001 /**
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements. See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership. The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License. You may obtain a copy of the License at
009 *
010 * http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */
018
019 package org.apache.hadoop.util;
020
021 import org.apache.commons.logging.*;
022
023 import org.apache.hadoop.conf.Configuration;
024
025 import java.util.ArrayList;
026 import java.util.Collection;
027 import java.util.Collections;
028 import java.util.List;
029 import java.util.concurrent.Executor;
030 import java.util.concurrent.Executors;
031
032 /**
033 * Provides convenience functions for dispatching calls through
034 * to plugins registered with a class. Classes that wish to provide
035 * plugin interfaces should use this class to load the plugin list
036 * from the Configuration and to dispatch calls to the loaded instances.
037 *
038 * Calls dispatched through this class are performed on a second thread
039 * so as to not block execution of the plugged service.
040 */
041 public class PluginDispatcher<T extends ServicePlugin> {
042 public static final Log LOG = LogFactory.getLog(PluginDispatcher.class.getName());
043
044 private final List<T> plugins;
045 private Executor executor;
046
047 /**
048 * Load a PluginDispatcher from the given Configuration. The start()
049 * callback will not be automatically called.
050 *
051 * @param conf the Configuration from which to load
052 * @param key the configuration key that lists class names to instantiate
053 * @param clazz the class or interface from which plugins must extend
054 */
055 public static <X extends ServicePlugin> PluginDispatcher<X> createFromConfiguration(
056 Configuration conf, String key, Class<X> clazz) {
057 List<X> plugins = new ArrayList<X>();
058 try {
059 plugins.addAll(conf.getInstances(key, clazz));
060 } catch (Throwable t) {
061 LOG.warn("Unable to load "+key+" plugins");
062 }
063 return new PluginDispatcher<X>(plugins);
064 }
065
066 PluginDispatcher(Collection<T> plugins) {
067 this.plugins = Collections.synchronizedList(new ArrayList<T>(plugins));
068 executor = Executors.newSingleThreadExecutor();
069 }
070
071 PluginDispatcher(Collection<T> plugins, Executor executor) {
072 this.plugins = Collections.synchronizedList(new ArrayList<T>(plugins));
073 this.executor = executor;
074 }
075
076 /**
077 * Dispatch a call to all active plugins.
078 *
079 * Exceptions will be caught and logged at WARN level.
080 *
081 * @param callback a function which will run once for each plugin, with
082 * that plugin as the argument
083 */
084 public void dispatchCall(final SingleArgumentRunnable<T> callback) {
085 executor.execute(new Runnable() {
086 public void run() {
087 for (T plugin : plugins) {
088 try {
089 callback.run(plugin);
090 } catch (Throwable t) {
091 LOG.warn("Uncaught exception dispatching to plugin " + plugin, t);
092 }
093 }
094 }});
095 }
096
097 /**
098 * Dispatches the start(...) hook common to all ServicePlugins. This
099 * also automatically removes any plugin that throws an exception while
100 * attempting to start.
101 *
102 * @param plugPoint passed to ServicePlugin.start()
103 */
104 public void dispatchStart(final Object plugPoint) {
105 dispatchCall(
106 new SingleArgumentRunnable<T>() {
107 public void run(T p) {
108 try {
109 p.start(plugPoint);
110 } catch (Throwable t) {
111 LOG.error("ServicePlugin " + p + " could not be started. " +
112 "Removing from future callbacks.", t);
113 plugins.remove(p);
114 }
115 }
116 });
117 }
118
119 /**
120 * Convenience function for dispatching the stop() hook common to all
121 * ServicePlugins.
122 */
123 public void dispatchStop() {
124 dispatchCall(
125 new SingleArgumentRunnable<T>() {
126 public void run(T p) {
127 try {
128 p.stop();
129 } catch (Throwable t) {
130 LOG.warn("ServicePlugin " + p + " could not be stopped", t);
131 }
132 }
133 });
134 }
135 }