HADOOP-8832. Port generic service plugin mechanism from HADOOP-5257 to branch-1. Backported by Brandon Li.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1@1388286 13f79535-47bb-0310-9956-ffa450edef68

CHANGES.txt

@@ -101,6 +101,9 @@ Release 1.2.0 - unreleased
 
     HDFS-1108 Log newly allocated blocks (hdfs-1108-hadoop-1-v5.patch) (sanjay) 
 
+    HADOOP-8832. Port generic service plugin mechanism from HADOOP-5257
+    to branch-1. (backported by Brandon Li via suresh) 
+
   OPTIMIZATIONS
 
     HDFS-2533. Backport: Remove needless synchronization on some FSDataSet

src/core/org/apache/hadoop/util/ServicePlugin.java

@@ -0,0 +1,46 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.util;
+
+import java.io.Closeable;
+
+/**
+ * Service plug-in interface.
+ *
+ * Service plug-ins may be used to expose functionality of datanodes or
+ * namenodes using arbitrary RPC protocols. Plug-ins are instantiated by the
+ * service instance, and are notified of service life-cycle events using the
+ * methods defined by this class.
+ *
+ * Service plug-ins are started after the service instance is started, and
+ * stopped before the service instance is stopped.
+ */
+public interface ServicePlugin extends Closeable {
+
+  /**
+   * This method is invoked when the service instance has been started.
+   *
+   * @param service The service instance invoking this method
+   */
+  void start(Object service);
+
+  /**
+   * This method is invoked when the service instance is about to be shut down.
+   */
+  void stop();
+}

src/test/org/apache/hadoop/conf/TestGetInstances.java

@@ -0,0 +1,74 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.conf;
+
+import java.util.List;
+
+import junit.framework.TestCase;
+
+public class TestGetInstances extends TestCase {
+  
+  interface SampleInterface {}
+  
+  interface ChildInterface extends SampleInterface {}
+  
+  static class SampleClass implements SampleInterface {
+    SampleClass() {}
+  }
+	
+  static class AnotherClass implements ChildInterface {
+    AnotherClass() {}
+  }
+  
+  /**
+   * Makes sure <code>Configuration.getInstances()</code> returns
+   * instances of the required type.
+   */
+  public void testGetInstances() throws Exception {
+    Configuration conf = new Configuration();
+    
+    List<SampleInterface> classes =
+      conf.getInstances("no.such.property", SampleInterface.class);
+    assertTrue(classes.isEmpty());
+
+    conf.set("empty.property", "");
+    classes = conf.getInstances("empty.property", SampleInterface.class);
+    assertTrue(classes.isEmpty());
+    
+    conf.setStrings("some.classes",
+        SampleClass.class.getName(), AnotherClass.class.getName());
+    classes = conf.getInstances("some.classes", SampleInterface.class);
+    assertEquals(2, classes.size());
+    
+    try {
+      conf.setStrings("some.classes",
+          SampleClass.class.getName(), AnotherClass.class.getName(),
+          String.class.getName());
+      conf.getInstances("some.classes", SampleInterface.class);
+      fail("java.lang.String does not implement SampleInterface");
+    } catch (RuntimeException e) {}
+    
+    try {
+      conf.setStrings("some.classes",
+          SampleClass.class.getName(), AnotherClass.class.getName(),
+          "no.such.Class");
+      conf.getInstances("some.classes", SampleInterface.class);
+      fail("no.such.Class does not exist");
+    } catch (RuntimeException e) {}
+  }
+}