1/* 2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 3 * 4 * This code is free software; you can redistribute it and/or modify it 5 * under the terms of the GNU General Public License version 2 only, as 6 * published by the Free Software Foundation. Oracle designates this 7 * particular file as subject to the "Classpath" exception as provided 8 * by Oracle in the LICENSE file that accompanied this code. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 */ 24 25/* 26 * This file is available under and governed by the GNU General Public 27 * License version 2 only, as published by the Free Software Foundation. 28 * However, the following notice accompanied the original version of this 29 * file: 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36package java.util.concurrent; 37 38import java.util.Map; 39import java.util.Objects; 40import java.util.function.BiConsumer; 41import java.util.function.BiFunction; 42import java.util.function.Function; 43 44/** 45 * A {@link java.util.Map} providing thread safety and atomicity 46 * guarantees. 47 * 48 * <p>To maintain the specified guarantees, default implementations of 49 * methods including {@link #putIfAbsent} inherited from {@link Map} 50 * must be overridden by implementations of this interface. Similarly, 51 * implementations of the collections returned by methods {@link 52 * #keySet}, {@link #values}, and {@link #entrySet} must override 53 * methods such as {@code removeIf} when necessary to 54 * preserve atomicity guarantees. 55 * 56 * <p>Memory consistency effects: As with other concurrent 57 * collections, actions in a thread prior to placing an object into a 58 * {@code ConcurrentMap} as a key or value 59 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 60 * actions subsequent to the access or removal of that object from 61 * the {@code ConcurrentMap} in another thread. 62 * 63 * <p>This interface is a member of the 64 * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework"> 65 * Java Collections Framework</a>. 66 * 67 * @since 1.5 68 * @author Doug Lea 69 * @param <K> the type of keys maintained by this map 70 * @param <V> the type of mapped values 71 */ 72public interface ConcurrentMap<K,V> extends Map<K,V> { 73 74 /** 75 * {@inheritDoc} 76 * 77 * @implNote This implementation assumes that the ConcurrentMap cannot 78 * contain null values and {@code get()} returning null unambiguously means 79 * the key is absent. Implementations which support null values 80 * <strong>must</strong> override this default implementation. 81 * 82 * @throws ClassCastException {@inheritDoc} 83 * @throws NullPointerException {@inheritDoc} 84 * @since 1.8 85 */ 86 @Override 87 default V getOrDefault(Object key, V defaultValue) { 88 V v; 89 return ((v = get(key)) != null) ? v : defaultValue; 90 } 91 92 /** 93 * {@inheritDoc} 94 * 95 * @implSpec The default implementation is equivalent to, for this 96 * {@code map}: 97 * <pre> {@code 98 * for (Map.Entry<K,V> entry : map.entrySet()) { 99 * action.accept(entry.getKey(), entry.getValue()); 100 * }}</pre> 101 * 102 * @implNote The default implementation assumes that 103 * {@code IllegalStateException} thrown by {@code getKey()} or 104 * {@code getValue()} indicates that the entry has been removed and cannot 105 * be processed. Operation continues for subsequent entries. 106 * 107 * @throws NullPointerException {@inheritDoc} 108 * @since 1.8 109 */ 110 @Override 111 default void forEach(BiConsumer<? super K, ? super V> action) { 112 Objects.requireNonNull(action); 113 for (Map.Entry<K,V> entry : entrySet()) { 114 K k; 115 V v; 116 try { 117 k = entry.getKey(); 118 v = entry.getValue(); 119 } catch (IllegalStateException ise) { 120 // this usually means the entry is no longer in the map. 121 continue; 122 } 123 action.accept(k, v); 124 } 125 } 126 127 /** 128 * If the specified key is not already associated 129 * with a value, associates it with the given value. 130 * This is equivalent to, for this {@code map}: 131 * <pre> {@code 132 * if (!map.containsKey(key)) 133 * return map.put(key, value); 134 * else 135 * return map.get(key);}</pre> 136 * 137 * except that the action is performed atomically. 138 * 139 * @implNote This implementation intentionally re-abstracts the 140 * inappropriate default provided in {@code Map}. 141 * 142 * @param key key with which the specified value is to be associated 143 * @param value value to be associated with the specified key 144 * @return the previous value associated with the specified key, or 145 * {@code null} if there was no mapping for the key. 146 * (A {@code null} return can also indicate that the map 147 * previously associated {@code null} with the key, 148 * if the implementation supports null values.) 149 * @throws UnsupportedOperationException if the {@code put} operation 150 * is not supported by this map 151 * @throws ClassCastException if the class of the specified key or value 152 * prevents it from being stored in this map 153 * @throws NullPointerException if the specified key or value is null, 154 * and this map does not permit null keys or values 155 * @throws IllegalArgumentException if some property of the specified key 156 * or value prevents it from being stored in this map 157 */ 158 V putIfAbsent(K key, V value); 159 160 /** 161 * Removes the entry for a key only if currently mapped to a given value. 162 * This is equivalent to, for this {@code map}: 163 * <pre> {@code 164 * if (map.containsKey(key) 165 * && Objects.equals(map.get(key), value)) { 166 * map.remove(key); 167 * return true; 168 * } else { 169 * return false; 170 * }}</pre> 171 * 172 * except that the action is performed atomically. 173 * 174 * @implNote This implementation intentionally re-abstracts the 175 * inappropriate default provided in {@code Map}. 176 * 177 * @param key key with which the specified value is associated 178 * @param value value expected to be associated with the specified key 179 * @return {@code true} if the value was removed 180 * @throws UnsupportedOperationException if the {@code remove} operation 181 * is not supported by this map 182 * @throws ClassCastException if the key or value is of an inappropriate 183 * type for this map 184 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 185 * @throws NullPointerException if the specified key or value is null, 186 * and this map does not permit null keys or values 187 * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>) 188 */ 189 boolean remove(Object key, Object value); 190 191 /** 192 * Replaces the entry for a key only if currently mapped to a given value. 193 * This is equivalent to, for this {@code map}: 194 * <pre> {@code 195 * if (map.containsKey(key) 196 * && Objects.equals(map.get(key), oldValue)) { 197 * map.put(key, newValue); 198 * return true; 199 * } else { 200 * return false; 201 * }}</pre> 202 * 203 * except that the action is performed atomically. 204 * 205 * @implNote This implementation intentionally re-abstracts the 206 * inappropriate default provided in {@code Map}. 207 * 208 * @param key key with which the specified value is associated 209 * @param oldValue value expected to be associated with the specified key 210 * @param newValue value to be associated with the specified key 211 * @return {@code true} if the value was replaced 212 * @throws UnsupportedOperationException if the {@code put} operation 213 * is not supported by this map 214 * @throws ClassCastException if the class of a specified key or value 215 * prevents it from being stored in this map 216 * @throws NullPointerException if a specified key or value is null, 217 * and this map does not permit null keys or values 218 * @throws IllegalArgumentException if some property of a specified key 219 * or value prevents it from being stored in this map 220 */ 221 boolean replace(K key, V oldValue, V newValue); 222 223 /** 224 * Replaces the entry for a key only if currently mapped to some value. 225 * This is equivalent to, for this {@code map}: 226 * <pre> {@code 227 * if (map.containsKey(key)) 228 * return map.put(key, value); 229 * else 230 * return null;}</pre> 231 * 232 * except that the action is performed atomically. 233 * 234 * @implNote This implementation intentionally re-abstracts the 235 * inappropriate default provided in {@code Map}. 236 * 237 * @param key key with which the specified value is associated 238 * @param value value to be associated with the specified key 239 * @return the previous value associated with the specified key, or 240 * {@code null} if there was no mapping for the key. 241 * (A {@code null} return can also indicate that the map 242 * previously associated {@code null} with the key, 243 * if the implementation supports null values.) 244 * @throws UnsupportedOperationException if the {@code put} operation 245 * is not supported by this map 246 * @throws ClassCastException if the class of the specified key or value 247 * prevents it from being stored in this map 248 * @throws NullPointerException if the specified key or value is null, 249 * and this map does not permit null keys or values 250 * @throws IllegalArgumentException if some property of the specified key 251 * or value prevents it from being stored in this map 252 */ 253 V replace(K key, V value); 254 255 /** 256 * {@inheritDoc} 257 * 258 * @implSpec 259 * <p>The default implementation is equivalent to, for this {@code map}: 260 * <pre> {@code 261 * for (Map.Entry<K,V> entry : map.entrySet()) { 262 * K k; 263 * V v; 264 * do { 265 * k = entry.getKey(); 266 * v = entry.getValue(); 267 * } while (!map.replace(k, v, function.apply(k, v))); 268 * }}</pre> 269 * 270 * The default implementation may retry these steps when multiple 271 * threads attempt updates including potentially calling the function 272 * repeatedly for a given key. 273 * 274 * <p>This implementation assumes that the ConcurrentMap cannot contain null 275 * values and {@code get()} returning null unambiguously means the key is 276 * absent. Implementations which support null values <strong>must</strong> 277 * override this default implementation. 278 * 279 * @throws UnsupportedOperationException {@inheritDoc} 280 * @throws NullPointerException {@inheritDoc} 281 * @throws ClassCastException {@inheritDoc} 282 * @throws IllegalArgumentException {@inheritDoc} 283 * @since 1.8 284 */ 285 @Override 286 default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) { 287 Objects.requireNonNull(function); 288 forEach((k,v) -> { 289 while (!replace(k, v, function.apply(k, v))) { 290 // v changed or k is gone 291 if ( (v = get(k)) == null) { 292 // k is no longer in the map. 293 break; 294 } 295 } 296 }); 297 } 298 299 /** 300 * {@inheritDoc} 301 * 302 * @implSpec 303 * The default implementation is equivalent to the following steps for this 304 * {@code map}: 305 * 306 * <pre> {@code 307 * V oldValue, newValue; 308 * return ((oldValue = map.get(key)) == null 309 * && (newValue = mappingFunction.apply(key)) != null 310 * && (oldValue = map.putIfAbsent(key, newValue)) == null) 311 * ? newValue 312 * : oldValue;}</pre> 313 * 314 * <p>This implementation assumes that the ConcurrentMap cannot contain null 315 * values and {@code get()} returning null unambiguously means the key is 316 * absent. Implementations which support null values <strong>must</strong> 317 * override this default implementation. 318 * 319 * @throws UnsupportedOperationException {@inheritDoc} 320 * @throws ClassCastException {@inheritDoc} 321 * @throws NullPointerException {@inheritDoc} 322 * @throws IllegalArgumentException {@inheritDoc} 323 * @since 1.8 324 */ 325 @Override 326 default V computeIfAbsent(K key, 327 Function<? super K, ? extends V> mappingFunction) { 328 Objects.requireNonNull(mappingFunction); 329 V oldValue, newValue; 330 return ((oldValue = get(key)) == null 331 && (newValue = mappingFunction.apply(key)) != null 332 && (oldValue = putIfAbsent(key, newValue)) == null) 333 ? newValue 334 : oldValue; 335 } 336 337 /** 338 * {@inheritDoc} 339 * 340 * @implSpec 341 * The default implementation is equivalent to performing the following 342 * steps for this {@code map}: 343 * 344 * <pre> {@code 345 * for (V oldValue; (oldValue = map.get(key)) != null; ) { 346 * V newValue = remappingFunction.apply(key, oldValue); 347 * if ((newValue == null) 348 * ? map.remove(key, oldValue) 349 * : map.replace(key, oldValue, newValue)) 350 * return newValue; 351 * } 352 * return null;}</pre> 353 * When multiple threads attempt updates, map operations and the 354 * remapping function may be called multiple times. 355 * 356 * <p>This implementation assumes that the ConcurrentMap cannot contain null 357 * values and {@code get()} returning null unambiguously means the key is 358 * absent. Implementations which support null values <strong>must</strong> 359 * override this default implementation. 360 * 361 * @throws UnsupportedOperationException {@inheritDoc} 362 * @throws ClassCastException {@inheritDoc} 363 * @throws NullPointerException {@inheritDoc} 364 * @throws IllegalArgumentException {@inheritDoc} 365 * @since 1.8 366 */ 367 @Override 368 default V computeIfPresent(K key, 369 BiFunction<? super K, ? super V, ? extends V> remappingFunction) { 370 Objects.requireNonNull(remappingFunction); 371 for (V oldValue; (oldValue = get(key)) != null; ) { 372 V newValue = remappingFunction.apply(key, oldValue); 373 if ((newValue == null) 374 ? remove(key, oldValue) 375 : replace(key, oldValue, newValue)) 376 return newValue; 377 } 378 return null; 379 } 380 381 /** 382 * {@inheritDoc} 383 * 384 * @implSpec 385 * The default implementation is equivalent to performing the following 386 * steps for this {@code map}: 387 * 388 * <pre> {@code 389 * for (;;) { 390 * V oldValue = map.get(key); 391 * V newValue = remappingFunction.apply(key, oldValue); 392 * if (newValue != null) { 393 * if ((oldValue != null) 394 * ? map.replace(key, oldValue, newValue) 395 * : map.putIfAbsent(key, newValue) == null) 396 * return newValue; 397 * } else if (oldValue == null || map.remove(key, oldValue)) { 398 * return null; 399 * } 400 * }}</pre> 401 * When multiple threads attempt updates, map operations and the 402 * remapping function may be called multiple times. 403 * 404 * <p>This implementation assumes that the ConcurrentMap cannot contain null 405 * values and {@code get()} returning null unambiguously means the key is 406 * absent. Implementations which support null values <strong>must</strong> 407 * override this default implementation. 408 * 409 * @throws UnsupportedOperationException {@inheritDoc} 410 * @throws ClassCastException {@inheritDoc} 411 * @throws NullPointerException {@inheritDoc} 412 * @throws IllegalArgumentException {@inheritDoc} 413 * @since 1.8 414 */ 415 @Override 416 default V compute(K key, 417 BiFunction<? super K, ? super V, ? extends V> remappingFunction) { 418 retry: for (;;) { 419 V oldValue = get(key); 420 // if putIfAbsent fails, opportunistically use its return value 421 haveOldValue: for (;;) { 422 V newValue = remappingFunction.apply(key, oldValue); 423 if (newValue != null) { 424 if (oldValue != null) { 425 if (replace(key, oldValue, newValue)) 426 return newValue; 427 } 428 else if ((oldValue = putIfAbsent(key, newValue)) == null) 429 return newValue; 430 else continue haveOldValue; 431 } else if (oldValue == null || remove(key, oldValue)) { 432 return null; 433 } 434 continue retry; 435 } 436 } 437 } 438 439 /** 440 * {@inheritDoc} 441 * 442 * @implSpec 443 * The default implementation is equivalent to performing the following 444 * steps for this {@code map}: 445 * 446 * <pre> {@code 447 * for (;;) { 448 * V oldValue = map.get(key); 449 * if (oldValue != null) { 450 * V newValue = remappingFunction.apply(oldValue, value); 451 * if (newValue != null) { 452 * if (map.replace(key, oldValue, newValue)) 453 * return newValue; 454 * } else if (map.remove(key, oldValue)) { 455 * return null; 456 * } 457 * } else if (map.putIfAbsent(key, value) == null) { 458 * return value; 459 * } 460 * }}</pre> 461 * When multiple threads attempt updates, map operations and the 462 * remapping function may be called multiple times. 463 * 464 * <p>This implementation assumes that the ConcurrentMap cannot contain null 465 * values and {@code get()} returning null unambiguously means the key is 466 * absent. Implementations which support null values <strong>must</strong> 467 * override this default implementation. 468 * 469 * @throws UnsupportedOperationException {@inheritDoc} 470 * @throws ClassCastException {@inheritDoc} 471 * @throws NullPointerException {@inheritDoc} 472 * @throws IllegalArgumentException {@inheritDoc} 473 * @since 1.8 474 */ 475 @Override 476 default V merge(K key, V value, 477 BiFunction<? super V, ? super V, ? extends V> remappingFunction) { 478 Objects.requireNonNull(remappingFunction); 479 Objects.requireNonNull(value); 480 retry: for (;;) { 481 V oldValue = get(key); 482 // if putIfAbsent fails, opportunistically use its return value 483 haveOldValue: for (;;) { 484 if (oldValue != null) { 485 V newValue = remappingFunction.apply(oldValue, value); 486 if (newValue != null) { 487 if (replace(key, oldValue, newValue)) 488 return newValue; 489 } else if (remove(key, oldValue)) { 490 return null; 491 } 492 continue retry; 493 } else { 494 if ((oldValue = putIfAbsent(key, value)) == null) 495 return value; 496 continue haveOldValue; 497 } 498 } 499 } 500 } 501} 502