Adding a Scala and a Java guide to Akka Extensions

akka-docs/java/code/akka/docs/extension/ExtensionDocTest.scala

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+package akka.docs.extension
+
+import org.scalatest.junit.JUnitSuite
+
+class ExtensionDocTest extends ExtensionDocTestBase with JUnitSuite

akka-docs/java/code/akka/docs/extension/ExtensionDocTestBase.java

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+package akka.docs.extension;
+
+//#imports
+import akka.actor.*;
+import java.util.concurrent.atomic.AtomicLong;
+
+//#imports
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class ExtensionDocTestBase {
+
+    //#extension
+    public static class CountExtensionImpl implements Extension {
+      //Since this Extension is a shared instance
+      // per ActorSystem we need to be threadsafe
+      private final AtomicLong counter = new AtomicLong(0);
+
+      //This is the operation this Extension provides
+      public long increment()  {
+          return counter.incrementAndGet();
+      }
+    }
+    //#extension
+
+    //#extensionid
+    static class CountExtensionId extends AbstractExtensionId<CountExtensionImpl> {
+        //This method will be called by Akka
+        // to instantiate our Extension
+        public CountExtensionImpl createExtension(ActorSystemImpl i) {
+          return new CountExtensionImpl();
+        }
+    }
+
+    //This will be the identifier of our CountExtension
+    public final static CountExtensionId CountExtension = new CountExtensionId();
+    //#extensionid
+
+    //#extensionid-provider
+    static class CountExtensionIdProvider implements ExtensionIdProvider {
+        public CountExtensionId lookup() {
+          return CountExtension; //The public static final
+        }
+      }
+    //#extensionid-provider
+
+    //#extension-usage-actor
+    static class MyActor extends UntypedActor {
+      public void onReceive(Object msg) {
+        CountExtension.get(getContext().system()).increment();
+      }
+    }
+    //#extension-usage-actor
+
+
+  @Test public void demonstrateHowToCreateAndUseAnAkkaExtensionInJava() {
+    final ActorSystem system = null;
+    try {
+      //#extension-usage
+      CountExtension.get(system).increment();
+      //#extension-usage
+    } catch(Exception e) {
+      //do nothing
+    }
+  }
+
+}