/**
* 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);
}
}
});
}
}