Adding a Scala and a Java guide to Akka Extensions

akka-docs/java/extending-akka.rst

@@ -0,0 +1,63 @@
+.. _extending-akka:
+
+Akka Extensions
+===============
+
+.. sidebar:: Contents
+
+   .. contents:: :local:
+
+If you want to add features to Akka, there is a very elegant, but powerful mechanism for doing so.
+It's called Akka Extensions and is comprised of 2 basic components: an ``Extension`` and an ``ExtensionId``.
+
+Extensions will only be loaded once per ``ActorSystem``, which will be managed by Akka.
+You can choose to have your Extension loaded on-demand or at ``ActorSystem`` creation time through the Akka configuration.
+Details on how to make that happens are below, in the "Loading from Configuration" section.
+
+.. warning::
+   
+    Since an extension is a way to hook into Akka itself, the implementor of the extension needs to
+    ensure the thread safety of his/her extension.
+
+
+Building an Extension
+---------------------
+
+So let's create a sample extension that just lets us count the number of times something has happened.
+
+First, we define what our ``Extension`` should do:
+
+.. includecode:: code/akka/docs/extension/ExtensionDocTestBase.java
+   :include: imports,extension
+
+Then we need to create an ``ExtensionId`` for our extension so we can grab ahold of it.
+
+.. includecode:: code/akka/docs/extension/ExtensionDocTestBase.java
+   :include: imports,extensionid 
+
+Wicked! Now all we need to do is to actually use it:
+
+.. includecode:: code/akka/docs/extension/ExtensionDocTestBase.java
+   :include: extension-usage
+
+Or from inside of an Akka Actor:
+
+.. includecode:: code/akka/docs/extension/ExtensionDocTestBase.java
+   :include: extension-usage-actor
+
+That's all there is to it!
+
+Loading from Configuration
+--------------------------
+
+To be able to load extensions from your Akka configuration you must add FQCNs of implementations of either ``ExtensionId`` or ``ExtensionIdProvider``
+in the "akka.extensions" section of the config you provide to your ``ActorSystem``.
+
+.. includecode:: code/akka/docs/extension/ExtensionDocTestBase.java
+   :include: extensionid-provider
+
+Applicability
+-------------
+
+The sky is the limit!
+By the way, did you know that Akka's ``Typed Actors``, ``Serialization`` and other features are implemented as Akka Extensions?