J. Duke | 319a3b9 | 2007-12-01 00:00:00 +0000 | [diff] [blame^] | 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. Sun designates this |
| 7 | * particular file as subject to the "Classpath" exception as provided |
| 8 | * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
| 21 | * CA 95054 USA or visit www.sun.com if you need additional information or |
| 22 | * have any 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/licenses/publicdomain |
| 34 | */ |
| 35 | |
| 36 | package java.util.concurrent.locks; |
| 37 | import java.util.concurrent.*; |
| 38 | import java.util.concurrent.atomic.*; |
| 39 | import java.util.*; |
| 40 | |
| 41 | /** |
| 42 | * An implementation of {@link ReadWriteLock} supporting similar |
| 43 | * semantics to {@link ReentrantLock}. |
| 44 | * <p>This class has the following properties: |
| 45 | * |
| 46 | * <ul> |
| 47 | * <li><b>Acquisition order</b> |
| 48 | * |
| 49 | * <p> This class does not impose a reader or writer preference |
| 50 | * ordering for lock access. However, it does support an optional |
| 51 | * <em>fairness</em> policy. |
| 52 | * |
| 53 | * <dl> |
| 54 | * <dt><b><i>Non-fair mode (default)</i></b> |
| 55 | * <dd>When constructed as non-fair (the default), the order of entry |
| 56 | * to the read and write lock is unspecified, subject to reentrancy |
| 57 | * constraints. A nonfair lock that is continuously contended may |
| 58 | * indefinitely postpone one or more reader or writer threads, but |
| 59 | * will normally have higher throughput than a fair lock. |
| 60 | * <p> |
| 61 | * |
| 62 | * <dt><b><i>Fair mode</i></b> |
| 63 | * <dd> When constructed as fair, threads contend for entry using an |
| 64 | * approximately arrival-order policy. When the currently held lock |
| 65 | * is released either the longest-waiting single writer thread will |
| 66 | * be assigned the write lock, or if there is a group of reader threads |
| 67 | * waiting longer than all waiting writer threads, that group will be |
| 68 | * assigned the read lock. |
| 69 | * |
| 70 | * <p>A thread that tries to acquire a fair read lock (non-reentrantly) |
| 71 | * will block if either the write lock is held, or there is a waiting |
| 72 | * writer thread. The thread will not acquire the read lock until |
| 73 | * after the oldest currently waiting writer thread has acquired and |
| 74 | * released the write lock. Of course, if a waiting writer abandons |
| 75 | * its wait, leaving one or more reader threads as the longest waiters |
| 76 | * in the queue with the write lock free, then those readers will be |
| 77 | * assigned the read lock. |
| 78 | * |
| 79 | * <p>A thread that tries to acquire a fair write lock (non-reentrantly) |
| 80 | * will block unless both the read lock and write lock are free (which |
| 81 | * implies there are no waiting threads). (Note that the non-blocking |
| 82 | * {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods |
| 83 | * do not honor this fair setting and will acquire the lock if it is |
| 84 | * possible, regardless of waiting threads.) |
| 85 | * <p> |
| 86 | * </dl> |
| 87 | * |
| 88 | * <li><b>Reentrancy</b> |
| 89 | * |
| 90 | * <p>This lock allows both readers and writers to reacquire read or |
| 91 | * write locks in the style of a {@link ReentrantLock}. Non-reentrant |
| 92 | * readers are not allowed until all write locks held by the writing |
| 93 | * thread have been released. |
| 94 | * |
| 95 | * <p>Additionally, a writer can acquire the read lock, but not |
| 96 | * vice-versa. Among other applications, reentrancy can be useful |
| 97 | * when write locks are held during calls or callbacks to methods that |
| 98 | * perform reads under read locks. If a reader tries to acquire the |
| 99 | * write lock it will never succeed. |
| 100 | * |
| 101 | * <li><b>Lock downgrading</b> |
| 102 | * <p>Reentrancy also allows downgrading from the write lock to a read lock, |
| 103 | * by acquiring the write lock, then the read lock and then releasing the |
| 104 | * write lock. However, upgrading from a read lock to the write lock is |
| 105 | * <b>not</b> possible. |
| 106 | * |
| 107 | * <li><b>Interruption of lock acquisition</b> |
| 108 | * <p>The read lock and write lock both support interruption during lock |
| 109 | * acquisition. |
| 110 | * |
| 111 | * <li><b>{@link Condition} support</b> |
| 112 | * <p>The write lock provides a {@link Condition} implementation that |
| 113 | * behaves in the same way, with respect to the write lock, as the |
| 114 | * {@link Condition} implementation provided by |
| 115 | * {@link ReentrantLock#newCondition} does for {@link ReentrantLock}. |
| 116 | * This {@link Condition} can, of course, only be used with the write lock. |
| 117 | * |
| 118 | * <p>The read lock does not support a {@link Condition} and |
| 119 | * {@code readLock().newCondition()} throws |
| 120 | * {@code UnsupportedOperationException}. |
| 121 | * |
| 122 | * <li><b>Instrumentation</b> |
| 123 | * <p>This class supports methods to determine whether locks |
| 124 | * are held or contended. These methods are designed for monitoring |
| 125 | * system state, not for synchronization control. |
| 126 | * </ul> |
| 127 | * |
| 128 | * <p>Serialization of this class behaves in the same way as built-in |
| 129 | * locks: a deserialized lock is in the unlocked state, regardless of |
| 130 | * its state when serialized. |
| 131 | * |
| 132 | * <p><b>Sample usages</b>. Here is a code sketch showing how to perform |
| 133 | * lock downgrading after updating a cache (exception handling is |
| 134 | * particularly tricky when handling multiple locks in a non-nested |
| 135 | * fashion): |
| 136 | * |
| 137 | * <pre> {@code |
| 138 | * class CachedData { |
| 139 | * Object data; |
| 140 | * volatile boolean cacheValid; |
| 141 | * final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); |
| 142 | * |
| 143 | * void processCachedData() { |
| 144 | * rwl.readLock().lock(); |
| 145 | * if (!cacheValid) { |
| 146 | * // Must release read lock before acquiring write lock |
| 147 | * rwl.readLock().unlock(); |
| 148 | * rwl.writeLock().lock(); |
| 149 | * try { |
| 150 | * // Recheck state because another thread might have |
| 151 | * // acquired write lock and changed state before we did. |
| 152 | * if (!cacheValid) { |
| 153 | * data = ... |
| 154 | * cacheValid = true; |
| 155 | * } |
| 156 | * // Downgrade by acquiring read lock before releasing write lock |
| 157 | * rwl.readLock().lock(); |
| 158 | * } finally { |
| 159 | * rwl.writeLock().unlock(); // Unlock write, still hold read |
| 160 | * } |
| 161 | * } |
| 162 | * |
| 163 | * try { |
| 164 | * use(data); |
| 165 | * } finally { |
| 166 | * rwl.readLock().unlock(); |
| 167 | * } |
| 168 | * } |
| 169 | * }}</pre> |
| 170 | * |
| 171 | * ReentrantReadWriteLocks can be used to improve concurrency in some |
| 172 | * uses of some kinds of Collections. This is typically worthwhile |
| 173 | * only when the collections are expected to be large, accessed by |
| 174 | * more reader threads than writer threads, and entail operations with |
| 175 | * overhead that outweighs synchronization overhead. For example, here |
| 176 | * is a class using a TreeMap that is expected to be large and |
| 177 | * concurrently accessed. |
| 178 | * |
| 179 | * <pre>{@code |
| 180 | * class RWDictionary { |
| 181 | * private final Map<String, Data> m = new TreeMap<String, Data>(); |
| 182 | * private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); |
| 183 | * private final Lock r = rwl.readLock(); |
| 184 | * private final Lock w = rwl.writeLock(); |
| 185 | * |
| 186 | * public Data get(String key) { |
| 187 | * r.lock(); |
| 188 | * try { return m.get(key); } |
| 189 | * finally { r.unlock(); } |
| 190 | * } |
| 191 | * public String[] allKeys() { |
| 192 | * r.lock(); |
| 193 | * try { return m.keySet().toArray(); } |
| 194 | * finally { r.unlock(); } |
| 195 | * } |
| 196 | * public Data put(String key, Data value) { |
| 197 | * w.lock(); |
| 198 | * try { return m.put(key, value); } |
| 199 | * finally { w.unlock(); } |
| 200 | * } |
| 201 | * public void clear() { |
| 202 | * w.lock(); |
| 203 | * try { m.clear(); } |
| 204 | * finally { w.unlock(); } |
| 205 | * } |
| 206 | * }}</pre> |
| 207 | * |
| 208 | * <h3>Implementation Notes</h3> |
| 209 | * |
| 210 | * <p>This lock supports a maximum of 65535 recursive write locks |
| 211 | * and 65535 read locks. Attempts to exceed these limits result in |
| 212 | * {@link Error} throws from locking methods. |
| 213 | * |
| 214 | * @since 1.5 |
| 215 | * @author Doug Lea |
| 216 | * |
| 217 | */ |
| 218 | public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable { |
| 219 | private static final long serialVersionUID = -6992448646407690164L; |
| 220 | /** Inner class providing readlock */ |
| 221 | private final ReentrantReadWriteLock.ReadLock readerLock; |
| 222 | /** Inner class providing writelock */ |
| 223 | private final ReentrantReadWriteLock.WriteLock writerLock; |
| 224 | /** Performs all synchronization mechanics */ |
| 225 | private final Sync sync; |
| 226 | |
| 227 | /** |
| 228 | * Creates a new {@code ReentrantReadWriteLock} with |
| 229 | * default (nonfair) ordering properties. |
| 230 | */ |
| 231 | public ReentrantReadWriteLock() { |
| 232 | this(false); |
| 233 | } |
| 234 | |
| 235 | /** |
| 236 | * Creates a new {@code ReentrantReadWriteLock} with |
| 237 | * the given fairness policy. |
| 238 | * |
| 239 | * @param fair {@code true} if this lock should use a fair ordering policy |
| 240 | */ |
| 241 | public ReentrantReadWriteLock(boolean fair) { |
| 242 | sync = (fair)? new FairSync() : new NonfairSync(); |
| 243 | readerLock = new ReadLock(this); |
| 244 | writerLock = new WriteLock(this); |
| 245 | } |
| 246 | |
| 247 | public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; } |
| 248 | public ReentrantReadWriteLock.ReadLock readLock() { return readerLock; } |
| 249 | |
| 250 | /** |
| 251 | * Synchronization implementation for ReentrantReadWriteLock. |
| 252 | * Subclassed into fair and nonfair versions. |
| 253 | */ |
| 254 | static abstract class Sync extends AbstractQueuedSynchronizer { |
| 255 | private static final long serialVersionUID = 6317671515068378041L; |
| 256 | |
| 257 | /* |
| 258 | * Read vs write count extraction constants and functions. |
| 259 | * Lock state is logically divided into two shorts: The lower |
| 260 | * one representing the exclusive (writer) lock hold count, |
| 261 | * and the upper the shared (reader) hold count. |
| 262 | */ |
| 263 | |
| 264 | static final int SHARED_SHIFT = 16; |
| 265 | static final int SHARED_UNIT = (1 << SHARED_SHIFT); |
| 266 | static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1; |
| 267 | static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1; |
| 268 | |
| 269 | /** Returns the number of shared holds represented in count */ |
| 270 | static int sharedCount(int c) { return c >>> SHARED_SHIFT; } |
| 271 | /** Returns the number of exclusive holds represented in count */ |
| 272 | static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; } |
| 273 | |
| 274 | /** |
| 275 | * A counter for per-thread read hold counts. |
| 276 | * Maintained as a ThreadLocal; cached in cachedHoldCounter |
| 277 | */ |
| 278 | static final class HoldCounter { |
| 279 | int count; |
| 280 | // Use id, not reference, to avoid garbage retention |
| 281 | final long tid = Thread.currentThread().getId(); |
| 282 | /** Decrement if positive; return previous value */ |
| 283 | int tryDecrement() { |
| 284 | int c = count; |
| 285 | if (c > 0) |
| 286 | count = c - 1; |
| 287 | return c; |
| 288 | } |
| 289 | } |
| 290 | |
| 291 | /** |
| 292 | * ThreadLocal subclass. Easiest to explicitly define for sake |
| 293 | * of deserialization mechanics. |
| 294 | */ |
| 295 | static final class ThreadLocalHoldCounter |
| 296 | extends ThreadLocal<HoldCounter> { |
| 297 | public HoldCounter initialValue() { |
| 298 | return new HoldCounter(); |
| 299 | } |
| 300 | } |
| 301 | |
| 302 | /** |
| 303 | * The number of read locks held by current thread. |
| 304 | * Initialized only in constructor and readObject. |
| 305 | */ |
| 306 | transient ThreadLocalHoldCounter readHolds; |
| 307 | |
| 308 | /** |
| 309 | * The hold count of the last thread to successfully acquire |
| 310 | * readLock. This saves ThreadLocal lookup in the common case |
| 311 | * where the next thread to release is the last one to |
| 312 | * acquire. This is non-volatile since it is just used |
| 313 | * as a heuristic, and would be great for threads to cache. |
| 314 | */ |
| 315 | transient HoldCounter cachedHoldCounter; |
| 316 | |
| 317 | Sync() { |
| 318 | readHolds = new ThreadLocalHoldCounter(); |
| 319 | setState(getState()); // ensures visibility of readHolds |
| 320 | } |
| 321 | |
| 322 | /* |
| 323 | * Acquires and releases use the same code for fair and |
| 324 | * nonfair locks, but differ in whether/how they allow barging |
| 325 | * when queues are non-empty. |
| 326 | */ |
| 327 | |
| 328 | /** |
| 329 | * Returns true if the current thread, when trying to acquire |
| 330 | * the read lock, and otherwise eligible to do so, should block |
| 331 | * because of policy for overtaking other waiting threads. |
| 332 | */ |
| 333 | abstract boolean readerShouldBlock(); |
| 334 | |
| 335 | /** |
| 336 | * Returns true if the current thread, when trying to acquire |
| 337 | * the write lock, and otherwise eligible to do so, should block |
| 338 | * because of policy for overtaking other waiting threads. |
| 339 | */ |
| 340 | abstract boolean writerShouldBlock(); |
| 341 | |
| 342 | /* |
| 343 | * Note that tryRelease and tryAcquire can be called by |
| 344 | * Conditions. So it is possible that their arguments contain |
| 345 | * both read and write holds that are all released during a |
| 346 | * condition wait and re-established in tryAcquire. |
| 347 | */ |
| 348 | |
| 349 | protected final boolean tryRelease(int releases) { |
| 350 | if (!isHeldExclusively()) |
| 351 | throw new IllegalMonitorStateException(); |
| 352 | int nextc = getState() - releases; |
| 353 | boolean free = exclusiveCount(nextc) == 0; |
| 354 | if (free) |
| 355 | setExclusiveOwnerThread(null); |
| 356 | setState(nextc); |
| 357 | return free; |
| 358 | } |
| 359 | |
| 360 | protected final boolean tryAcquire(int acquires) { |
| 361 | /* |
| 362 | * Walkthrough: |
| 363 | * 1. If read count nonzero or write count nonzero |
| 364 | * and owner is a different thread, fail. |
| 365 | * 2. If count would saturate, fail. (This can only |
| 366 | * happen if count is already nonzero.) |
| 367 | * 3. Otherwise, this thread is eligible for lock if |
| 368 | * it is either a reentrant acquire or |
| 369 | * queue policy allows it. If so, update state |
| 370 | * and set owner. |
| 371 | */ |
| 372 | Thread current = Thread.currentThread(); |
| 373 | int c = getState(); |
| 374 | int w = exclusiveCount(c); |
| 375 | if (c != 0) { |
| 376 | // (Note: if c != 0 and w == 0 then shared count != 0) |
| 377 | if (w == 0 || current != getExclusiveOwnerThread()) |
| 378 | return false; |
| 379 | if (w + exclusiveCount(acquires) > MAX_COUNT) |
| 380 | throw new Error("Maximum lock count exceeded"); |
| 381 | // Reentrant acquire |
| 382 | setState(c + acquires); |
| 383 | return true; |
| 384 | } |
| 385 | if (writerShouldBlock() || |
| 386 | !compareAndSetState(c, c + acquires)) |
| 387 | return false; |
| 388 | setExclusiveOwnerThread(current); |
| 389 | return true; |
| 390 | } |
| 391 | |
| 392 | protected final boolean tryReleaseShared(int unused) { |
| 393 | HoldCounter rh = cachedHoldCounter; |
| 394 | Thread current = Thread.currentThread(); |
| 395 | if (rh == null || rh.tid != current.getId()) |
| 396 | rh = readHolds.get(); |
| 397 | if (rh.tryDecrement() <= 0) |
| 398 | throw new IllegalMonitorStateException(); |
| 399 | for (;;) { |
| 400 | int c = getState(); |
| 401 | int nextc = c - SHARED_UNIT; |
| 402 | if (compareAndSetState(c, nextc)) |
| 403 | return nextc == 0; |
| 404 | } |
| 405 | } |
| 406 | |
| 407 | protected final int tryAcquireShared(int unused) { |
| 408 | /* |
| 409 | * Walkthrough: |
| 410 | * 1. If write lock held by another thread, fail. |
| 411 | * 2. If count saturated, throw error. |
| 412 | * 3. Otherwise, this thread is eligible for |
| 413 | * lock wrt state, so ask if it should block |
| 414 | * because of queue policy. If not, try |
| 415 | * to grant by CASing state and updating count. |
| 416 | * Note that step does not check for reentrant |
| 417 | * acquires, which is postponed to full version |
| 418 | * to avoid having to check hold count in |
| 419 | * the more typical non-reentrant case. |
| 420 | * 4. If step 3 fails either because thread |
| 421 | * apparently not eligible or CAS fails, |
| 422 | * chain to version with full retry loop. |
| 423 | */ |
| 424 | Thread current = Thread.currentThread(); |
| 425 | int c = getState(); |
| 426 | if (exclusiveCount(c) != 0 && |
| 427 | getExclusiveOwnerThread() != current) |
| 428 | return -1; |
| 429 | if (sharedCount(c) == MAX_COUNT) |
| 430 | throw new Error("Maximum lock count exceeded"); |
| 431 | if (!readerShouldBlock() && |
| 432 | compareAndSetState(c, c + SHARED_UNIT)) { |
| 433 | HoldCounter rh = cachedHoldCounter; |
| 434 | if (rh == null || rh.tid != current.getId()) |
| 435 | cachedHoldCounter = rh = readHolds.get(); |
| 436 | rh.count++; |
| 437 | return 1; |
| 438 | } |
| 439 | return fullTryAcquireShared(current); |
| 440 | } |
| 441 | |
| 442 | /** |
| 443 | * Full version of acquire for reads, that handles CAS misses |
| 444 | * and reentrant reads not dealt with in tryAcquireShared. |
| 445 | */ |
| 446 | final int fullTryAcquireShared(Thread current) { |
| 447 | /* |
| 448 | * This code is in part redundant with that in |
| 449 | * tryAcquireShared but is simpler overall by not |
| 450 | * complicating tryAcquireShared with interactions between |
| 451 | * retries and lazily reading hold counts. |
| 452 | */ |
| 453 | HoldCounter rh = cachedHoldCounter; |
| 454 | if (rh == null || rh.tid != current.getId()) |
| 455 | rh = readHolds.get(); |
| 456 | for (;;) { |
| 457 | int c = getState(); |
| 458 | int w = exclusiveCount(c); |
| 459 | if ((w != 0 && getExclusiveOwnerThread() != current) || |
| 460 | ((rh.count | w) == 0 && readerShouldBlock())) |
| 461 | return -1; |
| 462 | if (sharedCount(c) == MAX_COUNT) |
| 463 | throw new Error("Maximum lock count exceeded"); |
| 464 | if (compareAndSetState(c, c + SHARED_UNIT)) { |
| 465 | cachedHoldCounter = rh; // cache for release |
| 466 | rh.count++; |
| 467 | return 1; |
| 468 | } |
| 469 | } |
| 470 | } |
| 471 | |
| 472 | /** |
| 473 | * Performs tryLock for write, enabling barging in both modes. |
| 474 | * This is identical in effect to tryAcquire except for lack |
| 475 | * of calls to writerShouldBlock |
| 476 | */ |
| 477 | final boolean tryWriteLock() { |
| 478 | Thread current = Thread.currentThread(); |
| 479 | int c = getState(); |
| 480 | if (c != 0) { |
| 481 | int w = exclusiveCount(c); |
| 482 | if (w == 0 ||current != getExclusiveOwnerThread()) |
| 483 | return false; |
| 484 | if (w == MAX_COUNT) |
| 485 | throw new Error("Maximum lock count exceeded"); |
| 486 | } |
| 487 | if (!compareAndSetState(c, c + 1)) |
| 488 | return false; |
| 489 | setExclusiveOwnerThread(current); |
| 490 | return true; |
| 491 | } |
| 492 | |
| 493 | /** |
| 494 | * Performs tryLock for read, enabling barging in both modes. |
| 495 | * This is identical in effect to tryAcquireShared except for |
| 496 | * lack of calls to readerShouldBlock |
| 497 | */ |
| 498 | final boolean tryReadLock() { |
| 499 | Thread current = Thread.currentThread(); |
| 500 | for (;;) { |
| 501 | int c = getState(); |
| 502 | if (exclusiveCount(c) != 0 && |
| 503 | getExclusiveOwnerThread() != current) |
| 504 | return false; |
| 505 | if (sharedCount(c) == MAX_COUNT) |
| 506 | throw new Error("Maximum lock count exceeded"); |
| 507 | if (compareAndSetState(c, c + SHARED_UNIT)) { |
| 508 | HoldCounter rh = cachedHoldCounter; |
| 509 | if (rh == null || rh.tid != current.getId()) |
| 510 | cachedHoldCounter = rh = readHolds.get(); |
| 511 | rh.count++; |
| 512 | return true; |
| 513 | } |
| 514 | } |
| 515 | } |
| 516 | |
| 517 | protected final boolean isHeldExclusively() { |
| 518 | // While we must in general read state before owner, |
| 519 | // we don't need to do so to check if current thread is owner |
| 520 | return getExclusiveOwnerThread() == Thread.currentThread(); |
| 521 | } |
| 522 | |
| 523 | // Methods relayed to outer class |
| 524 | |
| 525 | final ConditionObject newCondition() { |
| 526 | return new ConditionObject(); |
| 527 | } |
| 528 | |
| 529 | final Thread getOwner() { |
| 530 | // Must read state before owner to ensure memory consistency |
| 531 | return ((exclusiveCount(getState()) == 0)? |
| 532 | null : |
| 533 | getExclusiveOwnerThread()); |
| 534 | } |
| 535 | |
| 536 | final int getReadLockCount() { |
| 537 | return sharedCount(getState()); |
| 538 | } |
| 539 | |
| 540 | final boolean isWriteLocked() { |
| 541 | return exclusiveCount(getState()) != 0; |
| 542 | } |
| 543 | |
| 544 | final int getWriteHoldCount() { |
| 545 | return isHeldExclusively() ? exclusiveCount(getState()) : 0; |
| 546 | } |
| 547 | |
| 548 | final int getReadHoldCount() { |
| 549 | return getReadLockCount() == 0? 0 : readHolds.get().count; |
| 550 | } |
| 551 | |
| 552 | /** |
| 553 | * Reconstitute this lock instance from a stream |
| 554 | * @param s the stream |
| 555 | */ |
| 556 | private void readObject(java.io.ObjectInputStream s) |
| 557 | throws java.io.IOException, ClassNotFoundException { |
| 558 | s.defaultReadObject(); |
| 559 | readHolds = new ThreadLocalHoldCounter(); |
| 560 | setState(0); // reset to unlocked state |
| 561 | } |
| 562 | |
| 563 | final int getCount() { return getState(); } |
| 564 | } |
| 565 | |
| 566 | /** |
| 567 | * Nonfair version of Sync |
| 568 | */ |
| 569 | final static class NonfairSync extends Sync { |
| 570 | private static final long serialVersionUID = -8159625535654395037L; |
| 571 | final boolean writerShouldBlock() { |
| 572 | return false; // writers can always barge |
| 573 | } |
| 574 | final boolean readerShouldBlock() { |
| 575 | /* As a heuristic to avoid indefinite writer starvation, |
| 576 | * block if the thread that momentarily appears to be head |
| 577 | * of queue, if one exists, is a waiting writer. This is |
| 578 | * only a probabilistic effect since a new reader will not |
| 579 | * block if there is a waiting writer behind other enabled |
| 580 | * readers that have not yet drained from the queue. |
| 581 | */ |
| 582 | return apparentlyFirstQueuedIsExclusive(); |
| 583 | } |
| 584 | } |
| 585 | |
| 586 | /** |
| 587 | * Fair version of Sync |
| 588 | */ |
| 589 | final static class FairSync extends Sync { |
| 590 | private static final long serialVersionUID = -2274990926593161451L; |
| 591 | final boolean writerShouldBlock() { |
| 592 | return hasQueuedPredecessors(); |
| 593 | } |
| 594 | final boolean readerShouldBlock() { |
| 595 | return hasQueuedPredecessors(); |
| 596 | } |
| 597 | } |
| 598 | |
| 599 | /** |
| 600 | * The lock returned by method {@link ReentrantReadWriteLock#readLock}. |
| 601 | */ |
| 602 | public static class ReadLock implements Lock, java.io.Serializable { |
| 603 | private static final long serialVersionUID = -5992448646407690164L; |
| 604 | private final Sync sync; |
| 605 | |
| 606 | /** |
| 607 | * Constructor for use by subclasses |
| 608 | * |
| 609 | * @param lock the outer lock object |
| 610 | * @throws NullPointerException if the lock is null |
| 611 | */ |
| 612 | protected ReadLock(ReentrantReadWriteLock lock) { |
| 613 | sync = lock.sync; |
| 614 | } |
| 615 | |
| 616 | /** |
| 617 | * Acquires the read lock. |
| 618 | * |
| 619 | * <p>Acquires the read lock if the write lock is not held by |
| 620 | * another thread and returns immediately. |
| 621 | * |
| 622 | * <p>If the write lock is held by another thread then |
| 623 | * the current thread becomes disabled for thread scheduling |
| 624 | * purposes and lies dormant until the read lock has been acquired. |
| 625 | */ |
| 626 | public void lock() { |
| 627 | sync.acquireShared(1); |
| 628 | } |
| 629 | |
| 630 | /** |
| 631 | * Acquires the read lock unless the current thread is |
| 632 | * {@linkplain Thread#interrupt interrupted}. |
| 633 | * |
| 634 | * <p>Acquires the read lock if the write lock is not held |
| 635 | * by another thread and returns immediately. |
| 636 | * |
| 637 | * <p>If the write lock is held by another thread then the |
| 638 | * current thread becomes disabled for thread scheduling |
| 639 | * purposes and lies dormant until one of two things happens: |
| 640 | * |
| 641 | * <ul> |
| 642 | * |
| 643 | * <li>The read lock is acquired by the current thread; or |
| 644 | * |
| 645 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 646 | * the current thread. |
| 647 | * |
| 648 | * </ul> |
| 649 | * |
| 650 | * <p>If the current thread: |
| 651 | * |
| 652 | * <ul> |
| 653 | * |
| 654 | * <li>has its interrupted status set on entry to this method; or |
| 655 | * |
| 656 | * <li>is {@linkplain Thread#interrupt interrupted} while |
| 657 | * acquiring the read lock, |
| 658 | * |
| 659 | * </ul> |
| 660 | * |
| 661 | * then {@link InterruptedException} is thrown and the current |
| 662 | * thread's interrupted status is cleared. |
| 663 | * |
| 664 | * <p>In this implementation, as this method is an explicit |
| 665 | * interruption point, preference is given to responding to |
| 666 | * the interrupt over normal or reentrant acquisition of the |
| 667 | * lock. |
| 668 | * |
| 669 | * @throws InterruptedException if the current thread is interrupted |
| 670 | */ |
| 671 | public void lockInterruptibly() throws InterruptedException { |
| 672 | sync.acquireSharedInterruptibly(1); |
| 673 | } |
| 674 | |
| 675 | /** |
| 676 | * Acquires the read lock only if the write lock is not held by |
| 677 | * another thread at the time of invocation. |
| 678 | * |
| 679 | * <p>Acquires the read lock if the write lock is not held by |
| 680 | * another thread and returns immediately with the value |
| 681 | * {@code true}. Even when this lock has been set to use a |
| 682 | * fair ordering policy, a call to {@code tryLock()} |
| 683 | * <em>will</em> immediately acquire the read lock if it is |
| 684 | * available, whether or not other threads are currently |
| 685 | * waiting for the read lock. This "barging" behavior |
| 686 | * can be useful in certain circumstances, even though it |
| 687 | * breaks fairness. If you want to honor the fairness setting |
| 688 | * for this lock, then use {@link #tryLock(long, TimeUnit) |
| 689 | * tryLock(0, TimeUnit.SECONDS) } which is almost equivalent |
| 690 | * (it also detects interruption). |
| 691 | * |
| 692 | * <p>If the write lock is held by another thread then |
| 693 | * this method will return immediately with the value |
| 694 | * {@code false}. |
| 695 | * |
| 696 | * @return {@code true} if the read lock was acquired |
| 697 | */ |
| 698 | public boolean tryLock() { |
| 699 | return sync.tryReadLock(); |
| 700 | } |
| 701 | |
| 702 | /** |
| 703 | * Acquires the read lock if the write lock is not held by |
| 704 | * another thread within the given waiting time and the |
| 705 | * current thread has not been {@linkplain Thread#interrupt |
| 706 | * interrupted}. |
| 707 | * |
| 708 | * <p>Acquires the read lock if the write lock is not held by |
| 709 | * another thread and returns immediately with the value |
| 710 | * {@code true}. If this lock has been set to use a fair |
| 711 | * ordering policy then an available lock <em>will not</em> be |
| 712 | * acquired if any other threads are waiting for the |
| 713 | * lock. This is in contrast to the {@link #tryLock()} |
| 714 | * method. If you want a timed {@code tryLock} that does |
| 715 | * permit barging on a fair lock then combine the timed and |
| 716 | * un-timed forms together: |
| 717 | * |
| 718 | * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... } |
| 719 | * </pre> |
| 720 | * |
| 721 | * <p>If the write lock is held by another thread then the |
| 722 | * current thread becomes disabled for thread scheduling |
| 723 | * purposes and lies dormant until one of three things happens: |
| 724 | * |
| 725 | * <ul> |
| 726 | * |
| 727 | * <li>The read lock is acquired by the current thread; or |
| 728 | * |
| 729 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 730 | * the current thread; or |
| 731 | * |
| 732 | * <li>The specified waiting time elapses. |
| 733 | * |
| 734 | * </ul> |
| 735 | * |
| 736 | * <p>If the read lock is acquired then the value {@code true} is |
| 737 | * returned. |
| 738 | * |
| 739 | * <p>If the current thread: |
| 740 | * |
| 741 | * <ul> |
| 742 | * |
| 743 | * <li>has its interrupted status set on entry to this method; or |
| 744 | * |
| 745 | * <li>is {@linkplain Thread#interrupt interrupted} while |
| 746 | * acquiring the read lock, |
| 747 | * |
| 748 | * </ul> then {@link InterruptedException} is thrown and the |
| 749 | * current thread's interrupted status is cleared. |
| 750 | * |
| 751 | * <p>If the specified waiting time elapses then the value |
| 752 | * {@code false} is returned. If the time is less than or |
| 753 | * equal to zero, the method will not wait at all. |
| 754 | * |
| 755 | * <p>In this implementation, as this method is an explicit |
| 756 | * interruption point, preference is given to responding to |
| 757 | * the interrupt over normal or reentrant acquisition of the |
| 758 | * lock, and over reporting the elapse of the waiting time. |
| 759 | * |
| 760 | * @param timeout the time to wait for the read lock |
| 761 | * @param unit the time unit of the timeout argument |
| 762 | * @return {@code true} if the read lock was acquired |
| 763 | * @throws InterruptedException if the current thread is interrupted |
| 764 | * @throws NullPointerException if the time unit is null |
| 765 | * |
| 766 | */ |
| 767 | public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { |
| 768 | return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); |
| 769 | } |
| 770 | |
| 771 | /** |
| 772 | * Attempts to release this lock. |
| 773 | * |
| 774 | * <p> If the number of readers is now zero then the lock |
| 775 | * is made available for write lock attempts. |
| 776 | */ |
| 777 | public void unlock() { |
| 778 | sync.releaseShared(1); |
| 779 | } |
| 780 | |
| 781 | /** |
| 782 | * Throws {@code UnsupportedOperationException} because |
| 783 | * {@code ReadLocks} do not support conditions. |
| 784 | * |
| 785 | * @throws UnsupportedOperationException always |
| 786 | */ |
| 787 | public Condition newCondition() { |
| 788 | throw new UnsupportedOperationException(); |
| 789 | } |
| 790 | |
| 791 | /** |
| 792 | * Returns a string identifying this lock, as well as its lock state. |
| 793 | * The state, in brackets, includes the String {@code "Read locks ="} |
| 794 | * followed by the number of held read locks. |
| 795 | * |
| 796 | * @return a string identifying this lock, as well as its lock state |
| 797 | */ |
| 798 | public String toString() { |
| 799 | int r = sync.getReadLockCount(); |
| 800 | return super.toString() + |
| 801 | "[Read locks = " + r + "]"; |
| 802 | } |
| 803 | } |
| 804 | |
| 805 | /** |
| 806 | * The lock returned by method {@link ReentrantReadWriteLock#writeLock}. |
| 807 | */ |
| 808 | public static class WriteLock implements Lock, java.io.Serializable { |
| 809 | private static final long serialVersionUID = -4992448646407690164L; |
| 810 | private final Sync sync; |
| 811 | |
| 812 | /** |
| 813 | * Constructor for use by subclasses |
| 814 | * |
| 815 | * @param lock the outer lock object |
| 816 | * @throws NullPointerException if the lock is null |
| 817 | */ |
| 818 | protected WriteLock(ReentrantReadWriteLock lock) { |
| 819 | sync = lock.sync; |
| 820 | } |
| 821 | |
| 822 | /** |
| 823 | * Acquires the write lock. |
| 824 | * |
| 825 | * <p>Acquires the write lock if neither the read nor write lock |
| 826 | * are held by another thread |
| 827 | * and returns immediately, setting the write lock hold count to |
| 828 | * one. |
| 829 | * |
| 830 | * <p>If the current thread already holds the write lock then the |
| 831 | * hold count is incremented by one and the method returns |
| 832 | * immediately. |
| 833 | * |
| 834 | * <p>If the lock is held by another thread then the current |
| 835 | * thread becomes disabled for thread scheduling purposes and |
| 836 | * lies dormant until the write lock has been acquired, at which |
| 837 | * time the write lock hold count is set to one. |
| 838 | */ |
| 839 | public void lock() { |
| 840 | sync.acquire(1); |
| 841 | } |
| 842 | |
| 843 | /** |
| 844 | * Acquires the write lock unless the current thread is |
| 845 | * {@linkplain Thread#interrupt interrupted}. |
| 846 | * |
| 847 | * <p>Acquires the write lock if neither the read nor write lock |
| 848 | * are held by another thread |
| 849 | * and returns immediately, setting the write lock hold count to |
| 850 | * one. |
| 851 | * |
| 852 | * <p>If the current thread already holds this lock then the |
| 853 | * hold count is incremented by one and the method returns |
| 854 | * immediately. |
| 855 | * |
| 856 | * <p>If the lock is held by another thread then the current |
| 857 | * thread becomes disabled for thread scheduling purposes and |
| 858 | * lies dormant until one of two things happens: |
| 859 | * |
| 860 | * <ul> |
| 861 | * |
| 862 | * <li>The write lock is acquired by the current thread; or |
| 863 | * |
| 864 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 865 | * the current thread. |
| 866 | * |
| 867 | * </ul> |
| 868 | * |
| 869 | * <p>If the write lock is acquired by the current thread then the |
| 870 | * lock hold count is set to one. |
| 871 | * |
| 872 | * <p>If the current thread: |
| 873 | * |
| 874 | * <ul> |
| 875 | * |
| 876 | * <li>has its interrupted status set on entry to this method; |
| 877 | * or |
| 878 | * |
| 879 | * <li>is {@linkplain Thread#interrupt interrupted} while |
| 880 | * acquiring the write lock, |
| 881 | * |
| 882 | * </ul> |
| 883 | * |
| 884 | * then {@link InterruptedException} is thrown and the current |
| 885 | * thread's interrupted status is cleared. |
| 886 | * |
| 887 | * <p>In this implementation, as this method is an explicit |
| 888 | * interruption point, preference is given to responding to |
| 889 | * the interrupt over normal or reentrant acquisition of the |
| 890 | * lock. |
| 891 | * |
| 892 | * @throws InterruptedException if the current thread is interrupted |
| 893 | */ |
| 894 | public void lockInterruptibly() throws InterruptedException { |
| 895 | sync.acquireInterruptibly(1); |
| 896 | } |
| 897 | |
| 898 | /** |
| 899 | * Acquires the write lock only if it is not held by another thread |
| 900 | * at the time of invocation. |
| 901 | * |
| 902 | * <p>Acquires the write lock if neither the read nor write lock |
| 903 | * are held by another thread |
| 904 | * and returns immediately with the value {@code true}, |
| 905 | * setting the write lock hold count to one. Even when this lock has |
| 906 | * been set to use a fair ordering policy, a call to |
| 907 | * {@code tryLock()} <em>will</em> immediately acquire the |
| 908 | * lock if it is available, whether or not other threads are |
| 909 | * currently waiting for the write lock. This "barging" |
| 910 | * behavior can be useful in certain circumstances, even |
| 911 | * though it breaks fairness. If you want to honor the |
| 912 | * fairness setting for this lock, then use {@link |
| 913 | * #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) } |
| 914 | * which is almost equivalent (it also detects interruption). |
| 915 | * |
| 916 | * <p> If the current thread already holds this lock then the |
| 917 | * hold count is incremented by one and the method returns |
| 918 | * {@code true}. |
| 919 | * |
| 920 | * <p>If the lock is held by another thread then this method |
| 921 | * will return immediately with the value {@code false}. |
| 922 | * |
| 923 | * @return {@code true} if the lock was free and was acquired |
| 924 | * by the current thread, or the write lock was already held |
| 925 | * by the current thread; and {@code false} otherwise. |
| 926 | */ |
| 927 | public boolean tryLock( ) { |
| 928 | return sync.tryWriteLock(); |
| 929 | } |
| 930 | |
| 931 | /** |
| 932 | * Acquires the write lock if it is not held by another thread |
| 933 | * within the given waiting time and the current thread has |
| 934 | * not been {@linkplain Thread#interrupt interrupted}. |
| 935 | * |
| 936 | * <p>Acquires the write lock if neither the read nor write lock |
| 937 | * are held by another thread |
| 938 | * and returns immediately with the value {@code true}, |
| 939 | * setting the write lock hold count to one. If this lock has been |
| 940 | * set to use a fair ordering policy then an available lock |
| 941 | * <em>will not</em> be acquired if any other threads are |
| 942 | * waiting for the write lock. This is in contrast to the {@link |
| 943 | * #tryLock()} method. If you want a timed {@code tryLock} |
| 944 | * that does permit barging on a fair lock then combine the |
| 945 | * timed and un-timed forms together: |
| 946 | * |
| 947 | * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... } |
| 948 | * </pre> |
| 949 | * |
| 950 | * <p>If the current thread already holds this lock then the |
| 951 | * hold count is incremented by one and the method returns |
| 952 | * {@code true}. |
| 953 | * |
| 954 | * <p>If the lock is held by another thread then the current |
| 955 | * thread becomes disabled for thread scheduling purposes and |
| 956 | * lies dormant until one of three things happens: |
| 957 | * |
| 958 | * <ul> |
| 959 | * |
| 960 | * <li>The write lock is acquired by the current thread; or |
| 961 | * |
| 962 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 963 | * the current thread; or |
| 964 | * |
| 965 | * <li>The specified waiting time elapses |
| 966 | * |
| 967 | * </ul> |
| 968 | * |
| 969 | * <p>If the write lock is acquired then the value {@code true} is |
| 970 | * returned and the write lock hold count is set to one. |
| 971 | * |
| 972 | * <p>If the current thread: |
| 973 | * |
| 974 | * <ul> |
| 975 | * |
| 976 | * <li>has its interrupted status set on entry to this method; |
| 977 | * or |
| 978 | * |
| 979 | * <li>is {@linkplain Thread#interrupt interrupted} while |
| 980 | * acquiring the write lock, |
| 981 | * |
| 982 | * </ul> |
| 983 | * |
| 984 | * then {@link InterruptedException} is thrown and the current |
| 985 | * thread's interrupted status is cleared. |
| 986 | * |
| 987 | * <p>If the specified waiting time elapses then the value |
| 988 | * {@code false} is returned. If the time is less than or |
| 989 | * equal to zero, the method will not wait at all. |
| 990 | * |
| 991 | * <p>In this implementation, as this method is an explicit |
| 992 | * interruption point, preference is given to responding to |
| 993 | * the interrupt over normal or reentrant acquisition of the |
| 994 | * lock, and over reporting the elapse of the waiting time. |
| 995 | * |
| 996 | * @param timeout the time to wait for the write lock |
| 997 | * @param unit the time unit of the timeout argument |
| 998 | * |
| 999 | * @return {@code true} if the lock was free and was acquired |
| 1000 | * by the current thread, or the write lock was already held by the |
| 1001 | * current thread; and {@code false} if the waiting time |
| 1002 | * elapsed before the lock could be acquired. |
| 1003 | * |
| 1004 | * @throws InterruptedException if the current thread is interrupted |
| 1005 | * @throws NullPointerException if the time unit is null |
| 1006 | * |
| 1007 | */ |
| 1008 | public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { |
| 1009 | return sync.tryAcquireNanos(1, unit.toNanos(timeout)); |
| 1010 | } |
| 1011 | |
| 1012 | /** |
| 1013 | * Attempts to release this lock. |
| 1014 | * |
| 1015 | * <p>If the current thread is the holder of this lock then |
| 1016 | * the hold count is decremented. If the hold count is now |
| 1017 | * zero then the lock is released. If the current thread is |
| 1018 | * not the holder of this lock then {@link |
| 1019 | * IllegalMonitorStateException} is thrown. |
| 1020 | * |
| 1021 | * @throws IllegalMonitorStateException if the current thread does not |
| 1022 | * hold this lock. |
| 1023 | */ |
| 1024 | public void unlock() { |
| 1025 | sync.release(1); |
| 1026 | } |
| 1027 | |
| 1028 | /** |
| 1029 | * Returns a {@link Condition} instance for use with this |
| 1030 | * {@link Lock} instance. |
| 1031 | * <p>The returned {@link Condition} instance supports the same |
| 1032 | * usages as do the {@link Object} monitor methods ({@link |
| 1033 | * Object#wait() wait}, {@link Object#notify notify}, and {@link |
| 1034 | * Object#notifyAll notifyAll}) when used with the built-in |
| 1035 | * monitor lock. |
| 1036 | * |
| 1037 | * <ul> |
| 1038 | * |
| 1039 | * <li>If this write lock is not held when any {@link |
| 1040 | * Condition} method is called then an {@link |
| 1041 | * IllegalMonitorStateException} is thrown. (Read locks are |
| 1042 | * held independently of write locks, so are not checked or |
| 1043 | * affected. However it is essentially always an error to |
| 1044 | * invoke a condition waiting method when the current thread |
| 1045 | * has also acquired read locks, since other threads that |
| 1046 | * could unblock it will not be able to acquire the write |
| 1047 | * lock.) |
| 1048 | * |
| 1049 | * <li>When the condition {@linkplain Condition#await() waiting} |
| 1050 | * methods are called the write lock is released and, before |
| 1051 | * they return, the write lock is reacquired and the lock hold |
| 1052 | * count restored to what it was when the method was called. |
| 1053 | * |
| 1054 | * <li>If a thread is {@linkplain Thread#interrupt interrupted} while |
| 1055 | * waiting then the wait will terminate, an {@link |
| 1056 | * InterruptedException} will be thrown, and the thread's |
| 1057 | * interrupted status will be cleared. |
| 1058 | * |
| 1059 | * <li> Waiting threads are signalled in FIFO order. |
| 1060 | * |
| 1061 | * <li>The ordering of lock reacquisition for threads returning |
| 1062 | * from waiting methods is the same as for threads initially |
| 1063 | * acquiring the lock, which is in the default case not specified, |
| 1064 | * but for <em>fair</em> locks favors those threads that have been |
| 1065 | * waiting the longest. |
| 1066 | * |
| 1067 | * </ul> |
| 1068 | * |
| 1069 | * @return the Condition object |
| 1070 | */ |
| 1071 | public Condition newCondition() { |
| 1072 | return sync.newCondition(); |
| 1073 | } |
| 1074 | |
| 1075 | /** |
| 1076 | * Returns a string identifying this lock, as well as its lock |
| 1077 | * state. The state, in brackets includes either the String |
| 1078 | * {@code "Unlocked"} or the String {@code "Locked by"} |
| 1079 | * followed by the {@linkplain Thread#getName name} of the owning thread. |
| 1080 | * |
| 1081 | * @return a string identifying this lock, as well as its lock state |
| 1082 | */ |
| 1083 | public String toString() { |
| 1084 | Thread o = sync.getOwner(); |
| 1085 | return super.toString() + ((o == null) ? |
| 1086 | "[Unlocked]" : |
| 1087 | "[Locked by thread " + o.getName() + "]"); |
| 1088 | } |
| 1089 | |
| 1090 | /** |
| 1091 | * Queries if this write lock is held by the current thread. |
| 1092 | * Identical in effect to {@link |
| 1093 | * ReentrantReadWriteLock#isWriteLockedByCurrentThread}. |
| 1094 | * |
| 1095 | * @return {@code true} if the current thread holds this lock and |
| 1096 | * {@code false} otherwise |
| 1097 | * @since 1.6 |
| 1098 | */ |
| 1099 | public boolean isHeldByCurrentThread() { |
| 1100 | return sync.isHeldExclusively(); |
| 1101 | } |
| 1102 | |
| 1103 | /** |
| 1104 | * Queries the number of holds on this write lock by the current |
| 1105 | * thread. A thread has a hold on a lock for each lock action |
| 1106 | * that is not matched by an unlock action. Identical in effect |
| 1107 | * to {@link ReentrantReadWriteLock#getWriteHoldCount}. |
| 1108 | * |
| 1109 | * @return the number of holds on this lock by the current thread, |
| 1110 | * or zero if this lock is not held by the current thread |
| 1111 | * @since 1.6 |
| 1112 | */ |
| 1113 | public int getHoldCount() { |
| 1114 | return sync.getWriteHoldCount(); |
| 1115 | } |
| 1116 | } |
| 1117 | |
| 1118 | // Instrumentation and status |
| 1119 | |
| 1120 | /** |
| 1121 | * Returns {@code true} if this lock has fairness set true. |
| 1122 | * |
| 1123 | * @return {@code true} if this lock has fairness set true |
| 1124 | */ |
| 1125 | public final boolean isFair() { |
| 1126 | return sync instanceof FairSync; |
| 1127 | } |
| 1128 | |
| 1129 | /** |
| 1130 | * Returns the thread that currently owns the write lock, or |
| 1131 | * {@code null} if not owned. When this method is called by a |
| 1132 | * thread that is not the owner, the return value reflects a |
| 1133 | * best-effort approximation of current lock status. For example, |
| 1134 | * the owner may be momentarily {@code null} even if there are |
| 1135 | * threads trying to acquire the lock but have not yet done so. |
| 1136 | * This method is designed to facilitate construction of |
| 1137 | * subclasses that provide more extensive lock monitoring |
| 1138 | * facilities. |
| 1139 | * |
| 1140 | * @return the owner, or {@code null} if not owned |
| 1141 | */ |
| 1142 | protected Thread getOwner() { |
| 1143 | return sync.getOwner(); |
| 1144 | } |
| 1145 | |
| 1146 | /** |
| 1147 | * Queries the number of read locks held for this lock. This |
| 1148 | * method is designed for use in monitoring system state, not for |
| 1149 | * synchronization control. |
| 1150 | * @return the number of read locks held. |
| 1151 | */ |
| 1152 | public int getReadLockCount() { |
| 1153 | return sync.getReadLockCount(); |
| 1154 | } |
| 1155 | |
| 1156 | /** |
| 1157 | * Queries if the write lock is held by any thread. This method is |
| 1158 | * designed for use in monitoring system state, not for |
| 1159 | * synchronization control. |
| 1160 | * |
| 1161 | * @return {@code true} if any thread holds the write lock and |
| 1162 | * {@code false} otherwise |
| 1163 | */ |
| 1164 | public boolean isWriteLocked() { |
| 1165 | return sync.isWriteLocked(); |
| 1166 | } |
| 1167 | |
| 1168 | /** |
| 1169 | * Queries if the write lock is held by the current thread. |
| 1170 | * |
| 1171 | * @return {@code true} if the current thread holds the write lock and |
| 1172 | * {@code false} otherwise |
| 1173 | */ |
| 1174 | public boolean isWriteLockedByCurrentThread() { |
| 1175 | return sync.isHeldExclusively(); |
| 1176 | } |
| 1177 | |
| 1178 | /** |
| 1179 | * Queries the number of reentrant write holds on this lock by the |
| 1180 | * current thread. A writer thread has a hold on a lock for |
| 1181 | * each lock action that is not matched by an unlock action. |
| 1182 | * |
| 1183 | * @return the number of holds on the write lock by the current thread, |
| 1184 | * or zero if the write lock is not held by the current thread |
| 1185 | */ |
| 1186 | public int getWriteHoldCount() { |
| 1187 | return sync.getWriteHoldCount(); |
| 1188 | } |
| 1189 | |
| 1190 | /** |
| 1191 | * Queries the number of reentrant read holds on this lock by the |
| 1192 | * current thread. A reader thread has a hold on a lock for |
| 1193 | * each lock action that is not matched by an unlock action. |
| 1194 | * |
| 1195 | * @return the number of holds on the read lock by the current thread, |
| 1196 | * or zero if the read lock is not held by the current thread |
| 1197 | * @since 1.6 |
| 1198 | */ |
| 1199 | public int getReadHoldCount() { |
| 1200 | return sync.getReadHoldCount(); |
| 1201 | } |
| 1202 | |
| 1203 | /** |
| 1204 | * Returns a collection containing threads that may be waiting to |
| 1205 | * acquire the write lock. Because the actual set of threads may |
| 1206 | * change dynamically while constructing this result, the returned |
| 1207 | * collection is only a best-effort estimate. The elements of the |
| 1208 | * returned collection are in no particular order. This method is |
| 1209 | * designed to facilitate construction of subclasses that provide |
| 1210 | * more extensive lock monitoring facilities. |
| 1211 | * |
| 1212 | * @return the collection of threads |
| 1213 | */ |
| 1214 | protected Collection<Thread> getQueuedWriterThreads() { |
| 1215 | return sync.getExclusiveQueuedThreads(); |
| 1216 | } |
| 1217 | |
| 1218 | /** |
| 1219 | * Returns a collection containing threads that may be waiting to |
| 1220 | * acquire the read lock. Because the actual set of threads may |
| 1221 | * change dynamically while constructing this result, the returned |
| 1222 | * collection is only a best-effort estimate. The elements of the |
| 1223 | * returned collection are in no particular order. This method is |
| 1224 | * designed to facilitate construction of subclasses that provide |
| 1225 | * more extensive lock monitoring facilities. |
| 1226 | * |
| 1227 | * @return the collection of threads |
| 1228 | */ |
| 1229 | protected Collection<Thread> getQueuedReaderThreads() { |
| 1230 | return sync.getSharedQueuedThreads(); |
| 1231 | } |
| 1232 | |
| 1233 | /** |
| 1234 | * Queries whether any threads are waiting to acquire the read or |
| 1235 | * write lock. Note that because cancellations may occur at any |
| 1236 | * time, a {@code true} return does not guarantee that any other |
| 1237 | * thread will ever acquire a lock. This method is designed |
| 1238 | * primarily for use in monitoring of the system state. |
| 1239 | * |
| 1240 | * @return {@code true} if there may be other threads waiting to |
| 1241 | * acquire the lock |
| 1242 | */ |
| 1243 | public final boolean hasQueuedThreads() { |
| 1244 | return sync.hasQueuedThreads(); |
| 1245 | } |
| 1246 | |
| 1247 | /** |
| 1248 | * Queries whether the given thread is waiting to acquire either |
| 1249 | * the read or write lock. Note that because cancellations may |
| 1250 | * occur at any time, a {@code true} return does not guarantee |
| 1251 | * that this thread will ever acquire a lock. This method is |
| 1252 | * designed primarily for use in monitoring of the system state. |
| 1253 | * |
| 1254 | * @param thread the thread |
| 1255 | * @return {@code true} if the given thread is queued waiting for this lock |
| 1256 | * @throws NullPointerException if the thread is null |
| 1257 | */ |
| 1258 | public final boolean hasQueuedThread(Thread thread) { |
| 1259 | return sync.isQueued(thread); |
| 1260 | } |
| 1261 | |
| 1262 | /** |
| 1263 | * Returns an estimate of the number of threads waiting to acquire |
| 1264 | * either the read or write lock. The value is only an estimate |
| 1265 | * because the number of threads may change dynamically while this |
| 1266 | * method traverses internal data structures. This method is |
| 1267 | * designed for use in monitoring of the system state, not for |
| 1268 | * synchronization control. |
| 1269 | * |
| 1270 | * @return the estimated number of threads waiting for this lock |
| 1271 | */ |
| 1272 | public final int getQueueLength() { |
| 1273 | return sync.getQueueLength(); |
| 1274 | } |
| 1275 | |
| 1276 | /** |
| 1277 | * Returns a collection containing threads that may be waiting to |
| 1278 | * acquire either the read or write lock. Because the actual set |
| 1279 | * of threads may change dynamically while constructing this |
| 1280 | * result, the returned collection is only a best-effort estimate. |
| 1281 | * The elements of the returned collection are in no particular |
| 1282 | * order. This method is designed to facilitate construction of |
| 1283 | * subclasses that provide more extensive monitoring facilities. |
| 1284 | * |
| 1285 | * @return the collection of threads |
| 1286 | */ |
| 1287 | protected Collection<Thread> getQueuedThreads() { |
| 1288 | return sync.getQueuedThreads(); |
| 1289 | } |
| 1290 | |
| 1291 | /** |
| 1292 | * Queries whether any threads are waiting on the given condition |
| 1293 | * associated with the write lock. Note that because timeouts and |
| 1294 | * interrupts may occur at any time, a {@code true} return does |
| 1295 | * not guarantee that a future {@code signal} will awaken any |
| 1296 | * threads. This method is designed primarily for use in |
| 1297 | * monitoring of the system state. |
| 1298 | * |
| 1299 | * @param condition the condition |
| 1300 | * @return {@code true} if there are any waiting threads |
| 1301 | * @throws IllegalMonitorStateException if this lock is not held |
| 1302 | * @throws IllegalArgumentException if the given condition is |
| 1303 | * not associated with this lock |
| 1304 | * @throws NullPointerException if the condition is null |
| 1305 | */ |
| 1306 | public boolean hasWaiters(Condition condition) { |
| 1307 | if (condition == null) |
| 1308 | throw new NullPointerException(); |
| 1309 | if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) |
| 1310 | throw new IllegalArgumentException("not owner"); |
| 1311 | return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition); |
| 1312 | } |
| 1313 | |
| 1314 | /** |
| 1315 | * Returns an estimate of the number of threads waiting on the |
| 1316 | * given condition associated with the write lock. Note that because |
| 1317 | * timeouts and interrupts may occur at any time, the estimate |
| 1318 | * serves only as an upper bound on the actual number of waiters. |
| 1319 | * This method is designed for use in monitoring of the system |
| 1320 | * state, not for synchronization control. |
| 1321 | * |
| 1322 | * @param condition the condition |
| 1323 | * @return the estimated number of waiting threads |
| 1324 | * @throws IllegalMonitorStateException if this lock is not held |
| 1325 | * @throws IllegalArgumentException if the given condition is |
| 1326 | * not associated with this lock |
| 1327 | * @throws NullPointerException if the condition is null |
| 1328 | */ |
| 1329 | public int getWaitQueueLength(Condition condition) { |
| 1330 | if (condition == null) |
| 1331 | throw new NullPointerException(); |
| 1332 | if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) |
| 1333 | throw new IllegalArgumentException("not owner"); |
| 1334 | return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition); |
| 1335 | } |
| 1336 | |
| 1337 | /** |
| 1338 | * Returns a collection containing those threads that may be |
| 1339 | * waiting on the given condition associated with the write lock. |
| 1340 | * Because the actual set of threads may change dynamically while |
| 1341 | * constructing this result, the returned collection is only a |
| 1342 | * best-effort estimate. The elements of the returned collection |
| 1343 | * are in no particular order. This method is designed to |
| 1344 | * facilitate construction of subclasses that provide more |
| 1345 | * extensive condition monitoring facilities. |
| 1346 | * |
| 1347 | * @param condition the condition |
| 1348 | * @return the collection of threads |
| 1349 | * @throws IllegalMonitorStateException if this lock is not held |
| 1350 | * @throws IllegalArgumentException if the given condition is |
| 1351 | * not associated with this lock |
| 1352 | * @throws NullPointerException if the condition is null |
| 1353 | */ |
| 1354 | protected Collection<Thread> getWaitingThreads(Condition condition) { |
| 1355 | if (condition == null) |
| 1356 | throw new NullPointerException(); |
| 1357 | if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject)) |
| 1358 | throw new IllegalArgumentException("not owner"); |
| 1359 | return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition); |
| 1360 | } |
| 1361 | |
| 1362 | /** |
| 1363 | * Returns a string identifying this lock, as well as its lock state. |
| 1364 | * The state, in brackets, includes the String {@code "Write locks ="} |
| 1365 | * followed by the number of reentrantly held write locks, and the |
| 1366 | * String {@code "Read locks ="} followed by the number of held |
| 1367 | * read locks. |
| 1368 | * |
| 1369 | * @return a string identifying this lock, as well as its lock state |
| 1370 | */ |
| 1371 | public String toString() { |
| 1372 | int c = sync.getCount(); |
| 1373 | int w = Sync.exclusiveCount(c); |
| 1374 | int r = Sync.sharedCount(c); |
| 1375 | |
| 1376 | return super.toString() + |
| 1377 | "[Write locks = " + w + ", Read locks = " + r + "]"; |
| 1378 | } |
| 1379 | |
| 1380 | } |