org.apache.hadoop.util.PluginDispatcher.java Source code

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Here is the source code for org.apache.hadoop.util.PluginDispatcher.java

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/**
 * 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 org.apache.commons.logging.*;

import org.apache.hadoop.conf.Configuration;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

/**
 * Provides convenience functions for dispatching calls through
 * to plugins registered with a class. Classes that wish to provide
 * plugin interfaces should use this class to load the plugin list
 * from the Configuration and to dispatch calls to the loaded instances.
 *
 * Calls dispatched through this class are performed on a second thread
 * so as to not block execution of the plugged service.
 */
public class PluginDispatcher<T extends ServicePlugin> {
    public static final Log LOG = LogFactory.getLog(PluginDispatcher.class.getName());

    private final List<T> plugins;
    private Executor executor;

    /**
     * Load a PluginDispatcher from the given Configuration. The start()
     * callback will not be automatically called.
     *
     * @param conf the Configuration from which to load
     * @param key the configuration key that lists class names to instantiate
     * @param clazz the class or interface from which plugins must extend
     */
    public static <X extends ServicePlugin> PluginDispatcher<X> createFromConfiguration(Configuration conf,
            String key, Class<X> clazz) {
        List<X> plugins = new ArrayList<X>();
        try {
            plugins.addAll(conf.getInstances(key, clazz));
        } catch (Throwable t) {
            LOG.warn("Unable to load " + key + " plugins");
        }
        return new PluginDispatcher<X>(plugins);
    }

    PluginDispatcher(Collection<T> plugins) {
        this.plugins = Collections.synchronizedList(new ArrayList<T>(plugins));
        executor = Executors.newSingleThreadExecutor();
    }

    PluginDispatcher(Collection<T> plugins, Executor executor) {
        this.plugins = Collections.synchronizedList(new ArrayList<T>(plugins));
        this.executor = executor;
    }

    /**
     * Dispatch a call to all active plugins.
     *
     * Exceptions will be caught and logged at WARN level.
     *
     * @param callback a function which will run once for each plugin, with
     * that plugin as the argument
     */
    public void dispatchCall(final SingleArgumentRunnable<T> callback) {
        executor.execute(new Runnable() {
            public void run() {
                for (T plugin : plugins) {
                    try {
                        callback.run(plugin);
                    } catch (Throwable t) {
                        LOG.warn("Uncaught exception dispatching to plugin " + plugin, t);
                    }
                }
            }
        });
    }

    /**
     * Dispatches the start(...) hook common to all ServicePlugins. This
     * also automatically removes any plugin that throws an exception while
     * attempting to start.
     *
     * @param plugPoint passed to ServicePlugin.start()
     */
    public void dispatchStart(final Object plugPoint) {
        dispatchCall(new SingleArgumentRunnable<T>() {
            public void run(T p) {
                try {
                    p.start(plugPoint);
                } catch (Throwable t) {
                    LOG.error("ServicePlugin " + p + " could not be started. " + "Removing from future callbacks.",
                            t);
                    plugins.remove(p);
                }
            }
        });
    }

    /**
     * Convenience function for dispatching the stop() hook common to all
     * ServicePlugins.
     */
    public void dispatchStop() {
        dispatchCall(new SingleArgumentRunnable<T>() {
            public void run(T p) {
                try {
                    p.stop();
                } catch (Throwable t) {
                    LOG.warn("ServicePlugin " + p + " could not be stopped", t);
                }
            }
        });
    }
}