.. _dispatchers-java:

Dispatchers (Java)
===================

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   .. contents:: :local:

An Akka ``MessageDispatcher`` is what makes Akka Actors "tick", it is the engine of the machine so to speak.
All ``MessageDispatcher`` implementations are also an ``ExecutionContext``, which means that they can be used
to execute arbitrary code, for instance :ref:`futures-java`.

Default dispatcher
------------------

Every ``ActorSystem`` will have a default dispatcher that will be used in case nothing else is configured for an ``Actor``.
The default dispatcher can be configured, and is by default a ``Dispatcher`` with a "fork-join-executor", which gives excellent performance in most cases.

Setting the dispatcher for an Actor
-----------------------------------

So in case you want to give your ``Actor`` a different dispatcher than the default, you need to do two things, of which the first is:

.. includecode:: ../java/code/akka/docs/dispatcher/DispatcherDocTestBase.java#defining-dispatcher

.. note::
    The "dispatcherId" you specify in withDispatcher is in fact a path into your configuration.
    So in this example it's a top-level section, but you could for instance put it as a sub-section,
    where you'd use periods to denote sub-sections, like this: ``"foo.bar.my-dispatcher"``

And then you just need to configure that dispatcher in your configuration:

.. includecode:: ../scala/code/akka/docs/dispatcher/DispatcherDocSpec.scala#my-dispatcher-config

And here's another example that uses the "thread-pool-executor":

.. includecode:: ../scala/code/akka/docs/dispatcher/DispatcherDocSpec.scala#my-thread-pool-dispatcher-config

For more options, see the default-dispatcher section of the :ref:`configuration`.

Types of dispatchers
--------------------

There are 4 different types of message dispatchers:

* Dispatcher

  - Sharability: Unlimited

  - Mailboxes: Any, creates one per Actor

  - Use cases: Default dispatcher, Bulkheading

  - Driven by: ``java.util.concurrent.ExecutorService``
               specify using "executor" using "fork-join-executor",
               "thread-pool-executor" or the FQCN of
               an ``akka.dispatcher.ExecutorServiceConfigurator``

* PinnedDispatcher

  - Sharability: None

  - Mailboxes: Any, creates one per Actor

  - Use cases: Bulkheading

  - Driven by: Any ``akka.dispatch.ThreadPoolExecutorConfigurator``
               by default a "thread-pool-executor"

* BalancingDispatcher

  - Sharability: Actors of the same type only

  - Mailboxes: Any, creates one for all Actors

  - Use cases: Work-sharing

  - Driven by: ``java.util.concurrent.ExecutorService``
               specify using "executor" using "fork-join-executor",
               "thread-pool-executor" or the FQCN of
               an ``akka.dispatcher.ExecutorServiceConfigurator``

* CallingThreadDispatcher

  - Sharability: Unlimited

  - Mailboxes: Any, creates one per Actor per Thread (on demand)

  - Use cases: Testing

  - Driven by: The calling thread (duh)

More dispatcher configuration examples
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Configuring a ``PinnedDispatcher``:

.. includecode:: ../scala/code/akka/docs/dispatcher/DispatcherDocSpec.scala#my-pinned-dispatcher-config

And then using it:

.. includecode:: ../java/code/akka/docs/dispatcher/DispatcherDocTestBase.java#defining-pinned-dispatcher

Mailboxes
---------

An Akka ``Mailbox`` holds the messages that are destined for an ``Actor``.
Normally each ``Actor`` has its own mailbox, but with example a ``BalancingDispatcher`` all actors with the same ``BalancingDispatcher`` will share a single instance.

Builtin implementations
^^^^^^^^^^^^^^^^^^^^^^^

Akka comes shipped with a number of default mailbox implementations:

* UnboundedMailbox

  - Backed by a ``java.util.concurrent.ConcurrentLinkedQueue``

  - Blocking: No

  - Bounded: No

* BoundedMailbox

  - Backed by a ``java.util.concurrent.LinkedBlockingQueue``

  - Blocking: Yes

  - Bounded: Yes

* UnboundedPriorityMailbox

  - Backed by a ``java.util.concurrent.PriorityBlockingQueue``

  - Blocking: Yes

  - Bounded: No

* BoundedPriorityMailbox

  - Backed by a ``java.util.PriorityBlockingQueue`` wrapped in an ``akka.util.BoundedBlockingQueue``

  - Blocking: Yes

  - Bounded: Yes

* Durable mailboxes, see :ref:`durable-mailboxes`.

Mailbox configuration examples
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

How to create a PriorityMailbox:

.. includecode:: ../scala/code/akka/docs/dispatcher/DispatcherTestBase.java#prio-mailbox

And then add it to the configuration:

.. includecode:: ../scala/code/akka/docs/dispatcher/DispatcherDocSpec.scala#prio-dispatcher-config

And then an example on how you would use it:

.. includecode:: ../java/code/akka/docs/dispatcher/DispatcherDocTestBase.java#prio-dispatcher