Adding a Scala and a Java guide to Akka Extensions

akka-docs/scala/code/akka/docs/extension/ExtensionDocSpec.scala

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+package akka.docs.extension
+
+import org.scalatest.WordSpec
+import org.scalatest.matchers.MustMatchers
+
+//#imports
+import akka.actor._
+import java.util.concurrent.atomic.AtomicLong
+
+//#imports
+
+//#extension
+class CountExtensionImpl extends Extension {
+  //Since this Extension is a shared instance
+  // per ActorSystem we need to be threadsafe
+  private val counter = new AtomicLong(0)
+
+  //This is the operation this Extension provides
+  def increment() = counter.incrementAndGet()
+}
+//#extension
+
+//#extensionid
+object CountExtension
+  extends ExtensionId[CountExtensionImpl]
+  with ExtensionIdProvider {
+  //The lookup method is required by ExtensionIdProvider,
+  // so we return ourselves here, this allows us
+  // to configure our extension to be loaded when
+  // the ActorSystem starts up
+  override def lookup = CountExtension
+
+  //This method will be called by Akka
+  // to instantiate our Extension
+  override def createExtension(system: ActorSystemImpl) = new CountExtensionImpl
+}
+//#extensionid
+
+//#extension-usage-actor
+import akka.actor.Actor
+
+class MyActor extends Actor {
+  def receive = {
+    case someMessage ⇒
+      CountExtension(context.system).increment()
+  }
+}
+//#extension-usage-actor
+
+//#extension-usage-actor-trait
+import akka.actor.Actor
+
+trait Counting { self: Actor ⇒
+  def increment() = CountExtension(context.system).increment()
+}
+class MyCounterActor extends Actor with Counting {
+  def receive = {
+    case someMessage ⇒ increment()
+  }
+}
+//#extension-usage-actor-trait
+
+class ExtensionDocSpec extends WordSpec with MustMatchers {
+
+  "demonstrate how to create an extension in Scala" in {
+    val system: ActorSystem = null
+    intercept[Exception] {
+      //#extension-usage
+      CountExtension(system).increment
+      //#extension-usage
+    }
+  }
+
+}