1 /*
   2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   3  *
   4  * This code is free software; you can redistribute it and/or modify it
   5  * under the terms of the GNU General Public License version 2 only, as
   6  * published by the Free Software Foundation.  Oracle designates this
   7  * particular file as subject to the "Classpath" exception as provided
   8  * by Oracle in the LICENSE file that accompanied this code.
   9  *
  10  * This code is distributed in the hope that it will be useful, but WITHOUT
  11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  13  * version 2 for more details (a copy is included in the LICENSE file that
  14  * accompanied this code).
  15  *
  16  * You should have received a copy of the GNU General Public License version
  17  * 2 along with this work; if not, write to the Free Software Foundation,
  18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  19  *
  20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  21  * or visit www.oracle.com if you need additional information or have any
  22  * questions.
  23  */
  24 
  25 /*
  26  * This file is available under and governed by the GNU General Public
  27  * License version 2 only, as published by the Free Software Foundation.
  28  * However, the following notice accompanied the original version of this
  29  * file:
  30  *
  31  * Written by Doug Lea with assistance from members of JCP JSR-166
  32  * Expert Group and released to the public domain, as explained at
  33  * http://creativecommons.org/publicdomain/zero/1.0/
  34  */
  35 
  36 package java.util.concurrent;
  37 
  38 import java.util.Collection;
  39 import java.util.concurrent.locks.AbstractQueuedSynchronizer;
  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> {@code
  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  * }}</pre>
  95  *
  96  * <p>Before obtaining an item each thread must acquire a permit from
  97  * the semaphore, guaranteeing that an item is available for use. When
  98  * the thread has finished with the item it is returned back to the
  99  * pool and a permit is returned to the semaphore, allowing another
 100  * thread to acquire that item.  Note that no synchronization lock is
 101  * held when {@link #acquire} is called as that would prevent an item
 102  * from being returned to the pool.  The semaphore encapsulates the
 103  * synchronization needed to restrict access to the pool, separately
 104  * from any synchronization needed to maintain the consistency of the
 105  * pool itself.
 106  *
 107  * <p>A semaphore initialized to one, and which is used such that it
 108  * only has at most one permit available, can serve as a mutual
 109  * exclusion lock.  This is more commonly known as a <em>binary
 110  * semaphore</em>, because it only has two states: one permit
 111  * available, or zero permits available.  When used in this way, the
 112  * binary semaphore has the property (unlike many {@link java.util.concurrent.locks.Lock}
 113  * implementations), that the &quot;lock&quot; can be released by a
 114  * thread other than the owner (as semaphores have no notion of
 115  * ownership).  This can be useful in some specialized contexts, such
 116  * as deadlock recovery.
 117  *
 118  * <p>The constructor for this class optionally accepts a
 119  * <em>fairness</em> parameter. When set false, this class makes no
 120  * guarantees about the order in which threads acquire permits. In
 121  * particular, <em>barging</em> is permitted, that is, a thread
 122  * invoking {@link #acquire} can be allocated a permit ahead of a
 123  * thread that has been waiting - logically the new thread places itself at
 124  * the head of the queue of waiting threads. When fairness is set true, the
 125  * semaphore guarantees that threads invoking any of the {@link
 126  * #acquire() acquire} methods are selected to obtain permits in the order in
 127  * which their invocation of those methods was processed
 128  * (first-in-first-out; FIFO). Note that FIFO ordering necessarily
 129  * applies to specific internal points of execution within these
 130  * methods.  So, it is possible for one thread to invoke
 131  * {@code acquire} before another, but reach the ordering point after
 132  * the other, and similarly upon return from the method.
 133  * Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not
 134  * honor the fairness setting, but will take any permits that are
 135  * available.
 136  *
 137  * <p>Generally, semaphores used to control resource access should be
 138  * initialized as fair, to ensure that no thread is starved out from
 139  * accessing a resource. When using semaphores for other kinds of
 140  * synchronization control, the throughput advantages of non-fair
 141  * ordering often outweigh fairness considerations.
 142  *
 143  * <p>This class also provides convenience methods to {@link
 144  * #acquire(int) acquire} and {@link #release(int) release} multiple
 145  * permits at a time. These methods are generally more efficient and
 146  * effective than loops. However, they do not establish any preference
 147  * order. For example, if thread A invokes {@code s.acquire(3}) and
 148  * thread B invokes {@code s.acquire(2)}, and two permits become
 149  * available, then there is no guarantee that thread B will obtain
 150  * them unless its acquire came first and Semaphore {@code s} is in
 151  * fair mode.
 152  *
 153  * <p>Memory consistency effects: Actions in a thread prior to calling
 154  * a "release" method such as {@code release()}
 155  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 156  * actions following a successful "acquire" method such as {@code acquire()}
 157  * in another thread.
 158  *
 159  * @since 1.5
 160  * @author Doug Lea
 161  */
 162 public class Semaphore implements java.io.Serializable {
 163     private static final long serialVersionUID = -3222578661600680210L;
 164     /** All mechanics via AbstractQueuedSynchronizer subclass */
 165     private final Sync sync;
 166 
 167     /**
 168      * Synchronization implementation for semaphore.  Uses AQS state
 169      * to represent permits. Subclassed into fair and nonfair
 170      * versions.
 171      */
 172     abstract static class Sync extends AbstractQueuedSynchronizer {
 173         private static final long serialVersionUID = 1192457210091910933L;
 174 
 175         Sync(int permits) {
 176             setState(permits);
 177         }
 178 
 179         final int getPermits() {
 180             return getState();
 181         }
 182 
 183         final int nonfairTryAcquireShared(int acquires) {
 184             for (;;) {
 185                 int available = getState();
 186                 int remaining = available - acquires;
 187                 if (remaining < 0 ||
 188                     compareAndSetState(available, remaining))
 189                     return remaining;
 190             }
 191         }
 192 
 193         protected final boolean tryReleaseShared(int releases) {
 194             for (;;) {
 195                 int current = getState();
 196                 int next = current + releases;
 197                 if (next < current) // overflow
 198                     throw new Error("Maximum permit count exceeded");
 199                 if (compareAndSetState(current, next))
 200                     return true;
 201             }
 202         }
 203 
 204         final void reducePermits(int reductions) {
 205             for (;;) {
 206                 int current = getState();
 207                 int next = current - reductions;
 208                 if (next > current) // underflow
 209                     throw new Error("Permit count underflow");
 210                 if (compareAndSetState(current, next))
 211                     return;
 212             }
 213         }
 214 
 215         final int drainPermits() {
 216             for (;;) {
 217                 int current = getState();
 218                 if (current == 0 || compareAndSetState(current, 0))
 219                     return current;
 220             }
 221         }
 222     }
 223 
 224     /**
 225      * NonFair version
 226      */
 227     static final class NonfairSync extends Sync {
 228         private static final long serialVersionUID = -2694183684443567898L;
 229 
 230         NonfairSync(int permits) {
 231             super(permits);
 232         }
 233 
 234         protected int tryAcquireShared(int acquires) {
 235             return nonfairTryAcquireShared(acquires);
 236         }
 237     }
 238 
 239     /**
 240      * Fair version
 241      */
 242     static final class FairSync extends Sync {
 243         private static final long serialVersionUID = 2014338818796000944L;
 244 
 245         FairSync(int permits) {
 246             super(permits);
 247         }
 248 
 249         protected int tryAcquireShared(int acquires) {
 250             for (;;) {
 251                 if (hasQueuedPredecessors())
 252                     return -1;
 253                 int available = getState();
 254                 int remaining = available - acquires;
 255                 if (remaining < 0 ||
 256                     compareAndSetState(available, remaining))
 257                     return remaining;
 258             }
 259         }
 260     }
 261 
 262     /**
 263      * Creates a {@code Semaphore} with the given number of
 264      * permits and nonfair fairness setting.
 265      *
 266      * @param permits the initial number of permits available.
 267      *        This value may be negative, in which case releases
 268      *        must occur before any acquires will be granted.
 269      */
 270     public Semaphore(int permits) {
 271         sync = new NonfairSync(permits);
 272     }
 273 
 274     /**
 275      * Creates a {@code Semaphore} with the given number of
 276      * permits and the given fairness setting.
 277      *
 278      * @param permits the initial number of permits available.
 279      *        This value may be negative, in which case releases
 280      *        must occur before any acquires will be granted.
 281      * @param fair {@code true} if this semaphore will guarantee
 282      *        first-in first-out granting of permits under contention,
 283      *        else {@code false}
 284      */
 285     public Semaphore(int permits, boolean fair) {
 286         sync = fair ? new FairSync(permits) : new NonfairSync(permits);
 287     }
 288 
 289     /**
 290      * Acquires a permit from this semaphore, blocking until one is
 291      * available, or the thread is {@linkplain Thread#interrupt interrupted}.
 292      *
 293      * <p>Acquires a permit, if one is available and returns immediately,
 294      * reducing the number of available permits by one.
 295      *
 296      * <p>If no permit is available then the current thread becomes
 297      * disabled for thread scheduling purposes and lies dormant until
 298      * one of two things happens:
 299      * <ul>
 300      * <li>Some other thread invokes the {@link #release} method for this
 301      * semaphore and the current thread is next to be assigned a permit; or
 302      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
 303      * the current thread.
 304      * </ul>
 305      *
 306      * <p>If the current thread:
 307      * <ul>
 308      * <li>has its interrupted status set on entry to this method; or
 309      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
 310      * for a permit,
 311      * </ul>
 312      * then {@link InterruptedException} is thrown and the current thread's
 313      * interrupted status is cleared.
 314      *
 315      * @throws InterruptedException if the current thread is interrupted
 316      */
 317     public void acquire() throws InterruptedException {
 318         sync.acquireSharedInterruptibly(1);
 319     }
 320 
 321     /**
 322      * Acquires a permit from this semaphore, blocking until one is
 323      * available.
 324      *
 325      * <p>Acquires a permit, if one is available and returns immediately,
 326      * reducing the number of available permits by one.
 327      *
 328      * <p>If no permit is available then the current thread becomes
 329      * disabled for thread scheduling purposes and lies dormant until
 330      * some other thread invokes the {@link #release} method for this
 331      * semaphore and the current thread is next to be assigned a permit.
 332      *
 333      * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
 334      * while waiting for a permit then it will continue to wait, but the
 335      * time at which the thread is assigned a permit may change compared to
 336      * the time it would have received the permit had no interruption
 337      * occurred.  When the thread does return from this method its interrupt
 338      * status will be set.
 339      */
 340     public void acquireUninterruptibly() {
 341         sync.acquireShared(1);
 342     }
 343 
 344     /**
 345      * Acquires a permit from this semaphore, only if one is available at the
 346      * time of invocation.
 347      *
 348      * <p>Acquires a permit, if one is available and returns immediately,
 349      * with the value {@code true},
 350      * reducing the number of available permits by one.
 351      *
 352      * <p>If no permit is available then this method will return
 353      * immediately with the value {@code false}.
 354      *
 355      * <p>Even when this semaphore has been set to use a
 356      * fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
 357      * immediately acquire a permit if one is available, whether or not
 358      * other threads are currently waiting.
 359      * This &quot;barging&quot; behavior can be useful in certain
 360      * circumstances, even though it breaks fairness. If you want to honor
 361      * the fairness setting, then use
 362      * {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS)}
 363      * which is almost equivalent (it also detects interruption).
 364      *
 365      * @return {@code true} if a permit was acquired and {@code false}
 366      *         otherwise
 367      */
 368     public boolean tryAcquire() {
 369         return sync.nonfairTryAcquireShared(1) >= 0;
 370     }
 371 
 372     /**
 373      * Acquires a permit from this semaphore, if one becomes available
 374      * within the given waiting time and the current thread has not
 375      * been {@linkplain Thread#interrupt interrupted}.
 376      *
 377      * <p>Acquires a permit, if one is available and returns immediately,
 378      * with the value {@code true},
 379      * reducing the number of available permits by one.
 380      *
 381      * <p>If no permit is available then the current thread becomes
 382      * disabled for thread scheduling purposes and lies dormant until
 383      * one of three things happens:
 384      * <ul>
 385      * <li>Some other thread invokes the {@link #release} method for this
 386      * semaphore and the current thread is next to be assigned a permit; or
 387      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
 388      * the current thread; or
 389      * <li>The specified waiting time elapses.
 390      * </ul>
 391      *
 392      * <p>If a permit is acquired then the value {@code true} is returned.
 393      *
 394      * <p>If the current thread:
 395      * <ul>
 396      * <li>has its interrupted status set on entry to this method; or
 397      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
 398      * to acquire a permit,
 399      * </ul>
 400      * then {@link InterruptedException} is thrown and the current thread's
 401      * interrupted status is cleared.
 402      *
 403      * <p>If the specified waiting time elapses then the value {@code false}
 404      * is returned.  If the time is less than or equal to zero, the method
 405      * will not wait at all.
 406      *
 407      * @param timeout the maximum time to wait for a permit
 408      * @param unit the time unit of the {@code timeout} argument
 409      * @return {@code true} if a permit was acquired and {@code false}
 410      *         if the waiting time elapsed before a permit was acquired
 411      * @throws InterruptedException if the current thread is interrupted
 412      */
 413     public boolean tryAcquire(long timeout, TimeUnit unit)
 414         throws InterruptedException {
 415         return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
 416     }
 417 
 418     /**
 419      * Releases a permit, returning it to the semaphore.
 420      *
 421      * <p>Releases a permit, increasing the number of available permits by
 422      * one.  If any threads are trying to acquire a permit, then one is
 423      * selected and given the permit that was just released.  That thread
 424      * is (re)enabled for thread scheduling purposes.
 425      *
 426      * <p>There is no requirement that a thread that releases a permit must
 427      * have acquired that permit by calling {@link #acquire}.
 428      * Correct usage of a semaphore is established by programming convention
 429      * in the application.
 430      */
 431     public void release() {
 432         sync.releaseShared(1);
 433     }
 434 
 435     /**
 436      * Acquires the given number of permits from this semaphore,
 437      * blocking until all are available,
 438      * or the thread is {@linkplain Thread#interrupt interrupted}.
 439      *
 440      * <p>Acquires the given number of permits, if they are available,
 441      * and returns immediately, reducing the number of available permits
 442      * by the given amount. This method has the same effect as the
 443      * loop {@code for (int i = 0; i < permits; ++i) acquire();} except
 444      * that it atomically acquires the permits all at once:
 445      *
 446      * <p>If insufficient permits are available then the current thread becomes
 447      * disabled for thread scheduling purposes and lies dormant until
 448      * one of two things happens:
 449      * <ul>
 450      * <li>Some other thread invokes one of the {@link #release() release}
 451      * methods for this semaphore and the current thread is next to be assigned
 452      * permits and the number of available permits satisfies this request; or
 453      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
 454      * the current thread.
 455      * </ul>
 456      *
 457      * <p>If the current thread:
 458      * <ul>
 459      * <li>has its interrupted status set on entry to this method; or
 460      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
 461      * for a permit,
 462      * </ul>
 463      * then {@link InterruptedException} is thrown and the current thread's
 464      * interrupted status is cleared.
 465      * Any permits that were to be assigned to this thread are instead
 466      * assigned to other threads trying to acquire permits, as if
 467      * permits had been made available by a call to {@link #release()}.
 468      *
 469      * @param permits the number of permits to acquire
 470      * @throws InterruptedException if the current thread is interrupted
 471      * @throws IllegalArgumentException if {@code permits} is negative
 472      */
 473     public void acquire(int permits) throws InterruptedException {
 474         if (permits < 0) throw new IllegalArgumentException();
 475         sync.acquireSharedInterruptibly(permits);
 476     }
 477 
 478     /**
 479      * Acquires the given number of permits from this semaphore,
 480      * blocking until all are available.
 481      *
 482      * <p>Acquires the given number of permits, if they are available,
 483      * and returns immediately, reducing the number of available permits
 484      * by the given amount. This method has the same effect as the
 485      * loop {@code for (int i = 0; i < permits; ++i) acquireUninterruptibly();}
 486      * except that it atomically acquires the permits all at once:
 487      *
 488      * <p>If insufficient permits are available then the current thread becomes
 489      * disabled for thread scheduling purposes and lies dormant until
 490      * some other thread invokes one of the {@link #release() release}
 491      * methods for this semaphore and the current thread is next to be assigned
 492      * permits and the number of available permits satisfies this request.
 493      *
 494      * <p>If the current thread is {@linkplain Thread#interrupt interrupted}
 495      * while waiting for permits then it will continue to wait and its
 496      * position in the queue is not affected.  When the thread does return
 497      * from this method its interrupt status will be set.
 498      *
 499      * @param permits the number of permits to acquire
 500      * @throws IllegalArgumentException if {@code permits} is negative
 501      */
 502     public void acquireUninterruptibly(int permits) {
 503         if (permits < 0) throw new IllegalArgumentException();
 504         sync.acquireShared(permits);
 505     }
 506 
 507     /**
 508      * Acquires the given number of permits from this semaphore, only
 509      * if all are available at the time of invocation.
 510      *
 511      * <p>Acquires the given number of permits, if they are available, and
 512      * returns immediately, with the value {@code true},
 513      * reducing the number of available permits by the given amount.
 514      *
 515      * <p>If insufficient permits are available then this method will return
 516      * immediately with the value {@code false} and the number of available
 517      * permits is unchanged.
 518      *
 519      * <p>Even when this semaphore has been set to use a fair ordering
 520      * policy, a call to {@code tryAcquire} <em>will</em>
 521      * immediately acquire a permit if one is available, whether or
 522      * not other threads are currently waiting.  This
 523      * &quot;barging&quot; behavior can be useful in certain
 524      * circumstances, even though it breaks fairness. If you want to
 525      * honor the fairness setting, then use {@link #tryAcquire(int,
 526      * long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS)}
 527      * which is almost equivalent (it also detects interruption).
 528      *
 529      * @param permits the number of permits to acquire
 530      * @return {@code true} if the permits were acquired and
 531      *         {@code false} otherwise
 532      * @throws IllegalArgumentException if {@code permits} is negative
 533      */
 534     public boolean tryAcquire(int permits) {
 535         if (permits < 0) throw new IllegalArgumentException();
 536         return sync.nonfairTryAcquireShared(permits) >= 0;
 537     }
 538 
 539     /**
 540      * Acquires the given number of permits from this semaphore, if all
 541      * become available within the given waiting time and the current
 542      * thread has not been {@linkplain Thread#interrupt interrupted}.
 543      *
 544      * <p>Acquires the given number of permits, if they are available and
 545      * returns immediately, with the value {@code true},
 546      * reducing the number of available permits by the given amount.
 547      *
 548      * <p>If insufficient permits are available then
 549      * the current thread becomes disabled for thread scheduling
 550      * purposes and lies dormant until one of three things happens:
 551      * <ul>
 552      * <li>Some other thread invokes one of the {@link #release() release}
 553      * methods for this semaphore and the current thread is next to be assigned
 554      * permits and the number of available permits satisfies this request; or
 555      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
 556      * the current thread; or
 557      * <li>The specified waiting time elapses.
 558      * </ul>
 559      *
 560      * <p>If the permits are acquired then the value {@code true} is returned.
 561      *
 562      * <p>If the current thread:
 563      * <ul>
 564      * <li>has its interrupted status set on entry to this method; or
 565      * <li>is {@linkplain Thread#interrupt interrupted} while waiting
 566      * to acquire the permits,
 567      * </ul>
 568      * then {@link InterruptedException} is thrown and the current thread's
 569      * interrupted status is cleared.
 570      * Any permits that were to be assigned to this thread, are instead
 571      * assigned to other threads trying to acquire permits, as if
 572      * the permits had been made available by a call to {@link #release()}.
 573      *
 574      * <p>If the specified waiting time elapses then the value {@code false}
 575      * is returned.  If the time is less than or equal to zero, the method
 576      * will not wait at all.  Any permits that were to be assigned to this
 577      * thread, are instead assigned to other threads trying to acquire
 578      * permits, as if the permits had been made available by a call to
 579      * {@link #release()}.
 580      *
 581      * @param permits the number of permits to acquire
 582      * @param timeout the maximum time to wait for the permits
 583      * @param unit the time unit of the {@code timeout} argument
 584      * @return {@code true} if all permits were acquired and {@code false}
 585      *         if the waiting time elapsed before all permits were acquired
 586      * @throws InterruptedException if the current thread is interrupted
 587      * @throws IllegalArgumentException if {@code permits} is negative
 588      */
 589     public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
 590         throws InterruptedException {
 591         if (permits < 0) throw new IllegalArgumentException();
 592         return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
 593     }
 594 
 595     /**
 596      * Releases the given number of permits, returning them to the semaphore.
 597      *
 598      * <p>Releases the given number of permits, increasing the number of
 599      * available permits by that amount.
 600      * If any threads are trying to acquire permits, then one thread
 601      * is selected and given the permits that were just released.
 602      * If the number of available permits satisfies that thread's request
 603      * then that thread is (re)enabled for thread scheduling purposes;
 604      * otherwise the thread will wait until sufficient permits are available.
 605      * If there are still permits available
 606      * after this thread's request has been satisfied, then those permits
 607      * are assigned in turn to other threads trying to acquire permits.
 608      *
 609      * <p>There is no requirement that a thread that releases a permit must
 610      * have acquired that permit by calling {@link Semaphore#acquire acquire}.
 611      * Correct usage of a semaphore is established by programming convention
 612      * in the application.
 613      *
 614      * @param permits the number of permits to release
 615      * @throws IllegalArgumentException if {@code permits} is negative
 616      */
 617     public void release(int permits) {
 618         if (permits < 0) throw new IllegalArgumentException();
 619         sync.releaseShared(permits);
 620     }
 621 
 622     /**
 623      * Returns the current number of permits available in this semaphore.
 624      *
 625      * <p>This method is typically used for debugging and testing purposes.
 626      *
 627      * @return the number of permits available in this semaphore
 628      */
 629     public int availablePermits() {
 630         return sync.getPermits();
 631     }
 632 
 633     /**
 634      * Acquires and returns all permits that are immediately
 635      * available, or if negative permits are available, releases them.
 636      * Upon return, zero permits are available.
 637      *
 638      * @return the number of permits acquired or, if negative, the
 639      * number released
 640      */
 641     public int drainPermits() {
 642         return sync.drainPermits();
 643     }
 644 
 645     /**
 646      * Shrinks the number of available permits by the indicated
 647      * reduction. This method can be useful in subclasses that use
 648      * semaphores to track resources that become unavailable. This
 649      * method differs from {@code acquire} in that it does not block
 650      * waiting for permits to become available.
 651      *
 652      * @param reduction the number of permits to remove
 653      * @throws IllegalArgumentException if {@code reduction} is negative
 654      */
 655     protected void reducePermits(int reduction) {
 656         if (reduction < 0) throw new IllegalArgumentException();
 657         sync.reducePermits(reduction);
 658     }
 659 
 660     /**
 661      * Returns {@code true} if this semaphore has fairness set true.
 662      *
 663      * @return {@code true} if this semaphore has fairness set true
 664      */
 665     public boolean isFair() {
 666         return sync instanceof FairSync;
 667     }
 668 
 669     /**
 670      * Queries whether any threads are waiting to acquire. Note that
 671      * because cancellations may occur at any time, a {@code true}
 672      * return does not guarantee that any other thread will ever
 673      * acquire.  This method is designed primarily for use in
 674      * monitoring of the system state.
 675      *
 676      * @return {@code true} if there may be other threads waiting to
 677      *         acquire the lock
 678      */
 679     public final boolean hasQueuedThreads() {
 680         return sync.hasQueuedThreads();
 681     }
 682 
 683     /**
 684      * Returns an estimate of the number of threads waiting to acquire.
 685      * The value is only an estimate because the number of threads may
 686      * change dynamically while this method traverses internal data
 687      * structures.  This method is designed for use in monitoring
 688      * system state, not for synchronization control.
 689      *
 690      * @return the estimated number of threads waiting for this lock
 691      */
 692     public final int getQueueLength() {
 693         return sync.getQueueLength();
 694     }
 695 
 696     /**
 697      * Returns a collection containing threads that may be waiting to acquire.
 698      * Because the actual set of threads may change dynamically while
 699      * constructing this result, the returned collection is only a best-effort
 700      * estimate.  The elements of the returned collection are in no particular
 701      * order.  This method is designed to facilitate construction of
 702      * subclasses that provide more extensive monitoring facilities.
 703      *
 704      * @return the collection of threads
 705      */
 706     protected Collection<Thread> getQueuedThreads() {
 707         return sync.getQueuedThreads();
 708     }
 709 
 710     /**
 711      * Returns a string identifying this semaphore, as well as its state.
 712      * The state, in brackets, includes the String {@code "Permits ="}
 713      * followed by the number of permits.
 714      *
 715      * @return a string identifying this semaphore, as well as its state
 716      */
 717     public String toString() {
 718         return super.toString() + "[Permits = " + sync.getPermits() + "]";
 719     }
 720 }