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; |
| 37 | import java.util.*; |
| 38 | import java.util.concurrent.locks.*; |
| 39 | import java.util.concurrent.atomic.*; |
| 40 | |
| 41 | /** |
| 42 | * A counting semaphore. Conceptually, a semaphore maintains a set of |
| 43 | * permits. Each {@link #acquire} blocks if necessary until a permit is |
| 44 | * available, and then takes it. Each {@link #release} adds a permit, |
| 45 | * potentially releasing a blocking acquirer. |
| 46 | * However, no actual permit objects are used; the {@code Semaphore} just |
| 47 | * keeps a count of the number available and acts accordingly. |
| 48 | * |
| 49 | * <p>Semaphores are often used to restrict the number of threads than can |
| 50 | * access some (physical or logical) resource. For example, here is |
| 51 | * a class that uses a semaphore to control access to a pool of items: |
| 52 | * <pre> |
| 53 | * class Pool { |
| 54 | * private static final int MAX_AVAILABLE = 100; |
| 55 | * private final Semaphore available = new Semaphore(MAX_AVAILABLE, true); |
| 56 | * |
| 57 | * public Object getItem() throws InterruptedException { |
| 58 | * available.acquire(); |
| 59 | * return getNextAvailableItem(); |
| 60 | * } |
| 61 | * |
| 62 | * public void putItem(Object x) { |
| 63 | * if (markAsUnused(x)) |
| 64 | * available.release(); |
| 65 | * } |
| 66 | * |
| 67 | * // Not a particularly efficient data structure; just for demo |
| 68 | * |
| 69 | * protected Object[] items = ... whatever kinds of items being managed |
| 70 | * protected boolean[] used = new boolean[MAX_AVAILABLE]; |
| 71 | * |
| 72 | * protected synchronized Object getNextAvailableItem() { |
| 73 | * for (int i = 0; i < MAX_AVAILABLE; ++i) { |
| 74 | * if (!used[i]) { |
| 75 | * used[i] = true; |
| 76 | * return items[i]; |
| 77 | * } |
| 78 | * } |
| 79 | * return null; // not reached |
| 80 | * } |
| 81 | * |
| 82 | * protected synchronized boolean markAsUnused(Object item) { |
| 83 | * for (int i = 0; i < MAX_AVAILABLE; ++i) { |
| 84 | * if (item == items[i]) { |
| 85 | * if (used[i]) { |
| 86 | * used[i] = false; |
| 87 | * return true; |
| 88 | * } else |
| 89 | * return false; |
| 90 | * } |
| 91 | * } |
| 92 | * return false; |
| 93 | * } |
| 94 | * |
| 95 | * } |
| 96 | * </pre> |
| 97 | * |
| 98 | * <p>Before obtaining an item each thread must acquire a permit from |
| 99 | * the semaphore, guaranteeing that an item is available for use. When |
| 100 | * the thread has finished with the item it is returned back to the |
| 101 | * pool and a permit is returned to the semaphore, allowing another |
| 102 | * thread to acquire that item. Note that no synchronization lock is |
| 103 | * held when {@link #acquire} is called as that would prevent an item |
| 104 | * from being returned to the pool. The semaphore encapsulates the |
| 105 | * synchronization needed to restrict access to the pool, separately |
| 106 | * from any synchronization needed to maintain the consistency of the |
| 107 | * pool itself. |
| 108 | * |
| 109 | * <p>A semaphore initialized to one, and which is used such that it |
| 110 | * only has at most one permit available, can serve as a mutual |
| 111 | * exclusion lock. This is more commonly known as a <em>binary |
| 112 | * semaphore</em>, because it only has two states: one permit |
| 113 | * available, or zero permits available. When used in this way, the |
| 114 | * binary semaphore has the property (unlike many {@link Lock} |
| 115 | * implementations), that the "lock" can be released by a |
| 116 | * thread other than the owner (as semaphores have no notion of |
| 117 | * ownership). This can be useful in some specialized contexts, such |
| 118 | * as deadlock recovery. |
| 119 | * |
| 120 | * <p> The constructor for this class optionally accepts a |
| 121 | * <em>fairness</em> parameter. When set false, this class makes no |
| 122 | * guarantees about the order in which threads acquire permits. In |
| 123 | * particular, <em>barging</em> is permitted, that is, a thread |
| 124 | * invoking {@link #acquire} can be allocated a permit ahead of a |
| 125 | * thread that has been waiting - logically the new thread places itself at |
| 126 | * the head of the queue of waiting threads. When fairness is set true, the |
| 127 | * semaphore guarantees that threads invoking any of the {@link |
| 128 | * #acquire() acquire} methods are selected to obtain permits in the order in |
| 129 | * which their invocation of those methods was processed |
| 130 | * (first-in-first-out; FIFO). Note that FIFO ordering necessarily |
| 131 | * applies to specific internal points of execution within these |
| 132 | * methods. So, it is possible for one thread to invoke |
| 133 | * {@code acquire} before another, but reach the ordering point after |
| 134 | * the other, and similarly upon return from the method. |
| 135 | * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not |
| 136 | * honor the fairness setting, but will take any permits that are |
| 137 | * available. |
| 138 | * |
| 139 | * <p>Generally, semaphores used to control resource access should be |
| 140 | * initialized as fair, to ensure that no thread is starved out from |
| 141 | * accessing a resource. When using semaphores for other kinds of |
| 142 | * synchronization control, the throughput advantages of non-fair |
| 143 | * ordering often outweigh fairness considerations. |
| 144 | * |
| 145 | * <p>This class also provides convenience methods to {@link |
| 146 | * #acquire(int) acquire} and {@link #release(int) release} multiple |
| 147 | * permits at a time. Beware of the increased risk of indefinite |
| 148 | * postponement when these methods are used without fairness set true. |
| 149 | * |
| 150 | * <p>Memory consistency effects: Actions in a thread prior to calling |
| 151 | * a "release" method such as {@code release()} |
| 152 | * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
| 153 | * actions following a successful "acquire" method such as {@code acquire()} |
| 154 | * in another thread. |
| 155 | * |
| 156 | * @since 1.5 |
| 157 | * @author Doug Lea |
| 158 | * |
| 159 | */ |
| 160 | |
| 161 | public class Semaphore implements java.io.Serializable { |
| 162 | private static final long serialVersionUID = -3222578661600680210L; |
| 163 | /** All mechanics via AbstractQueuedSynchronizer subclass */ |
| 164 | private final Sync sync; |
| 165 | |
| 166 | /** |
| 167 | * Synchronization implementation for semaphore. Uses AQS state |
| 168 | * to represent permits. Subclassed into fair and nonfair |
| 169 | * versions. |
| 170 | */ |
| 171 | abstract static class Sync extends AbstractQueuedSynchronizer { |
| 172 | private static final long serialVersionUID = 1192457210091910933L; |
| 173 | |
| 174 | Sync(int permits) { |
| 175 | setState(permits); |
| 176 | } |
| 177 | |
| 178 | final int getPermits() { |
| 179 | return getState(); |
| 180 | } |
| 181 | |
| 182 | final int nonfairTryAcquireShared(int acquires) { |
| 183 | for (;;) { |
| 184 | int available = getState(); |
| 185 | int remaining = available - acquires; |
| 186 | if (remaining < 0 || |
| 187 | compareAndSetState(available, remaining)) |
| 188 | return remaining; |
| 189 | } |
| 190 | } |
| 191 | |
| 192 | protected final boolean tryReleaseShared(int releases) { |
| 193 | for (;;) { |
| 194 | int p = getState(); |
| 195 | if (compareAndSetState(p, p + releases)) |
| 196 | return true; |
| 197 | } |
| 198 | } |
| 199 | |
| 200 | final void reducePermits(int reductions) { |
| 201 | for (;;) { |
| 202 | int current = getState(); |
| 203 | int next = current - reductions; |
| 204 | if (compareAndSetState(current, next)) |
| 205 | return; |
| 206 | } |
| 207 | } |
| 208 | |
| 209 | final int drainPermits() { |
| 210 | for (;;) { |
| 211 | int current = getState(); |
| 212 | if (current == 0 || compareAndSetState(current, 0)) |
| 213 | return current; |
| 214 | } |
| 215 | } |
| 216 | } |
| 217 | |
| 218 | /** |
| 219 | * NonFair version |
| 220 | */ |
| 221 | final static class NonfairSync extends Sync { |
| 222 | private static final long serialVersionUID = -2694183684443567898L; |
| 223 | |
| 224 | NonfairSync(int permits) { |
| 225 | super(permits); |
| 226 | } |
| 227 | |
| 228 | protected int tryAcquireShared(int acquires) { |
| 229 | return nonfairTryAcquireShared(acquires); |
| 230 | } |
| 231 | } |
| 232 | |
| 233 | /** |
| 234 | * Fair version |
| 235 | */ |
| 236 | final static class FairSync extends Sync { |
| 237 | private static final long serialVersionUID = 2014338818796000944L; |
| 238 | |
| 239 | FairSync(int permits) { |
| 240 | super(permits); |
| 241 | } |
| 242 | |
| 243 | protected int tryAcquireShared(int acquires) { |
| 244 | for (;;) { |
| 245 | if (hasQueuedPredecessors()) |
| 246 | return -1; |
| 247 | int available = getState(); |
| 248 | int remaining = available - acquires; |
| 249 | if (remaining < 0 || |
| 250 | compareAndSetState(available, remaining)) |
| 251 | return remaining; |
| 252 | } |
| 253 | } |
| 254 | } |
| 255 | |
| 256 | /** |
| 257 | * Creates a {@code Semaphore} with the given number of |
| 258 | * permits and nonfair fairness setting. |
| 259 | * |
| 260 | * @param permits the initial number of permits available. |
| 261 | * This value may be negative, in which case releases |
| 262 | * must occur before any acquires will be granted. |
| 263 | */ |
| 264 | public Semaphore(int permits) { |
| 265 | sync = new NonfairSync(permits); |
| 266 | } |
| 267 | |
| 268 | /** |
| 269 | * Creates a {@code Semaphore} with the given number of |
| 270 | * permits and the given fairness setting. |
| 271 | * |
| 272 | * @param permits the initial number of permits available. |
| 273 | * This value may be negative, in which case releases |
| 274 | * must occur before any acquires will be granted. |
| 275 | * @param fair {@code true} if this semaphore will guarantee |
| 276 | * first-in first-out granting of permits under contention, |
| 277 | * else {@code false} |
| 278 | */ |
| 279 | public Semaphore(int permits, boolean fair) { |
| 280 | sync = (fair)? new FairSync(permits) : new NonfairSync(permits); |
| 281 | } |
| 282 | |
| 283 | /** |
| 284 | * Acquires a permit from this semaphore, blocking until one is |
| 285 | * available, or the thread is {@linkplain Thread#interrupt interrupted}. |
| 286 | * |
| 287 | * <p>Acquires a permit, if one is available and returns immediately, |
| 288 | * reducing the number of available permits by one. |
| 289 | * |
| 290 | * <p>If no permit is available then the current thread becomes |
| 291 | * disabled for thread scheduling purposes and lies dormant until |
| 292 | * one of two things happens: |
| 293 | * <ul> |
| 294 | * <li>Some other thread invokes the {@link #release} method for this |
| 295 | * semaphore and the current thread is next to be assigned a permit; or |
| 296 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 297 | * the current thread. |
| 298 | * </ul> |
| 299 | * |
| 300 | * <p>If the current thread: |
| 301 | * <ul> |
| 302 | * <li>has its interrupted status set on entry to this method; or |
| 303 | * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| 304 | * for a permit, |
| 305 | * </ul> |
| 306 | * then {@link InterruptedException} is thrown and the current thread's |
| 307 | * interrupted status is cleared. |
| 308 | * |
| 309 | * @throws InterruptedException if the current thread is interrupted |
| 310 | */ |
| 311 | public void acquire() throws InterruptedException { |
| 312 | sync.acquireSharedInterruptibly(1); |
| 313 | } |
| 314 | |
| 315 | /** |
| 316 | * Acquires a permit from this semaphore, blocking until one is |
| 317 | * available. |
| 318 | * |
| 319 | * <p>Acquires a permit, if one is available and returns immediately, |
| 320 | * reducing the number of available permits by one. |
| 321 | * |
| 322 | * <p>If no permit is available then the current thread becomes |
| 323 | * disabled for thread scheduling purposes and lies dormant until |
| 324 | * some other thread invokes the {@link #release} method for this |
| 325 | * semaphore and the current thread is next to be assigned a permit. |
| 326 | * |
| 327 | * <p>If the current thread is {@linkplain Thread#interrupt interrupted} |
| 328 | * while waiting for a permit then it will continue to wait, but the |
| 329 | * time at which the thread is assigned a permit may change compared to |
| 330 | * the time it would have received the permit had no interruption |
| 331 | * occurred. When the thread does return from this method its interrupt |
| 332 | * status will be set. |
| 333 | */ |
| 334 | public void acquireUninterruptibly() { |
| 335 | sync.acquireShared(1); |
| 336 | } |
| 337 | |
| 338 | /** |
| 339 | * Acquires a permit from this semaphore, only if one is available at the |
| 340 | * time of invocation. |
| 341 | * |
| 342 | * <p>Acquires a permit, if one is available and returns immediately, |
| 343 | * with the value {@code true}, |
| 344 | * reducing the number of available permits by one. |
| 345 | * |
| 346 | * <p>If no permit is available then this method will return |
| 347 | * immediately with the value {@code false}. |
| 348 | * |
| 349 | * <p>Even when this semaphore has been set to use a |
| 350 | * fair ordering policy, a call to {@code tryAcquire()} <em>will</em> |
| 351 | * immediately acquire a permit if one is available, whether or not |
| 352 | * other threads are currently waiting. |
| 353 | * This "barging" behavior can be useful in certain |
| 354 | * circumstances, even though it breaks fairness. If you want to honor |
| 355 | * the fairness setting, then use |
| 356 | * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) } |
| 357 | * which is almost equivalent (it also detects interruption). |
| 358 | * |
| 359 | * @return {@code true} if a permit was acquired and {@code false} |
| 360 | * otherwise |
| 361 | */ |
| 362 | public boolean tryAcquire() { |
| 363 | return sync.nonfairTryAcquireShared(1) >= 0; |
| 364 | } |
| 365 | |
| 366 | /** |
| 367 | * Acquires a permit from this semaphore, if one becomes available |
| 368 | * within the given waiting time and the current thread has not |
| 369 | * been {@linkplain Thread#interrupt interrupted}. |
| 370 | * |
| 371 | * <p>Acquires a permit, if one is available and returns immediately, |
| 372 | * with the value {@code true}, |
| 373 | * reducing the number of available permits by one. |
| 374 | * |
| 375 | * <p>If no permit is available then the current thread becomes |
| 376 | * disabled for thread scheduling purposes and lies dormant until |
| 377 | * one of three things happens: |
| 378 | * <ul> |
| 379 | * <li>Some other thread invokes the {@link #release} method for this |
| 380 | * semaphore and the current thread is next to be assigned a permit; or |
| 381 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 382 | * the current thread; or |
| 383 | * <li>The specified waiting time elapses. |
| 384 | * </ul> |
| 385 | * |
| 386 | * <p>If a permit is acquired then the value {@code true} is returned. |
| 387 | * |
| 388 | * <p>If the current thread: |
| 389 | * <ul> |
| 390 | * <li>has its interrupted status set on entry to this method; or |
| 391 | * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| 392 | * to acquire a permit, |
| 393 | * </ul> |
| 394 | * then {@link InterruptedException} is thrown and the current thread's |
| 395 | * interrupted status is cleared. |
| 396 | * |
| 397 | * <p>If the specified waiting time elapses then the value {@code false} |
| 398 | * is returned. If the time is less than or equal to zero, the method |
| 399 | * will not wait at all. |
| 400 | * |
| 401 | * @param timeout the maximum time to wait for a permit |
| 402 | * @param unit the time unit of the {@code timeout} argument |
| 403 | * @return {@code true} if a permit was acquired and {@code false} |
| 404 | * if the waiting time elapsed before a permit was acquired |
| 405 | * @throws InterruptedException if the current thread is interrupted |
| 406 | */ |
| 407 | public boolean tryAcquire(long timeout, TimeUnit unit) |
| 408 | throws InterruptedException { |
| 409 | return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); |
| 410 | } |
| 411 | |
| 412 | /** |
| 413 | * Releases a permit, returning it to the semaphore. |
| 414 | * |
| 415 | * <p>Releases a permit, increasing the number of available permits by |
| 416 | * one. If any threads are trying to acquire a permit, then one is |
| 417 | * selected and given the permit that was just released. That thread |
| 418 | * is (re)enabled for thread scheduling purposes. |
| 419 | * |
| 420 | * <p>There is no requirement that a thread that releases a permit must |
| 421 | * have acquired that permit by calling {@link #acquire}. |
| 422 | * Correct usage of a semaphore is established by programming convention |
| 423 | * in the application. |
| 424 | */ |
| 425 | public void release() { |
| 426 | sync.releaseShared(1); |
| 427 | } |
| 428 | |
| 429 | /** |
| 430 | * Acquires the given number of permits from this semaphore, |
| 431 | * blocking until all are available, |
| 432 | * or the thread is {@linkplain Thread#interrupt interrupted}. |
| 433 | * |
| 434 | * <p>Acquires the given number of permits, if they are available, |
| 435 | * and returns immediately, reducing the number of available permits |
| 436 | * by the given amount. |
| 437 | * |
| 438 | * <p>If insufficient permits are available then the current thread becomes |
| 439 | * disabled for thread scheduling purposes and lies dormant until |
| 440 | * one of two things happens: |
| 441 | * <ul> |
| 442 | * <li>Some other thread invokes one of the {@link #release() release} |
| 443 | * methods for this semaphore, the current thread is next to be assigned |
| 444 | * permits and the number of available permits satisfies this request; or |
| 445 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 446 | * the current thread. |
| 447 | * </ul> |
| 448 | * |
| 449 | * <p>If the current thread: |
| 450 | * <ul> |
| 451 | * <li>has its interrupted status set on entry to this method; or |
| 452 | * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| 453 | * for a permit, |
| 454 | * </ul> |
| 455 | * then {@link InterruptedException} is thrown and the current thread's |
| 456 | * interrupted status is cleared. |
| 457 | * Any permits that were to be assigned to this thread are instead |
| 458 | * assigned to other threads trying to acquire permits, as if |
| 459 | * permits had been made available by a call to {@link #release()}. |
| 460 | * |
| 461 | * @param permits the number of permits to acquire |
| 462 | * @throws InterruptedException if the current thread is interrupted |
| 463 | * @throws IllegalArgumentException if {@code permits} is negative |
| 464 | */ |
| 465 | public void acquire(int permits) throws InterruptedException { |
| 466 | if (permits < 0) throw new IllegalArgumentException(); |
| 467 | sync.acquireSharedInterruptibly(permits); |
| 468 | } |
| 469 | |
| 470 | /** |
| 471 | * Acquires the given number of permits from this semaphore, |
| 472 | * blocking until all are available. |
| 473 | * |
| 474 | * <p>Acquires the given number of permits, if they are available, |
| 475 | * and returns immediately, reducing the number of available permits |
| 476 | * by the given amount. |
| 477 | * |
| 478 | * <p>If insufficient permits are available then the current thread becomes |
| 479 | * disabled for thread scheduling purposes and lies dormant until |
| 480 | * some other thread invokes one of the {@link #release() release} |
| 481 | * methods for this semaphore, the current thread is next to be assigned |
| 482 | * permits and the number of available permits satisfies this request. |
| 483 | * |
| 484 | * <p>If the current thread is {@linkplain Thread#interrupt interrupted} |
| 485 | * while waiting for permits then it will continue to wait and its |
| 486 | * position in the queue is not affected. When the thread does return |
| 487 | * from this method its interrupt status will be set. |
| 488 | * |
| 489 | * @param permits the number of permits to acquire |
| 490 | * @throws IllegalArgumentException if {@code permits} is negative |
| 491 | * |
| 492 | */ |
| 493 | public void acquireUninterruptibly(int permits) { |
| 494 | if (permits < 0) throw new IllegalArgumentException(); |
| 495 | sync.acquireShared(permits); |
| 496 | } |
| 497 | |
| 498 | /** |
| 499 | * Acquires the given number of permits from this semaphore, only |
| 500 | * if all are available at the time of invocation. |
| 501 | * |
| 502 | * <p>Acquires the given number of permits, if they are available, and |
| 503 | * returns immediately, with the value {@code true}, |
| 504 | * reducing the number of available permits by the given amount. |
| 505 | * |
| 506 | * <p>If insufficient permits are available then this method will return |
| 507 | * immediately with the value {@code false} and the number of available |
| 508 | * permits is unchanged. |
| 509 | * |
| 510 | * <p>Even when this semaphore has been set to use a fair ordering |
| 511 | * policy, a call to {@code tryAcquire} <em>will</em> |
| 512 | * immediately acquire a permit if one is available, whether or |
| 513 | * not other threads are currently waiting. This |
| 514 | * "barging" behavior can be useful in certain |
| 515 | * circumstances, even though it breaks fairness. If you want to |
| 516 | * honor the fairness setting, then use {@link #tryAcquire(int, |
| 517 | * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) } |
| 518 | * which is almost equivalent (it also detects interruption). |
| 519 | * |
| 520 | * @param permits the number of permits to acquire |
| 521 | * @return {@code true} if the permits were acquired and |
| 522 | * {@code false} otherwise |
| 523 | * @throws IllegalArgumentException if {@code permits} is negative |
| 524 | */ |
| 525 | public boolean tryAcquire(int permits) { |
| 526 | if (permits < 0) throw new IllegalArgumentException(); |
| 527 | return sync.nonfairTryAcquireShared(permits) >= 0; |
| 528 | } |
| 529 | |
| 530 | /** |
| 531 | * Acquires the given number of permits from this semaphore, if all |
| 532 | * become available within the given waiting time and the current |
| 533 | * thread has not been {@linkplain Thread#interrupt interrupted}. |
| 534 | * |
| 535 | * <p>Acquires the given number of permits, if they are available and |
| 536 | * returns immediately, with the value {@code true}, |
| 537 | * reducing the number of available permits by the given amount. |
| 538 | * |
| 539 | * <p>If insufficient permits are available then |
| 540 | * the current thread becomes disabled for thread scheduling |
| 541 | * purposes and lies dormant until one of three things happens: |
| 542 | * <ul> |
| 543 | * <li>Some other thread invokes one of the {@link #release() release} |
| 544 | * methods for this semaphore, the current thread is next to be assigned |
| 545 | * permits and the number of available permits satisfies this request; or |
| 546 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 547 | * the current thread; or |
| 548 | * <li>The specified waiting time elapses. |
| 549 | * </ul> |
| 550 | * |
| 551 | * <p>If the permits are acquired then the value {@code true} is returned. |
| 552 | * |
| 553 | * <p>If the current thread: |
| 554 | * <ul> |
| 555 | * <li>has its interrupted status set on entry to this method; or |
| 556 | * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| 557 | * to acquire the permits, |
| 558 | * </ul> |
| 559 | * then {@link InterruptedException} is thrown and the current thread's |
| 560 | * interrupted status is cleared. |
| 561 | * Any permits that were to be assigned to this thread, are instead |
| 562 | * assigned to other threads trying to acquire permits, as if |
| 563 | * the permits had been made available by a call to {@link #release()}. |
| 564 | * |
| 565 | * <p>If the specified waiting time elapses then the value {@code false} |
| 566 | * is returned. If the time is less than or equal to zero, the method |
| 567 | * will not wait at all. Any permits that were to be assigned to this |
| 568 | * thread, are instead assigned to other threads trying to acquire |
| 569 | * permits, as if the permits had been made available by a call to |
| 570 | * {@link #release()}. |
| 571 | * |
| 572 | * @param permits the number of permits to acquire |
| 573 | * @param timeout the maximum time to wait for the permits |
| 574 | * @param unit the time unit of the {@code timeout} argument |
| 575 | * @return {@code true} if all permits were acquired and {@code false} |
| 576 | * if the waiting time elapsed before all permits were acquired |
| 577 | * @throws InterruptedException if the current thread is interrupted |
| 578 | * @throws IllegalArgumentException if {@code permits} is negative |
| 579 | */ |
| 580 | public boolean tryAcquire(int permits, long timeout, TimeUnit unit) |
| 581 | throws InterruptedException { |
| 582 | if (permits < 0) throw new IllegalArgumentException(); |
| 583 | return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout)); |
| 584 | } |
| 585 | |
| 586 | /** |
| 587 | * Releases the given number of permits, returning them to the semaphore. |
| 588 | * |
| 589 | * <p>Releases the given number of permits, increasing the number of |
| 590 | * available permits by that amount. |
| 591 | * If any threads are trying to acquire permits, then one |
| 592 | * is selected and given the permits that were just released. |
| 593 | * If the number of available permits satisfies that thread's request |
| 594 | * then that thread is (re)enabled for thread scheduling purposes; |
| 595 | * otherwise the thread will wait until sufficient permits are available. |
| 596 | * If there are still permits available |
| 597 | * after this thread's request has been satisfied, then those permits |
| 598 | * are assigned in turn to other threads trying to acquire permits. |
| 599 | * |
| 600 | * <p>There is no requirement that a thread that releases a permit must |
| 601 | * have acquired that permit by calling {@link Semaphore#acquire acquire}. |
| 602 | * Correct usage of a semaphore is established by programming convention |
| 603 | * in the application. |
| 604 | * |
| 605 | * @param permits the number of permits to release |
| 606 | * @throws IllegalArgumentException if {@code permits} is negative |
| 607 | */ |
| 608 | public void release(int permits) { |
| 609 | if (permits < 0) throw new IllegalArgumentException(); |
| 610 | sync.releaseShared(permits); |
| 611 | } |
| 612 | |
| 613 | /** |
| 614 | * Returns the current number of permits available in this semaphore. |
| 615 | * |
| 616 | * <p>This method is typically used for debugging and testing purposes. |
| 617 | * |
| 618 | * @return the number of permits available in this semaphore |
| 619 | */ |
| 620 | public int availablePermits() { |
| 621 | return sync.getPermits(); |
| 622 | } |
| 623 | |
| 624 | /** |
| 625 | * Acquires and returns all permits that are immediately available. |
| 626 | * |
| 627 | * @return the number of permits acquired |
| 628 | */ |
| 629 | public int drainPermits() { |
| 630 | return sync.drainPermits(); |
| 631 | } |
| 632 | |
| 633 | /** |
| 634 | * Shrinks the number of available permits by the indicated |
| 635 | * reduction. This method can be useful in subclasses that use |
| 636 | * semaphores to track resources that become unavailable. This |
| 637 | * method differs from {@code acquire} in that it does not block |
| 638 | * waiting for permits to become available. |
| 639 | * |
| 640 | * @param reduction the number of permits to remove |
| 641 | * @throws IllegalArgumentException if {@code reduction} is negative |
| 642 | */ |
| 643 | protected void reducePermits(int reduction) { |
| 644 | if (reduction < 0) throw new IllegalArgumentException(); |
| 645 | sync.reducePermits(reduction); |
| 646 | } |
| 647 | |
| 648 | /** |
| 649 | * Returns {@code true} if this semaphore has fairness set true. |
| 650 | * |
| 651 | * @return {@code true} if this semaphore has fairness set true |
| 652 | */ |
| 653 | public boolean isFair() { |
| 654 | return sync instanceof FairSync; |
| 655 | } |
| 656 | |
| 657 | /** |
| 658 | * Queries whether any threads are waiting to acquire. Note that |
| 659 | * because cancellations may occur at any time, a {@code true} |
| 660 | * return does not guarantee that any other thread will ever |
| 661 | * acquire. This method is designed primarily for use in |
| 662 | * monitoring of the system state. |
| 663 | * |
| 664 | * @return {@code true} if there may be other threads waiting to |
| 665 | * acquire the lock |
| 666 | */ |
| 667 | public final boolean hasQueuedThreads() { |
| 668 | return sync.hasQueuedThreads(); |
| 669 | } |
| 670 | |
| 671 | /** |
| 672 | * Returns an estimate of the number of threads waiting to acquire. |
| 673 | * The value is only an estimate because the number of threads may |
| 674 | * change dynamically while this method traverses internal data |
| 675 | * structures. This method is designed for use in monitoring of the |
| 676 | * system state, not for synchronization control. |
| 677 | * |
| 678 | * @return the estimated number of threads waiting for this lock |
| 679 | */ |
| 680 | public final int getQueueLength() { |
| 681 | return sync.getQueueLength(); |
| 682 | } |
| 683 | |
| 684 | /** |
| 685 | * Returns a collection containing threads that may be waiting to acquire. |
| 686 | * Because the actual set of threads may change dynamically while |
| 687 | * constructing this result, the returned collection is only a best-effort |
| 688 | * estimate. The elements of the returned collection are in no particular |
| 689 | * order. This method is designed to facilitate construction of |
| 690 | * subclasses that provide more extensive monitoring facilities. |
| 691 | * |
| 692 | * @return the collection of threads |
| 693 | */ |
| 694 | protected Collection<Thread> getQueuedThreads() { |
| 695 | return sync.getQueuedThreads(); |
| 696 | } |
| 697 | |
| 698 | /** |
| 699 | * Returns a string identifying this semaphore, as well as its state. |
| 700 | * The state, in brackets, includes the String {@code "Permits ="} |
| 701 | * followed by the number of permits. |
| 702 | * |
| 703 | * @return a string identifying this semaphore, as well as its state |
| 704 | */ |
| 705 | public String toString() { |
| 706 | return super.toString() + "[Permits = " + sync.getPermits() + "]"; |
| 707 | } |
| 708 | } |