001 /*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements. See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License. You may obtain a copy of the License at
008 *
009 * http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017 package org.apache.commons.lang3.concurrent;
018
019 import java.util.Collections;
020 import java.util.HashMap;
021 import java.util.Map;
022 import java.util.NoSuchElementException;
023 import java.util.Set;
024 import java.util.concurrent.ExecutorService;
025
026 /**
027 * <p>
028 * A specialized {@link BackgroundInitializer} implementation that can deal with
029 * multiple background initialization tasks.
030 * </p>
031 * <p>
032 * This class has a similar purpose as {@link BackgroundInitializer}. However,
033 * it is not limited to a single background initialization task. Rather it
034 * manages an arbitrary number of {@code BackgroundInitializer} objects,
035 * executes them, and waits until they are completely initialized. This is
036 * useful for applications that have to perform multiple initialization tasks
037 * that can run in parallel (i.e. that do not depend on each other). This class
038 * takes care about the management of an {@code ExecutorService} and shares it
039 * with the {@code BackgroundInitializer} objects it is responsible for; so the
040 * using application need not bother with these details.
041 * </p>
042 * <p>
043 * The typical usage scenario for {@code MultiBackgroundInitializer} is as
044 * follows:
045 * <ul>
046 * <li>Create a new instance of the class. Optionally pass in a pre-configured
047 * {@code ExecutorService}. Alternatively {@code MultiBackgroundInitializer} can
048 * create a temporary {@code ExecutorService} and delete it after initialization
049 * is complete.</li>
050 * <li>Create specialized {@link BackgroundInitializer} objects for the
051 * initialization tasks to be performed and add them to the {@code
052 * MultiBackgroundInitializer} using the
053 * {@link #addInitializer(String, BackgroundInitializer)} method.</li>
054 * <li>After all initializers have been added, call the {@link #start()} method.
055 * </li>
056 * <li>When access to the result objects produced by the {@code
057 * BackgroundInitializer} objects is needed call the {@link #get()} method. The
058 * object returned here provides access to all result objects created during
059 * initialization. It also stores information about exceptions that have
060 * occurred.</li>
061 * </ul>
062 * </p>
063 * <p>
064 * {@code MultiBackgroundInitializer} starts a special controller task that
065 * starts all {@code BackgroundInitializer} objects added to the instance.
066 * Before the an initializer is started it is checked whether this initializer
067 * already has an {@code ExecutorService} set. If this is the case, this {@code
068 * ExecutorService} is used for running the background task. Otherwise the
069 * current {@code ExecutorService} of this {@code MultiBackgroundInitializer} is
070 * shared with the initializer.
071 * </p>
072 * <p>
073 * The easiest way of using this class is to let it deal with the management of
074 * an {@code ExecutorService} itself: If no external {@code ExecutorService} is
075 * provided, the class creates a temporary {@code ExecutorService} (that is
076 * capable of executing all background tasks in parallel) and destroys it at the
077 * end of background processing.
078 * </p>
079 * <p>
080 * Alternatively an external {@code ExecutorService} can be provided - either at
081 * construction time or later by calling the
082 * {@link #setExternalExecutor(ExecutorService)} method. In this case all
083 * background tasks are scheduled at this external {@code ExecutorService}.
084 * <strong>Important note:</strong> When using an external {@code
085 * ExecutorService} be sure that the number of threads managed by the service is
086 * large enough. Otherwise a deadlock can happen! This is the case in the
087 * following scenario: {@code MultiBackgroundInitializer} starts a task that
088 * starts all registered {@code BackgroundInitializer} objects and waits for
089 * their completion. If for instance a single threaded {@code ExecutorService}
090 * is used, none of the background tasks can be executed, and the task created
091 * by {@code MultiBackgroundInitializer} waits forever.
092 * </p>
093 *
094 * @version $Id: MultiBackgroundInitializer.java 889215 2009-12-10 11:56:38Z bayard $
095 */
096 public class MultiBackgroundInitializer
097 extends
098 BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults> {
099 /** A map with the child initializers. */
100 private final Map<String, BackgroundInitializer<?>> childInitializers = new HashMap<String, BackgroundInitializer<?>>();
101
102 /**
103 * Creates a new instance of {@code MultiBackgroundInitializer}.
104 */
105 public MultiBackgroundInitializer() {
106 super();
107 }
108
109 /**
110 * Creates a new instance of {@code MultiBackgroundInitializer} and
111 * initializes it with the given external {@code ExecutorService}.
112 *
113 * @param exec the {@code ExecutorService} for executing the background
114 * tasks
115 */
116 public MultiBackgroundInitializer(ExecutorService exec) {
117 super(exec);
118 }
119
120 /**
121 * Adds a new {@code BackgroundInitializer} to this object. When this
122 * {@code MultiBackgroundInitializer} is started, the given initializer will
123 * be processed. This method must not be called after {@link #start()} has
124 * been invoked.
125 *
126 * @param name the name of the initializer (must not be <b>null</b>)
127 * @param init the {@code BackgroundInitializer} to add (must not be
128 * <b>null</b>)
129 * @throws IllegalArgumentException if a required parameter is missing
130 * @throws IllegalStateException if {@code start()} has already been called
131 */
132 public void addInitializer(String name, BackgroundInitializer<?> init) {
133 if (name == null) {
134 throw new IllegalArgumentException(
135 "Name of child initializer must not be null!");
136 }
137 if (init == null) {
138 throw new IllegalArgumentException(
139 "Child initializer must not be null!");
140 }
141
142 synchronized (this) {
143 if (isStarted()) {
144 throw new IllegalStateException(
145 "addInitializer() must not be called after start()!");
146 }
147 childInitializers.put(name, init);
148 }
149 }
150
151 /**
152 * Returns the number of tasks needed for executing all child {@code
153 * BackgroundInitializer} objects in parallel. This implementation sums up
154 * the required tasks for all child initializers (which is necessary if one
155 * of the child initializers is itself a {@code MultiBackgroundInitializer}
156 * ). Then it adds 1 for the control task that waits for the completion of
157 * the children.
158 *
159 * @return the number of tasks required for background processing
160 */
161 @Override
162 protected int getTaskCount() {
163 int result = 1;
164
165 for (BackgroundInitializer<?> bi : childInitializers.values()) {
166 result += bi.getTaskCount();
167 }
168
169 return result;
170 }
171
172 /**
173 * Creates the results object. This implementation starts all child {@code
174 * BackgroundInitializer} objects. Then it collects their results and
175 * creates a {@code MultiBackgroundInitializerResults} object with this
176 * data. If a child initializer throws a checked exceptions, it is added to
177 * the results object. Unchecked exceptions are propagated.
178 *
179 * @return the results object
180 * @throws Exception if an error occurs
181 */
182 @Override
183 protected MultiBackgroundInitializerResults initialize() throws Exception {
184 Map<String, BackgroundInitializer<?>> inits;
185 synchronized (this) {
186 // create a snapshot to operate on
187 inits = new HashMap<String, BackgroundInitializer<?>>(
188 childInitializers);
189 }
190
191 // start the child initializers
192 ExecutorService exec = getActiveExecutor();
193 for (BackgroundInitializer<?> bi : inits.values()) {
194 if (bi.getExternalExecutor() == null) {
195 // share the executor service if necessary
196 bi.setExternalExecutor(exec);
197 }
198 bi.start();
199 }
200
201 // collect the results
202 Map<String, Object> results = new HashMap<String, Object>();
203 Map<String, ConcurrentException> excepts = new HashMap<String, ConcurrentException>();
204 for (Map.Entry<String, BackgroundInitializer<?>> e : inits.entrySet()) {
205 try {
206 results.put(e.getKey(), e.getValue().get());
207 } catch (ConcurrentException cex) {
208 excepts.put(e.getKey(), cex);
209 }
210 }
211
212 return new MultiBackgroundInitializerResults(inits, results, excepts);
213 }
214
215 /**
216 * A data class for storing the results of the background initialization
217 * performed by {@code MultiBackgroundInitializer}. Objects of this inner
218 * class are returned by {@link MultiBackgroundInitializer#initialize()}.
219 * They allow access to all result objects produced by the
220 * {@link BackgroundInitializer} objects managed by the owning instance. It
221 * is also possible to retrieve status information about single
222 * {@link BackgroundInitializer}s, i.e. whether they completed normally or
223 * caused an exception.
224 */
225 public static class MultiBackgroundInitializerResults {
226 /** A map with the child initializers. */
227 private final Map<String, BackgroundInitializer<?>> initializers;
228
229 /** A map with the result objects. */
230 private final Map<String, Object> resultObjects;
231
232 /** A map with the exceptions. */
233 private final Map<String, ConcurrentException> exceptions;
234
235 /**
236 * Creates a new instance of {@code MultiBackgroundInitializerResults}
237 * and initializes it with maps for the {@code BackgroundInitializer}
238 * objects, their result objects and the exceptions thrown by them.
239 *
240 * @param inits the {@code BackgroundInitializer} objects
241 * @param results the result objects
242 * @param excepts the exceptions
243 */
244 private MultiBackgroundInitializerResults(
245 Map<String, BackgroundInitializer<?>> inits,
246 Map<String, Object> results,
247 Map<String, ConcurrentException> excepts) {
248 initializers = inits;
249 resultObjects = results;
250 exceptions = excepts;
251 }
252
253 /**
254 * Returns the {@code BackgroundInitializer} with the given name. If the
255 * name cannot be resolved, an exception is thrown.
256 *
257 * @param name the name of the {@code BackgroundInitializer}
258 * @return the {@code BackgroundInitializer} with this name
259 * @throws NoSuchElementException if the name cannot be resolved
260 */
261 public BackgroundInitializer<?> getInitializer(String name) {
262 return checkName(name);
263 }
264
265 /**
266 * Returns the result object produced by the {@code
267 * BackgroundInitializer} with the given name. This is the object
268 * returned by the initializer's {@code initialize()} method. If this
269 * {@code BackgroundInitializer} caused an exception, <b>null</b> is
270 * returned. If the name cannot be resolved, an exception is thrown.
271 *
272 * @param name the name of the {@code BackgroundInitializer}
273 * @return the result object produced by this {@code
274 * BackgroundInitializer}
275 * @throws NoSuchElementException if the name cannot be resolved
276 */
277 public Object getResultObject(String name) {
278 checkName(name);
279 return resultObjects.get(name);
280 }
281
282 /**
283 * Returns a flag whether the {@code BackgroundInitializer} with the
284 * given name caused an exception.
285 *
286 * @param name the name of the {@code BackgroundInitializer}
287 * @return a flag whether this initializer caused an exception
288 * @throws NoSuchElementException if the name cannot be resolved
289 */
290 public boolean isException(String name) {
291 checkName(name);
292 return exceptions.containsKey(name);
293 }
294
295 /**
296 * Returns the {@code ConcurrentException} object that was thrown by the
297 * {@code BackgroundInitializer} with the given name. If this
298 * initializer did not throw an exception, the return value is
299 * <b>null</b>. If the name cannot be resolved, an exception is thrown.
300 *
301 * @param name the name of the {@code BackgroundInitializer}
302 * @return the exception thrown by this initializer
303 * @throws NoSuchElementException if the name cannot be resolved
304 */
305 public ConcurrentException getException(String name) {
306 checkName(name);
307 return exceptions.get(name);
308 }
309
310 /**
311 * Returns a set with the names of all {@code BackgroundInitializer}
312 * objects managed by the {@code MultiBackgroundInitializer}.
313 *
314 * @return an (unmodifiable) set with the names of the managed {@code
315 * BackgroundInitializer} objects
316 */
317 public Set<String> initializerNames() {
318 return Collections.unmodifiableSet(initializers.keySet());
319 }
320
321 /**
322 * Checks whether an initializer with the given name exists. If not,
323 * throws an exception. If it exists, the associated child initializer
324 * is returned.
325 *
326 * @param name the name to check
327 * @return the initializer with this name
328 * @throws NoSuchElementException if the name is unknown
329 */
330 private BackgroundInitializer<?> checkName(String name) {
331 BackgroundInitializer<?> init = initializers.get(name);
332 if (init == null) {
333 throw new NoSuchElementException(
334 "No child initializer with name " + name);
335 }
336
337 return init;
338 }
339 }
340 }