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
019package org.apache.hadoop.util;
020
021import org.apache.commons.logging.*;
022
023import org.apache.hadoop.conf.Configuration;
024
025import java.util.ArrayList;
026import java.util.Collection;
027import java.util.Collections;
028import java.util.List;
029import java.util.concurrent.Executor;
030import 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 */
041public 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}