DOC: Extension sample for application specific settings. See #1702

* Added sample and documentation for how to create extension for application specific config
* Simplified java usage of ExtensionIdProvider, more aligned with how it is done in scala

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

@@ -0,0 +1,85 @@
+/**
+ * Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.docs.extension;
+
+//#imports
+import akka.actor.Extension;
+import akka.actor.AbstractExtensionId;
+import akka.actor.ExtensionIdProvider;
+import akka.actor.ActorSystem;
+import akka.actor.ActorSystemImpl;
+import akka.util.Duration;
+import com.typesafe.config.Config;
+import java.util.concurrent.TimeUnit;
+
+//#imports
+
+import akka.actor.UntypedActor;
+import org.junit.Test;
+
+public class SettingsExtensionDocTestBase {
+
+  //#extension
+  public static class SettingsImpl implements Extension {
+
+    public final String DB_URI;
+    public final Duration CIRCUIT_BREAKER_TIMEOUT;
+
+    public SettingsImpl(Config config) {
+      DB_URI = config.getString(config.getString("myapp.db.uri"));
+      CIRCUIT_BREAKER_TIMEOUT = Duration.create(config.getMilliseconds("myapp.circuit-breaker.timeout"),
+          TimeUnit.MILLISECONDS);
+    }
+
+  }
+
+  //#extension
+
+  //#extensionid
+  public static class Settings extends AbstractExtensionId<SettingsImpl> implements ExtensionIdProvider {
+    public final static Settings instance = new Settings();
+
+    public Settings lookup() {
+      return Settings.instance;
+    }
+
+    public SettingsImpl createExtension(ActorSystemImpl system) {
+      return new SettingsImpl(system.settings().config());
+    }
+  }
+
+  //#extensionid
+
+  //#extension-usage-actor
+  public static class MyActor extends UntypedActor {
+    Connection connection = connect(Settings.instance.get(getContext().system()).DB_URI,
+        Settings.instance.get(getContext().system()).CIRCUIT_BREAKER_TIMEOUT);
+
+    //#extension-usage-actor
+
+    public Connection connect(String dbUri, Duration circuitBreakerTimeout) {
+      return new Connection();
+    }
+
+    public void onReceive(Object msg) {
+    }
+
+  }
+
+  public static class Connection {
+  }
+
+  @Test
+  public void demonstrateHowToCreateAndUseAnAkkaExtensionInJava() {
+    final ActorSystem system = null;
+    try {
+      //#extension-usage
+      String dbUri = Settings.instance.get(system).DB_URI;
+      //#extension-usage
+    } catch (Exception e) {
+      //do nothing
+    }
+  }
+
+}