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/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 &quot;lock&quot; 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 current = getState();
 195                 int next = current + releases;
 196                 if (next < current) // overflow
 197                     throw new Error("Maximum permit count exceeded");
 198                 if (compareAndSetState(current, next))
 199                     return true;
 200             }
 201         }
 202 
 203         final void reducePermits(int reductions) {
 204             for (;;) {
 205                 int current = getState();
 206                 int next = current - reductions;
 207                 if (next > current) // underflow
 208                     throw new Error("Permit count underflow");
 209                 if (compareAndSetState(current, next))
 210                     return;
 211             }
 212         }
 213 
 214         final int drainPermits() {
 215             for (;;) {
 216                 int current = getState();
 217                 if (current == 0 || compareAndSetState(current, 0))
 218                     return current;
 219             }
 220         }
 221     }
 222 
 223     /**
 224      * NonFair version
 225      */
 226     final static class NonfairSync extends Sync {
 227         private static final long serialVersionUID = -2694183684443567898L;
 228 
 229         NonfairSync(int permits) {
 230             super(permits);
 231         }
 232 
 233         protected int tryAcquireShared(int acquires) {
 234             return nonfairTryAcquireShared(acquires);
 235         }
 236     }
 237 
 238     /**
 239      * Fair version
 240      */
 241     final static class FairSync extends Sync {
 242         private static final long serialVersionUID = 2014338818796000944L;
 243 
 244         FairSync(int permits) {
 245             super(permits);
 246         }
 247 
 248         protected int tryAcquireShared(int acquires) {
 249             for (;;) {
 250                 if (hasQueuedPredecessors())
 251                     return -1;
 252                 int available = getState();
 253                 int remaining = available - acquires;
 254                 if (remaining < 0 ||
 255                     compareAndSetState(available, remaining))
 256                     return remaining;
 257             }
 258         }
 259     }
 260 
 261     /**
 262      * Creates a {@code Semaphore} with the given number of
 263      * permits and nonfair fairness setting.
 264      *
 265      * @param permits the initial number of permits available.
 266      *        This value may be negative, in which case releases
 267      *        must occur before any acquires will be granted.
 268      */
 269     public Semaphore(int permits) {
 270         sync = new NonfairSync(permits);
 271     }
 272 
 273     /**
 274      * Creates a {@code Semaphore} with the given number of
 275      * permits and the given fairness setting.
 276      *
 277      * @param permits the initial number of permits available.
 278      *        This value may be negative, in which case releases
 279      *        must occur before any acquires will be granted.
 280      * @param fair {@code true} if this semaphore will guarantee
 281      *        first-in first-out granting of permits under contention,
 282      *        else {@code false}
 283      */
 284     public Semaphore(int permits, boolean fair) {
 285         sync = (fair)? new FairSync(permits) : new NonfairSync(permits);
 286     }
 287 
 288     /**
 289      * Acquires a permit from this semaphore, blocking until one is
 290      * available, or the thread is {@linkplain Thread#interrupt interrupted}.
 291      *
 292      * <p>Acquires a permit, if one is available and returns immediately,
 293      * reducing the number of available permits by one.
 294      *
 295      * <p>If no permit is available then the current thread becomes
 296      * disabled for thread scheduling purposes and lies dormant until
 297      * one of two things happens:
 298      * <ul>
 299      * <li>Some other thread invokes the {@link #release} method for this
 300      * semaphore and the current thread is next to be assigned a permit; or
 301      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
 302      * the current thread.
 303      * </ul>
 304      *
 305      * <p>If the current thread:
 306      * <ul>
 307      * <li>has its interrupted status set on entry to this method; or
 308      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
 309      * for a permit,
 310      * </ul>
 311      * then {@link InterruptedException} is thrown and the current thread's
 312      * interrupted status is cleared.
 313      *
 314      * @throws InterruptedException if the current thread is interrupted
 315      */
 316     public void acquire() throws InterruptedException {
 317         sync.acquireSharedInterruptibly(1);
 318     }
 319 
 320     /**
 321      * Acquires a permit from this semaphore, blocking until one is
 322      * available.
 323      *
 324      * <p>Acquires a permit, if one is available and returns immediately,
 325      * reducing the number of available permits by one.
 326      *
 327      * <p>If no permit is available then the current thread becomes
 328      * disabled for thread scheduling purposes and lies dormant until
 329      * some other thread invokes the {@link #release} method for this
 330      * semaphore and the current thread is next to be assigned a permit.
 331      *
 332      * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
 333      * while waiting for a permit then it will continue to wait, but the
 334      * time at which the thread is assigned a permit may change compared to
 335      * the time it would have received the permit had no interruption
 336      * occurred.  When the thread does return from this method its interrupt
 337      * status will be set.
 338      */
 339     public void acquireUninterruptibly() {
 340         sync.acquireShared(1);
 341     }
 342 
 343     /**
 344      * Acquires a permit from this semaphore, only if one is available at the
 345      * time of invocation.
 346      *
 347      * <p>Acquires a permit, if one is available and returns immediately,
 348      * with the value {@code true},
 349      * reducing the number of available permits by one.
 350      *
 351      * <p>If no permit is available then this method will return
 352      * immediately with the value {@code false}.
 353      *
 354      * <p>Even when this semaphore has been set to use a
 355      * fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
 356      * immediately acquire a permit if one is available, whether or not
 357      * other threads are currently waiting.
 358      * This &quot;barging&quot; behavior can be useful in certain
 359      * circumstances, even though it breaks fairness. If you want to honor
 360      * the fairness setting, then use
 361      * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
 362      * which is almost equivalent (it also detects interruption).
 363      *
 364      * @return {@code true} if a permit was acquired and {@code false}
 365      *         otherwise
 366      */
 367     public boolean tryAcquire() {
 368         return sync.nonfairTryAcquireShared(1) >= 0;
 369     }
 370 
 371     /**
 372      * Acquires a permit from this semaphore, if one becomes available
 373      * within the given waiting time and the current thread has not
 374      * been {@linkplain Thread#interrupt interrupted}.
 375      *
 376      * <p>Acquires a permit, if one is available and returns immediately,
 377      * with the value {@code true},
 378      * reducing the number of available permits by one.
 379      *
 380      * <p>If no permit is available then the current thread becomes
 381      * disabled for thread scheduling purposes and lies dormant until
 382      * one of three things happens:
 383      * <ul>
 384      * <li>Some other thread invokes the {@link #release} method for this
 385      * semaphore and the current thread is next to be assigned a permit; or
 386      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
 387      * the current thread; or
 388      * <li>The specified waiting time elapses.
 389      * </ul>
 390      *
 391      * <p>If a permit is acquired then the value {@code true} is returned.
 392      *
 393      * <p>If the current thread:
 394      * <ul>
 395      * <li>has its interrupted status set on entry to this method; or
 396      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
 397      * to acquire a permit,
 398      * </ul>
 399      * then {@link InterruptedException} is thrown and the current thread's
 400      * interrupted status is cleared.
 401      *
 402      * <p>If the specified waiting time elapses then the value {@code false}
 403      * is returned.  If the time is less than or equal to zero, the method
 404      * will not wait at all.
 405      *
 406      * @param timeout the maximum time to wait for a permit
 407      * @param unit the time unit of the {@code timeout} argument
 408      * @return {@code true} if a permit was acquired and {@code false}
 409      *         if the waiting time elapsed before a permit was acquired
 410      * @throws InterruptedException if the current thread is interrupted
 411      */
 412     public boolean tryAcquire(long timeout, TimeUnit unit)
 413         throws InterruptedException {
 414         return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
 415     }
 416 
 417     /**
 418      * Releases a permit, returning it to the semaphore.
 419      *
 420      * <p>Releases a permit, increasing the number of available permits by
 421      * one.  If any threads are trying to acquire a permit, then one is
 422      * selected and given the permit that was just released.  That thread
 423      * is (re)enabled for thread scheduling purposes.
 424      *
 425      * <p>There is no requirement that a thread that releases a permit must
 426      * have acquired that permit by calling {@link #acquire}.
 427      * Correct usage of a semaphore is established by programming convention
 428      * in the application.
 429      */
 430     public void release() {
 431         sync.releaseShared(1);
 432     }
 433 
 434     /**
 435      * Acquires the given number of permits from this semaphore,
 436      * blocking until all are available,
 437      * or the thread is {@linkplain Thread#interrupt interrupted}.
 438      *
 439      * <p>Acquires the given number of permits, if they are available,
 440      * and returns immediately, reducing the number of available permits
 441      * by the given amount.
 442      *
 443      * <p>If insufficient permits are available then the current thread becomes
 444      * disabled for thread scheduling purposes and lies dormant until
 445      * one of two things happens:
 446      * <ul>
 447      * <li>Some other thread invokes one of the {@link #release() release}
 448      * methods for this semaphore, the current thread is next to be assigned
 449      * permits and the number of available permits satisfies this request; or
 450      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
 451      * the current thread.
 452      * </ul>
 453      *
 454      * <p>If the current thread:
 455      * <ul>
 456      * <li>has its interrupted status set on entry to this method; or
 457      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
 458      * for a permit,
 459      * </ul>
 460      * then {@link InterruptedException} is thrown and the current thread's
 461      * interrupted status is cleared.
 462      * Any permits that were to be assigned to this thread are instead
 463      * assigned to other threads trying to acquire permits, as if
 464      * permits had been made available by a call to {@link #release()}.
 465      *
 466      * @param permits the number of permits to acquire
 467      * @throws InterruptedException if the current thread is interrupted
 468      * @throws IllegalArgumentException if {@code permits} is negative
 469      */
 470     public void acquire(int permits) throws InterruptedException {
 471         if (permits < 0) throw new IllegalArgumentException();
 472         sync.acquireSharedInterruptibly(permits);
 473     }
 474 
 475     /**
 476      * Acquires the given number of permits from this semaphore,
 477      * blocking until all are available.
 478      *
 479      * <p>Acquires the given number of permits, if they are available,
 480      * and returns immediately, reducing the number of available permits
 481      * by the given amount.
 482      *
 483      * <p>If insufficient permits are available then the current thread becomes
 484      * disabled for thread scheduling purposes and lies dormant until
 485      * some other thread invokes one of the {@link #release() release}
 486      * methods for this semaphore, the current thread is next to be assigned
 487      * permits and the number of available permits satisfies this request.
 488      *
 489      * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
 490      * while waiting for permits then it will continue to wait and its
 491      * position in the queue is not affected.  When the thread does return
 492      * from this method its interrupt status will be set.
 493      *
 494      * @param permits the number of permits to acquire
 495      * @throws IllegalArgumentException if {@code permits} is negative
 496      *
 497      */
 498     public void acquireUninterruptibly(int permits) {
 499         if (permits < 0) throw new IllegalArgumentException();
 500         sync.acquireShared(permits);
 501     }
 502 
 503     /**
 504      * Acquires the given number of permits from this semaphore, only
 505      * if all are available at the time of invocation.
 506      *
 507      * <p>Acquires the given number of permits, if they are available, and
 508      * returns immediately, with the value {@code true},
 509      * reducing the number of available permits by the given amount.
 510      *
 511      * <p>If insufficient permits are available then this method will return
 512      * immediately with the value {@code false} and the number of available
 513      * permits is unchanged.
 514      *
 515      * <p>Even when this semaphore has been set to use a fair ordering
 516      * policy, a call to {@code tryAcquire} <em>will</em>
 517      * immediately acquire a permit if one is available, whether or
 518      * not other threads are currently waiting.  This
 519      * &quot;barging&quot; behavior can be useful in certain
 520      * circumstances, even though it breaks fairness. If you want to
 521      * honor the fairness setting, then use {@link #tryAcquire(int,
 522      * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }
 523      * which is almost equivalent (it also detects interruption).
 524      *
 525      * @param permits the number of permits to acquire
 526      * @return {@code true} if the permits were acquired and
 527      *         {@code false} otherwise
 528      * @throws IllegalArgumentException if {@code permits} is negative
 529      */
 530     public boolean tryAcquire(int permits) {
 531         if (permits < 0) throw new IllegalArgumentException();
 532         return sync.nonfairTryAcquireShared(permits) >= 0;
 533     }
 534 
 535     /**
 536      * Acquires the given number of permits from this semaphore, if all
 537      * become available within the given waiting time and the current
 538      * thread has not been {@linkplain Thread#interrupt interrupted}.
 539      *
 540      * <p>Acquires the given number of permits, if they are available and
 541      * returns immediately, with the value {@code true},
 542      * reducing the number of available permits by the given amount.
 543      *
 544      * <p>If insufficient permits are available then
 545      * the current thread becomes disabled for thread scheduling
 546      * purposes and lies dormant until one of three things happens:
 547      * <ul>
 548      * <li>Some other thread invokes one of the {@link #release() release}
 549      * methods for this semaphore, the current thread is next to be assigned
 550      * permits and the number of available permits satisfies this request; or
 551      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
 552      * the current thread; or
 553      * <li>The specified waiting time elapses.
 554      * </ul>
 555      *
 556      * <p>If the permits are acquired then the value {@code true} is returned.
 557      *
 558      * <p>If the current thread:
 559      * <ul>
 560      * <li>has its interrupted status set on entry to this method; or
 561      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
 562      * to acquire the permits,
 563      * </ul>
 564      * then {@link InterruptedException} is thrown and the current thread's
 565      * interrupted status is cleared.
 566      * Any permits that were to be assigned to this thread, are instead
 567      * assigned to other threads trying to acquire permits, as if
 568      * the permits had been made available by a call to {@link #release()}.
 569      *
 570      * <p>If the specified waiting time elapses then the value {@code false}
 571      * is returned.  If the time is less than or equal to zero, the method
 572      * will not wait at all.  Any permits that were to be assigned to this
 573      * thread, are instead assigned to other threads trying to acquire
 574      * permits, as if the permits had been made available by a call to
 575      * {@link #release()}.
 576      *
 577      * @param permits the number of permits to acquire
 578      * @param timeout the maximum time to wait for the permits
 579      * @param unit the time unit of the {@code timeout} argument
 580      * @return {@code true} if all permits were acquired and {@code false}
 581      *         if the waiting time elapsed before all permits were acquired
 582      * @throws InterruptedException if the current thread is interrupted
 583      * @throws IllegalArgumentException if {@code permits} is negative
 584      */
 585     public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
 586         throws InterruptedException {
 587         if (permits < 0) throw new IllegalArgumentException();
 588         return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
 589     }
 590 
 591     /**
 592      * Releases the given number of permits, returning them to the semaphore.
 593      *
 594      * <p>Releases the given number of permits, increasing the number of
 595      * available permits by that amount.
 596      * If any threads are trying to acquire permits, then one
 597      * is selected and given the permits that were just released.
 598      * If the number of available permits satisfies that thread's request
 599      * then that thread is (re)enabled for thread scheduling purposes;
 600      * otherwise the thread will wait until sufficient permits are available.
 601      * If there are still permits available
 602      * after this thread's request has been satisfied, then those permits
 603      * are assigned in turn to other threads trying to acquire permits.
 604      *
 605      * <p>There is no requirement that a thread that releases a permit must
 606      * have acquired that permit by calling {@link Semaphore#acquire acquire}.
 607      * Correct usage of a semaphore is established by programming convention
 608      * in the application.
 609      *
 610      * @param permits the number of permits to release
 611      * @throws IllegalArgumentException if {@code permits} is negative
 612      */
 613     public void release(int permits) {
 614         if (permits < 0) throw new IllegalArgumentException();
 615         sync.releaseShared(permits);
 616     }
 617 
 618     /**
 619      * Returns the current number of permits available in this semaphore.
 620      *
 621      * <p>This method is typically used for debugging and testing purposes.
 622      *
 623      * @return the number of permits available in this semaphore
 624      */
 625     public int availablePermits() {
 626         return sync.getPermits();
 627     }
 628 
 629     /**
 630      * Acquires and returns all permits that are immediately available.
 631      *
 632      * @return the number of permits acquired
 633      */
 634     public int drainPermits() {
 635         return sync.drainPermits();
 636     }
 637 
 638     /**
 639      * Shrinks the number of available permits by the indicated
 640      * reduction. This method can be useful in subclasses that use
 641      * semaphores to track resources that become unavailable. This
 642      * method differs from {@code acquire} in that it does not block
 643      * waiting for permits to become available.
 644      *
 645      * @param reduction the number of permits to remove
 646      * @throws IllegalArgumentException if {@code reduction} is negative
 647      */
 648     protected void reducePermits(int reduction) {
 649         if (reduction < 0) throw new IllegalArgumentException();
 650         sync.reducePermits(reduction);
 651     }
 652 
 653     /**
 654      * Returns {@code true} if this semaphore has fairness set true.
 655      *
 656      * @return {@code true} if this semaphore has fairness set true
 657      */
 658     public boolean isFair() {
 659         return sync instanceof FairSync;
 660     }
 661 
 662     /**
 663      * Queries whether any threads are waiting to acquire. Note that
 664      * because cancellations may occur at any time, a {@code true}
 665      * return does not guarantee that any other thread will ever
 666      * acquire.  This method is designed primarily for use in
 667      * monitoring of the system state.
 668      *
 669      * @return {@code true} if there may be other threads waiting to
 670      *         acquire the lock
 671      */
 672     public final boolean hasQueuedThreads() {
 673         return sync.hasQueuedThreads();
 674     }
 675 
 676     /**
 677      * Returns an estimate of the number of threads waiting to acquire.
 678      * The value is only an estimate because the number of threads may
 679      * change dynamically while this method traverses internal data
 680      * structures.  This method is designed for use in monitoring of the
 681      * system state, not for synchronization control.
 682      *
 683      * @return the estimated number of threads waiting for this lock
 684      */
 685     public final int getQueueLength() {
 686         return sync.getQueueLength();
 687     }
 688 
 689     /**
 690      * Returns a collection containing threads that may be waiting to acquire.
 691      * Because the actual set of threads may change dynamically while
 692      * constructing this result, the returned collection is only a best-effort
 693      * estimate.  The elements of the returned collection are in no particular
 694      * order.  This method is designed to facilitate construction of
 695      * subclasses that provide more extensive monitoring facilities.
 696      *
 697      * @return the collection of threads
 698      */
 699     protected Collection<Thread> getQueuedThreads() {
 700         return sync.getQueuedThreads();
 701     }
 702 
 703     /**
 704      * Returns a string identifying this semaphore, as well as its state.
 705      * The state, in brackets, includes the String {@code "Permits ="}
 706      * followed by the number of permits.
 707      *
 708      * @return a string identifying this semaphore, as well as its state
 709      */
 710     public String toString() {
 711         return super.toString() + "[Permits = " + sync.getPermits() + "]";
 712     }
 713 }