
the sending of a `ReceiveTimeout` message is triggered.
When specified, the receive function should be able to handle an `akka.actor.ReceiveTimeout` message.
1 millisecond is the minimum supported timeout.

Please note that the receive timeout might fire and enqueue the `ReceiveTimeout` message right after
another message was enqueued; hence it is **not guaranteed** that upon reception of the receive
timeout there must have been an idle period beforehand as configured via this method.

Once set, the receive timeout stays in effect (i.e. continues firing repeatedly after inactivity
periods). Pass in `Duration.Undefined` to switch off this feature.

.. includecode:: code/docs/actor/MyReceivedTimeoutUntypedActor.java#receive-timeout

.. _stopping-actors-java:

Stopping actors
===============

Actors are stopped by invoking the :meth:`stop` method of a ``ActorRefFactory``,
i.e. ``ActorContext`` or ``ActorSystem``. Typically the context is used for stopping
child actors and the system for stopping top level actors. The actual termination of
the actor is performed asynchronously, i.e. :meth:`stop` may return before the actor is
stopped.

Processing of the current message, if any, will continue before the actor is stopped,
but additional messages in the mailbox will not be processed. By default these
messages are sent to the :obj:`deadLetters` of the :obj:`ActorSystem`, but that
depends on the mailbox implementation.

Termination of an actor proceeds in two steps: first the actor suspends its
mailbox processing and sends a stop command to all its children, then it keeps
processing the termination messages from its children until the last one is
gone, finally terminating itself (invoking :meth:`postStop`, dumping mailbox,
publishing :class:`Terminated` on the :ref:`DeathWatch <deathwatch-java>`, telling
its supervisor). This procedure ensures that actor system sub-trees terminate
in an orderly fashion, propagating the stop command to the leaves and
collecting their confirmation back to the stopped supervisor. If one of the
actors does not respond (i.e. processing a message for extended periods of time
and therefore not receiving the stop command), this whole process will be
stuck.

Upon :meth:`ActorSystem.shutdown()`, the system guardian actors will be
stopped, and the aforementioned process will ensure proper termination of the
whole system.

The :meth:`postStop()` hook is invoked after an actor is fully stopped. This
enables cleaning up of resources:

.. code-block:: java

  @Override
  public void postStop() {
    // close some file or database connection
  }

.. note::

  Since stopping an actor is asynchronous, you cannot immediately reuse the
  name of the child you just stopped; this will result in an
  :class:`InvalidActorNameException`. Instead, :meth:`watch()` the terminating
  actor and create its replacement in response to the :class:`Terminated`
  message which will eventually arrive.

.. _poison-pill-java:

PoisonPill
----------

You can also send an actor the ``akka.actor.PoisonPill`` message, which will
stop the actor when the message is processed. ``PoisonPill`` is enqueued as
ordinary messages and will be handled after messages that were already queued
in the mailbox.

Use it like this:

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java
   :include: import-actors

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java
   :include: poison-pill

Graceful Stop
-------------

:meth:`gracefulStop` is useful if you need to wait for termination or compose ordered
termination of several actors:

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java
   :include: import-gracefulStop

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java
   :include: gracefulStop

When ``gracefulStop()`` returns successfully, the actor’s ``postStop()`` hook
will have been executed: there exists a happens-before edge between the end of
``postStop()`` and the return of ``gracefulStop()``.

.. warning::

  Keep in mind that an actor stopping and its name being deregistered are
  separate events which happen asynchronously from each other. Therefore it may
  be that you will find the name still in use after ``gracefulStop()``
  returned. In order to guarantee proper deregistration, only reuse names from
  within a supervisor you control and only in response to a :class:`Terminated`
  message, i.e. not for top-level actors.

.. _UntypedActor.HotSwap:

HotSwap
=======

Upgrade
-------

Akka supports hotswapping the Actor’s message loop (e.g. its implementation) at
runtime. Use the ``getContext().become`` method from within the Actor.
The hotswapped code is kept in a Stack which can be pushed (replacing or adding
at the top) and popped.

.. warning::

  Please note that the actor will revert to its original behavior when restarted by its Supervisor.

To hotswap the Actor using ``getContext().become``:

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java
   :include: import-procedure

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java
   :include: hot-swap-actor

This variant of the :meth:`become` method is useful for many different things,
such as to implement a Finite State Machine (FSM). It will replace the current
behavior (i.e. the top of the behavior stack), which means that you do not use
:meth:`unbecome`, instead always the next behavior is explicitly installed.

The other way of using :meth:`become` does not replace but add to the top of
the behavior stack. In this case care must be taken to ensure that the number
of “pop” operations (i.e. :meth:`unbecome`) matches the number of “push” ones
in the long run, otherwise this amounts to a memory leak (which is why this
behavior is not the default). 

.. includecode:: code/docs/actor/UntypedActorSwapper.java#swapper

Stash
=====

The ``UntypedActorWithStash`` class enables an actor to temporarily stash away messages
that can not or should not be handled using the actor's current
behavior. Upon changing the actor's message handler, i.e., right
before invoking ``getContext().become()`` or ``getContext().unbecome()``, all
stashed messages can be "unstashed", thereby prepending them to the actor's
mailbox. This way, the stashed messages can be processed in the same
order as they have been received originally.

.. warning::

  Please note that the stash can only be used together with actors
  that have a deque-based mailbox. For this, configure the
  ``mailbox-type`` of the dispatcher to be a deque-based mailbox, such as
  ``akka.dispatch.UnboundedDequeBasedMailbox`` (see :ref:`dispatchers-java`).

Here is an example of the ``UntypedActorWithStash`` class in action:

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java#stash

Invoking ``stash()`` adds the current message (the message that the
actor received last) to the actor's stash. It is typically invoked
when handling the default case in the actor's message handler to stash
messages that aren't handled by the other cases. It is illegal to
stash the same message twice; to do so results in an
``IllegalStateException`` being thrown. The stash may also be bounded
in which case invoking ``stash()`` may lead to a capacity violation,
which results in a ``StashOverflowException``. The capacity of the
stash can be configured using the ``stash-capacity`` setting (an ``Int``) of the
dispatcher's configuration.

Invoking ``unstashAll()`` enqueues messages from the stash to the
actor's mailbox until the capacity of the mailbox (if any) has been
reached (note that messages from the stash are prepended to the
mailbox). In case a bounded mailbox overflows, a
``MessageQueueAppendFailedException`` is thrown.
The stash is guaranteed to be empty after calling ``unstashAll()``.

The stash is backed by a ``scala.collection.immutable.Vector``. As a
result, even a very large number of messages may be stashed without a
major impact on performance.

Note that the stash is part of the ephemeral actor state, unlike the
mailbox. Therefore, it should be managed like other parts of the
actor's state which have the same property. The :class:`Stash` trait’s
implementation of :meth:`preRestart` will call ``unstashAll()``, which is
usually the desired behavior.

.. _killing-actors-java:

Killing an Actor
================

You can kill an actor by sending a ``Kill`` message. This will cause the actor
to throw a :class:`ActorKilledException`, triggering a failure. The actor will
suspend operation and its supervisor will be asked how to handle the failure,
which may mean resuming the actor, restarting it or terminating it completely.
See :ref:`supervision-directives` for more information.

Use ``Kill`` like this:

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java
   :include: import-actors

.. includecode:: code/docs/actor/UntypedActorDocTestBase.java
   :include: kill

Actors and exceptions
=====================

It can happen that while a message is being processed by an actor, that some
kind of exception is thrown, e.g. a database exception.

What happens to the Message
---------------------------

If an exception is thrown while a message is being processed (i.e. taken out of
its mailbox and handed over to the current behavior), then this message will be
lost. It is important to understand that it is not put back on the mailbox. So
if you want to retry processing of a message, you need to deal with it yourself
by catching the exception and retry your flow. Make sure that you put a bound
on the number of retries since you don't want a system to livelock (so
consuming a lot of cpu cycles without making progress). Another possibility
would be to have a look at the :ref:`PeekMailbox pattern <mailbox-acking>`.

What happens to the mailbox
---------------------------

If an exception is thrown while a message is being processed, nothing happens to
the mailbox. If the actor is restarted, the same mailbox will be there. So all
messages on that mailbox will be there as well.

What happens to the actor
-------------------------

If code within an actor throws an exception, that actor is suspended and the
supervision process is started (see :ref:`supervision`). Depending on the
supervisor’s decision the actor is resumed (as if nothing happened), restarted
(wiping out its internal state and starting from scratch) or terminated.

Initialization patterns
=======================

The rich lifecycle hooks of Actors provide a useful toolkit to implement various initialization patterns. During the
lifetime of an ``ActorRef``, an actor can potentially go through several restarts, where the old instance is replaced by
a fresh one, invisibly to the outside observer who only sees the ``ActorRef``.

One may think about the new instances as "incarnations". Initialization might be necessary for every incarnation
of an actor, but sometimes one needs initialization to happen only at the birth of the first instance when the
``ActorRef`` is created. The following sections provide patterns for different initialization needs.

Initialization via constructor
------------------------------

Using the constructor for initialization has various benefits. First of all, it makes it possible to use ``val`` fields to store
any state that does not change during the life of the actor instance, making the implementation of the actor more robust.
The constructor is invoked for every incarnation of the actor, therefore the internals of the actor can always assume
that proper initialization happened. This is also the drawback of this approach, as there are cases when one would
like to avoid reinitializing internals on restart. For example, it is often useful to preserve child actors across
restarts. The following section provides a pattern for this case.

Initialization via preStart
---------------------------

The method ``preStart()`` of an actor is only called once directly during the initialization of the first instance, that
is, at creation of its ``ActorRef``. In the case of restarts, ``preStart()`` is called from ``postRestart()``, therefore
if not overridden, ``preStart()`` is called on every incarnation. However, overriding ``postRestart()`` one can disable
this behavior, and ensure that there is only one call to ``preStart()``.

One useful usage of this pattern is to disable creation of new ``ActorRefs`` for children during restarts. This can be
achieved by overriding ``preRestart()``:

.. includecode:: code/docs/actor/InitializationDocSpecJava.java#preStartInit

Please note, that the child actors are *still restarted*, but no new ``ActorRef`` is created. One can recursively apply
the same principles for the children, ensuring that their ``preStart()`` method is called only at the creation of their
refs.

For more information see :ref:`supervision-restart`.

Initialization via message passing
----------------------------------

There are cases when it is impossible to pass all the information needed for actor initialization in the constructor,
for example in the presence of circular dependencies. In this case the actor should listen for an initialization message,
and use ``become()`` or a finite state-machine state transition to encode the initialized and uninitialized states
of the actor.

.. includecode:: code/docs/actor/InitializationDocSpecJava.java#messageInit

If the actor may receive messages before it has been initialized, a useful tool can be the ``Stash`` to save messages
until the initialization finishes, and replaying them after the actor became initialized.

.. warning::

  This pattern should be used with care, and applied only when none of the patterns above are applicable. One of
  the potential issues is that messages might be lost when sent to remote actors. Also, publishing an ``ActorRef`` in
  an uninitialized state might lead to the condition that it receives a user message before the initialization has been
  done.
