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 */ 017package org.apache.commons.lang3.concurrent; 018 019import java.util.Collections; 020import java.util.HashMap; 021import java.util.Map; 022import java.util.NoSuchElementException; 023import java.util.Set; 024import java.util.concurrent.ExecutorService; 025 026import org.apache.commons.lang3.Validate; 027 028/** 029 * <p> 030 * A specialized {@link BackgroundInitializer} implementation that can deal with 031 * multiple background initialization tasks. 032 * </p> 033 * <p> 034 * This class has a similar purpose as {@link BackgroundInitializer}. However, 035 * it is not limited to a single background initialization task. Rather it 036 * manages an arbitrary number of {@code BackgroundInitializer} objects, 037 * executes them, and waits until they are completely initialized. This is 038 * useful for applications that have to perform multiple initialization tasks 039 * that can run in parallel (i.e. that do not depend on each other). This class 040 * takes care about the management of an {@code ExecutorService} and shares it 041 * with the {@code BackgroundInitializer} objects it is responsible for; so the 042 * using application need not bother with these details. 043 * </p> 044 * <p> 045 * The typical usage scenario for {@code MultiBackgroundInitializer} is as 046 * follows: 047 * </p> 048 * <ul> 049 * <li>Create a new instance of the class. Optionally pass in a pre-configured 050 * {@code ExecutorService}. Alternatively {@code MultiBackgroundInitializer} can 051 * create a temporary {@code ExecutorService} and delete it after initialization 052 * is complete.</li> 053 * <li>Create specialized {@link BackgroundInitializer} objects for the 054 * initialization tasks to be performed and add them to the {@code 055 * MultiBackgroundInitializer} using the 056 * {@link #addInitializer(String, BackgroundInitializer)} method.</li> 057 * <li>After all initializers have been added, call the {@link #start()} method. 058 * </li> 059 * <li>When access to the result objects produced by the {@code 060 * BackgroundInitializer} objects is needed call the {@link #get()} method. The 061 * object returned here provides access to all result objects created during 062 * initialization. It also stores information about exceptions that have 063 * occurred.</li> 064 * </ul> 065 * <p> 066 * {@code MultiBackgroundInitializer} starts a special controller task that 067 * starts all {@code BackgroundInitializer} objects added to the instance. 068 * Before the an initializer is started it is checked whether this initializer 069 * already has an {@code ExecutorService} set. If this is the case, this {@code 070 * ExecutorService} is used for running the background task. Otherwise the 071 * current {@code ExecutorService} of this {@code MultiBackgroundInitializer} is 072 * shared with the initializer. 073 * </p> 074 * <p> 075 * The easiest way of using this class is to let it deal with the management of 076 * an {@code ExecutorService} itself: If no external {@code ExecutorService} is 077 * provided, the class creates a temporary {@code ExecutorService} (that is 078 * capable of executing all background tasks in parallel) and destroys it at the 079 * end of background processing. 080 * </p> 081 * <p> 082 * Alternatively an external {@code ExecutorService} can be provided - either at 083 * construction time or later by calling the 084 * {@link #setExternalExecutor(ExecutorService)} method. In this case all 085 * background tasks are scheduled at this external {@code ExecutorService}. 086 * <strong>Important note:</strong> When using an external {@code 087 * ExecutorService} be sure that the number of threads managed by the service is 088 * large enough. Otherwise a deadlock can happen! This is the case in the 089 * following scenario: {@code MultiBackgroundInitializer} starts a task that 090 * starts all registered {@code BackgroundInitializer} objects and waits for 091 * their completion. If for instance a single threaded {@code ExecutorService} 092 * is used, none of the background tasks can be executed, and the task created 093 * by {@code MultiBackgroundInitializer} waits forever. 094 * </p> 095 * 096 * @since 3.0 097 */ 098public class MultiBackgroundInitializer 099 extends 100 BackgroundInitializer<MultiBackgroundInitializer.MultiBackgroundInitializerResults> { 101 /** A map with the child initializers. */ 102 private final Map<String, BackgroundInitializer<?>> childInitializers = 103 new HashMap<>(); 104 105 /** 106 * Creates a new instance of {@code MultiBackgroundInitializer}. 107 */ 108 public MultiBackgroundInitializer() { 109 } 110 111 /** 112 * Creates a new instance of {@code MultiBackgroundInitializer} and 113 * initializes it with the given external {@code ExecutorService}. 114 * 115 * @param exec the {@code ExecutorService} for executing the background 116 * tasks 117 */ 118 public MultiBackgroundInitializer(final ExecutorService exec) { 119 super(exec); 120 } 121 122 /** 123 * Adds a new {@code BackgroundInitializer} to this object. When this 124 * {@code MultiBackgroundInitializer} is started, the given initializer will 125 * be processed. This method must not be called after {@link #start()} has 126 * been invoked. 127 * 128 * @param name the name of the initializer (must not be <b>null</b>) 129 * @param backgroundInitializer the {@code BackgroundInitializer} to add (must not be 130 * <b>null</b>) 131 * @throws IllegalArgumentException if a required parameter is missing 132 * @throws IllegalStateException if {@code start()} has already been called 133 */ 134 public void addInitializer(final String name, final BackgroundInitializer<?> backgroundInitializer) { 135 Validate.notNull(name, "name"); 136 Validate.notNull(backgroundInitializer, "backgroundInitializer"); 137 138 synchronized (this) { 139 if (isStarted()) { 140 throw new IllegalStateException("addInitializer() must not be called after start()!"); 141 } 142 childInitializers.put(name, backgroundInitializer); 143 } 144 } 145 146 /** 147 * Returns the number of tasks needed for executing all child {@code 148 * BackgroundInitializer} objects in parallel. This implementation sums up 149 * the required tasks for all child initializers (which is necessary if one 150 * of the child initializers is itself a {@code MultiBackgroundInitializer} 151 * ). Then it adds 1 for the control task that waits for the completion of 152 * the children. 153 * 154 * @return the number of tasks required for background processing 155 */ 156 @Override 157 protected int getTaskCount() { 158 int result = 1; 159 160 for (final BackgroundInitializer<?> bi : childInitializers.values()) { 161 result += bi.getTaskCount(); 162 } 163 164 return result; 165 } 166 167 /** 168 * Creates the results object. This implementation starts all child {@code 169 * BackgroundInitializer} objects. Then it collects their results and 170 * creates a {@code MultiBackgroundInitializerResults} object with this 171 * data. If a child initializer throws a checked exceptions, it is added to 172 * the results object. Unchecked exceptions are propagated. 173 * 174 * @return the results object 175 * @throws Exception if an error occurs 176 */ 177 @Override 178 protected MultiBackgroundInitializerResults initialize() throws Exception { 179 final Map<String, BackgroundInitializer<?>> inits; 180 synchronized (this) { 181 // create a snapshot to operate on 182 inits = new HashMap<>( 183 childInitializers); 184 } 185 186 // start the child initializers 187 final ExecutorService exec = getActiveExecutor(); 188 for (final BackgroundInitializer<?> bi : inits.values()) { 189 if (bi.getExternalExecutor() == null) { 190 // share the executor service if necessary 191 bi.setExternalExecutor(exec); 192 } 193 bi.start(); 194 } 195 196 // collect the results 197 final Map<String, Object> results = new HashMap<>(); 198 final Map<String, ConcurrentException> excepts = new HashMap<>(); 199 for (final Map.Entry<String, BackgroundInitializer<?>> e : inits.entrySet()) { 200 try { 201 results.put(e.getKey(), e.getValue().get()); 202 } catch (final ConcurrentException cex) { 203 excepts.put(e.getKey(), cex); 204 } 205 } 206 207 return new MultiBackgroundInitializerResults(inits, results, excepts); 208 } 209 210 /** 211 * A data class for storing the results of the background initialization 212 * performed by {@code MultiBackgroundInitializer}. Objects of this inner 213 * class are returned by {@link MultiBackgroundInitializer#initialize()}. 214 * They allow access to all result objects produced by the 215 * {@link BackgroundInitializer} objects managed by the owning instance. It 216 * is also possible to retrieve status information about single 217 * {@link BackgroundInitializer}s, i.e. whether they completed normally or 218 * caused an exception. 219 */ 220 public static class MultiBackgroundInitializerResults { 221 /** A map with the child initializers. */ 222 private final Map<String, BackgroundInitializer<?>> initializers; 223 224 /** A map with the result objects. */ 225 private final Map<String, Object> resultObjects; 226 227 /** A map with the exceptions. */ 228 private final Map<String, ConcurrentException> exceptions; 229 230 /** 231 * Creates a new instance of {@code MultiBackgroundInitializerResults} 232 * and initializes it with maps for the {@code BackgroundInitializer} 233 * objects, their result objects and the exceptions thrown by them. 234 * 235 * @param inits the {@code BackgroundInitializer} objects 236 * @param results the result objects 237 * @param excepts the exceptions 238 */ 239 private MultiBackgroundInitializerResults( 240 final Map<String, BackgroundInitializer<?>> inits, 241 final Map<String, Object> results, 242 final Map<String, ConcurrentException> excepts) { 243 initializers = inits; 244 resultObjects = results; 245 exceptions = excepts; 246 } 247 248 /** 249 * Returns the {@code BackgroundInitializer} with the given name. If the 250 * name cannot be resolved, an exception is thrown. 251 * 252 * @param name the name of the {@code BackgroundInitializer} 253 * @return the {@code BackgroundInitializer} with this name 254 * @throws NoSuchElementException if the name cannot be resolved 255 */ 256 public BackgroundInitializer<?> getInitializer(final String name) { 257 return checkName(name); 258 } 259 260 /** 261 * Returns the result object produced by the {@code 262 * BackgroundInitializer} with the given name. This is the object 263 * returned by the initializer's {@code initialize()} method. If this 264 * {@code BackgroundInitializer} caused an exception, <b>null</b> is 265 * returned. If the name cannot be resolved, an exception is thrown. 266 * 267 * @param name the name of the {@code BackgroundInitializer} 268 * @return the result object produced by this {@code 269 * BackgroundInitializer} 270 * @throws NoSuchElementException if the name cannot be resolved 271 */ 272 public Object getResultObject(final String name) { 273 checkName(name); 274 return resultObjects.get(name); 275 } 276 277 /** 278 * Returns a flag whether the {@code BackgroundInitializer} with the 279 * given name caused an exception. 280 * 281 * @param name the name of the {@code BackgroundInitializer} 282 * @return a flag whether this initializer caused an exception 283 * @throws NoSuchElementException if the name cannot be resolved 284 */ 285 public boolean isException(final String name) { 286 checkName(name); 287 return exceptions.containsKey(name); 288 } 289 290 /** 291 * Returns the {@code ConcurrentException} object that was thrown by the 292 * {@code BackgroundInitializer} with the given name. If this 293 * initializer did not throw an exception, the return value is 294 * <b>null</b>. If the name cannot be resolved, an exception is thrown. 295 * 296 * @param name the name of the {@code BackgroundInitializer} 297 * @return the exception thrown by this initializer 298 * @throws NoSuchElementException if the name cannot be resolved 299 */ 300 public ConcurrentException getException(final String name) { 301 checkName(name); 302 return exceptions.get(name); 303 } 304 305 /** 306 * Returns a set with the names of all {@code BackgroundInitializer} 307 * objects managed by the {@code MultiBackgroundInitializer}. 308 * 309 * @return an (unmodifiable) set with the names of the managed {@code 310 * BackgroundInitializer} objects 311 */ 312 public Set<String> initializerNames() { 313 return Collections.unmodifiableSet(initializers.keySet()); 314 } 315 316 /** 317 * Returns a flag whether the whole initialization was successful. This 318 * is the case if no child initializer has thrown an exception. 319 * 320 * @return a flag whether the initialization was successful 321 */ 322 public boolean isSuccessful() { 323 return exceptions.isEmpty(); 324 } 325 326 /** 327 * Checks whether an initializer with the given name exists. If not, 328 * throws an exception. If it exists, the associated child initializer 329 * is returned. 330 * 331 * @param name the name to check 332 * @return the initializer with this name 333 * @throws NoSuchElementException if the name is unknown 334 */ 335 private BackgroundInitializer<?> checkName(final String name) { 336 final BackgroundInitializer<?> init = initializers.get(name); 337 if (init == null) { 338 throw new NoSuchElementException( 339 "No child initializer with name " + name); 340 } 341 342 return init; 343 } 344 } 345}