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.locks;
  37 import java.util.*;
  38 import java.util.concurrent.*;
  39 import java.util.concurrent.atomic.*;
  40 
  41 /**
  42  * A reentrant mutual exclusion {@link Lock} with the same basic
  43  * behavior and semantics as the implicit monitor lock accessed using
  44  * {@code synchronized} methods and statements, but with extended
  45  * capabilities.
  46  *
  47  * <p>A {@code ReentrantLock} is <em>owned</em> by the thread last
  48  * successfully locking, but not yet unlocking it. A thread invoking
  49  * {@code lock} will return, successfully acquiring the lock, when
  50  * the lock is not owned by another thread. The method will return
  51  * immediately if the current thread already owns the lock. This can
  52  * be checked using methods {@link #isHeldByCurrentThread}, and {@link
  53  * #getHoldCount}.
  54  *
  55  * <p>The constructor for this class accepts an optional
  56  * <em>fairness</em> parameter.  When set {@code true}, under
  57  * contention, locks favor granting access to the longest-waiting
  58  * thread.  Otherwise this lock does not guarantee any particular
  59  * access order.  Programs using fair locks accessed by many threads
  60  * may display lower overall throughput (i.e., are slower; often much
  61  * slower) than those using the default setting, but have smaller
  62  * variances in times to obtain locks and guarantee lack of
  63  * starvation. Note however, that fairness of locks does not guarantee
  64  * fairness of thread scheduling. Thus, one of many threads using a
  65  * fair lock may obtain it multiple times in succession while other
  66  * active threads are not progressing and not currently holding the
  67  * lock.
  68  * Also note that the untimed {@link #tryLock() tryLock} method does not
  69  * honor the fairness setting. It will succeed if the lock
  70  * is available even if other threads are waiting.
  71  *
  72  * <p>It is recommended practice to <em>always</em> immediately
  73  * follow a call to {@code lock} with a {@code try} block, most
  74  * typically in a before/after construction such as:
  75  *
  76  * <pre>
  77  * class X {
  78  *   private final ReentrantLock lock = new ReentrantLock();
  79  *   // ...
  80  *
  81  *   public void m() {
  82  *     lock.lock();  // block until condition holds
  83  *     try {
  84  *       // ... method body
  85  *     } finally {
  86  *       lock.unlock()
  87  *     }
  88  *   }
  89  * }
  90  * </pre>
  91  *
  92  * <p>In addition to implementing the {@link Lock} interface, this
  93  * class defines methods {@code isLocked} and
  94  * {@code getLockQueueLength}, as well as some associated
  95  * {@code protected} access methods that may be useful for
  96  * instrumentation and monitoring.
  97  *
  98  * <p>Serialization of this class behaves in the same way as built-in
  99  * locks: a deserialized lock is in the unlocked state, regardless of
 100  * its state when serialized.
 101  *
 102  * <p>This lock supports a maximum of 2147483647 recursive locks by
 103  * the same thread. Attempts to exceed this limit result in
 104  * {@link Error} throws from locking methods.
 105  *
 106  * @since 1.5
 107  * @author Doug Lea
 108  */
 109 public class ReentrantLock implements Lock, java.io.Serializable {
 110     private static final long serialVersionUID = 7373984872572414699L;
 111     /** Synchronizer providing all implementation mechanics */
 112     private final Sync sync;
 113 
 114     /**
 115      * Base of synchronization control for this lock. Subclassed
 116      * into fair and nonfair versions below. Uses AQS state to
 117      * represent the number of holds on the lock.
 118      */
 119     abstract static class Sync extends AbstractQueuedSynchronizer {
 120         private static final long serialVersionUID = -5179523762034025860L;
 121 
 122         /**
 123          * Performs {@link Lock#lock}. The main reason for subclassing
 124          * is to allow fast path for nonfair version.
 125          */
 126         abstract void lock();
 127 
 128         /**
 129          * Performs non-fair tryLock.  tryAcquire is
 130          * implemented in subclasses, but both need nonfair
 131          * try for trylock method.
 132          */
 133         final boolean nonfairTryAcquire(int acquires) {
 134             final Thread current = Thread.currentThread();
 135             int c = getState();
 136             if (c == 0) {
 137                 if (compareAndSetState(0, acquires)) {
 138                     setExclusiveOwnerThread(current);
 139                     return true;
 140                 }
 141             }
 142             else if (current == getExclusiveOwnerThread()) {
 143                 int nextc = c + acquires;
 144                 if (nextc < 0) // overflow
 145                     throw new Error("Maximum lock count exceeded");
 146                 setState(nextc);
 147                 return true;
 148             }
 149             return false;
 150         }
 151 
 152         protected final boolean tryRelease(int releases) {
 153             int c = getState() - releases;
 154             if (Thread.currentThread() != getExclusiveOwnerThread())
 155                 throw new IllegalMonitorStateException();
 156             boolean free = false;
 157             if (c == 0) {
 158                 free = true;
 159                 setExclusiveOwnerThread(null);
 160             }
 161             setState(c);
 162             return free;
 163         }
 164 
 165         protected final boolean isHeldExclusively() {
 166             // While we must in general read state before owner,
 167             // we don't need to do so to check if current thread is owner
 168             return getExclusiveOwnerThread() == Thread.currentThread();
 169         }
 170 
 171         final ConditionObject newCondition() {
 172             return new ConditionObject();
 173         }
 174 
 175         // Methods relayed from outer class
 176 
 177         final Thread getOwner() {
 178             return getState() == 0 ? null : getExclusiveOwnerThread();
 179         }
 180 
 181         final int getHoldCount() {
 182             return isHeldExclusively() ? getState() : 0;
 183         }
 184 
 185         final boolean isLocked() {
 186             return getState() != 0;
 187         }
 188 
 189         /**
 190          * Reconstitutes this lock instance from a stream.
 191          * @param s the stream
 192          */
 193         private void readObject(java.io.ObjectInputStream s)
 194             throws java.io.IOException, ClassNotFoundException {
 195             s.defaultReadObject();
 196             setState(0); // reset to unlocked state
 197         }
 198     }
 199 
 200     /**
 201      * Sync object for non-fair locks
 202      */
 203     static final class NonfairSync extends Sync {
 204         private static final long serialVersionUID = 7316153563782823691L;
 205 
 206         /**
 207          * Performs lock.  Try immediate barge, backing up to normal
 208          * acquire on failure.
 209          */
 210         final void lock() {
 211             if (compareAndSetState(0, 1))
 212                 setExclusiveOwnerThread(Thread.currentThread());
 213             else
 214                 acquire(1);
 215         }
 216 
 217         protected final boolean tryAcquire(int acquires) {
 218             return nonfairTryAcquire(acquires);
 219         }
 220     }
 221 
 222     /**
 223      * Sync object for fair locks
 224      */
 225     static final class FairSync extends Sync {
 226         private static final long serialVersionUID = -3000897897090466540L;
 227 
 228         final void lock() {
 229             acquire(1);
 230         }
 231 
 232         /**
 233          * Fair version of tryAcquire.  Don't grant access unless
 234          * recursive call or no waiters or is first.
 235          */
 236         protected final boolean tryAcquire(int acquires) {
 237             final Thread current = Thread.currentThread();
 238             int c = getState();
 239             if (c == 0) {
 240                 if (!hasQueuedPredecessors() &&
 241                     compareAndSetState(0, acquires)) {
 242                     setExclusiveOwnerThread(current);
 243                     return true;
 244                 }
 245             }
 246             else if (current == getExclusiveOwnerThread()) {
 247                 int nextc = c + acquires;
 248                 if (nextc < 0)
 249                     throw new Error("Maximum lock count exceeded");
 250                 setState(nextc);
 251                 return true;
 252             }
 253             return false;
 254         }
 255     }
 256 
 257     /**
 258      * Creates an instance of {@code ReentrantLock}.
 259      * This is equivalent to using {@code ReentrantLock(false)}.
 260      */
 261     public ReentrantLock() {
 262         sync = new NonfairSync();
 263     }
 264 
 265     /**
 266      * Creates an instance of {@code ReentrantLock} with the
 267      * given fairness policy.
 268      *
 269      * @param fair {@code true} if this lock should use a fair ordering policy
 270      */
 271     public ReentrantLock(boolean fair) {
 272         sync = fair ? new FairSync() : new NonfairSync();
 273     }
 274 
 275     /**
 276      * Acquires the lock.
 277      *
 278      * <p>Acquires the lock if it is not held by another thread and returns
 279      * immediately, setting the lock hold count to one.
 280      *
 281      * <p>If the current thread already holds the lock then the hold
 282      * count is incremented by one and the method returns immediately.
 283      *
 284      * <p>If the lock is held by another thread then the
 285      * current thread becomes disabled for thread scheduling
 286      * purposes and lies dormant until the lock has been acquired,
 287      * at which time the lock hold count is set to one.
 288      */
 289     public void lock() {
 290         sync.lock();
 291     }
 292 
 293     /**
 294      * Acquires the lock unless the current thread is
 295      * {@linkplain Thread#interrupt interrupted}.
 296      *
 297      * <p>Acquires the lock if it is not held by another thread and returns
 298      * immediately, setting the lock hold count to one.
 299      *
 300      * <p>If the current thread already holds this lock then the hold count
 301      * is incremented by one and the method returns immediately.
 302      *
 303      * <p>If the lock is held by another thread then the
 304      * current thread becomes disabled for thread scheduling
 305      * purposes and lies dormant until one of two things happens:
 306      *
 307      * <ul>
 308      *
 309      * <li>The lock is acquired by the current thread; or
 310      *
 311      * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
 312      * current thread.
 313      *
 314      * </ul>
 315      *
 316      * <p>If the lock is acquired by the current thread then the lock hold
 317      * count is set to one.
 318      *
 319      * <p>If the current thread:
 320      *
 321      * <ul>
 322      *
 323      * <li>has its interrupted status set on entry to this method; or
 324      *
 325      * <li>is {@linkplain Thread#interrupt interrupted} while acquiring
 326      * the lock,
 327      *
 328      * </ul>
 329      *
 330      * then {@link InterruptedException} is thrown and the current thread's
 331      * interrupted status is cleared.
 332      *
 333      * <p>In this implementation, as this method is an explicit
 334      * interruption point, preference is given to responding to the
 335      * interrupt over normal or reentrant acquisition of the lock.
 336      *
 337      * @throws InterruptedException if the current thread is interrupted
 338      */
 339     public void lockInterruptibly() throws InterruptedException {
 340         sync.acquireInterruptibly(1);
 341     }
 342 
 343     /**
 344      * Acquires the lock only if it is not held by another thread at the time
 345      * of invocation.
 346      *
 347      * <p>Acquires the lock if it is not held by another thread and
 348      * returns immediately with the value {@code true}, setting the
 349      * lock hold count to one. Even when this lock has been set to use a
 350      * fair ordering policy, a call to {@code tryLock()} <em>will</em>
 351      * immediately acquire the lock if it is available, whether or not
 352      * other threads are currently waiting for the lock.
 353      * This &quot;barging&quot; behavior can be useful in certain
 354      * circumstances, even though it breaks fairness. If you want to honor
 355      * the fairness setting for this lock, then use
 356      * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
 357      * which is almost equivalent (it also detects interruption).
 358      *
 359      * <p> If the current thread already holds this lock then the hold
 360      * count is incremented by one and the method returns {@code true}.
 361      *
 362      * <p>If the lock is held by another thread then this method will return
 363      * immediately with the value {@code false}.
 364      *
 365      * @return {@code true} if the lock was free and was acquired by the
 366      *         current thread, or the lock was already held by the current
 367      *         thread; and {@code false} otherwise
 368      */
 369     public boolean tryLock() {
 370         return sync.nonfairTryAcquire(1);
 371     }
 372 
 373     /**
 374      * Acquires the lock if it is not held by another thread within the given
 375      * waiting time and the current thread has not been
 376      * {@linkplain Thread#interrupt interrupted}.
 377      *
 378      * <p>Acquires the lock if it is not held by another thread and returns
 379      * immediately with the value {@code true}, setting the lock hold count
 380      * to one. If this lock has been set to use a fair ordering policy then
 381      * an available lock <em>will not</em> be acquired if any other threads
 382      * are waiting for the lock. This is in contrast to the {@link #tryLock()}
 383      * method. If you want a timed {@code tryLock} that does permit barging on
 384      * a fair lock then combine the timed and un-timed forms together:
 385      *
 386      * <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
 387      * </pre>
 388      *
 389      * <p>If the current thread
 390      * already holds this lock then the hold count is incremented by one and
 391      * the method returns {@code true}.
 392      *
 393      * <p>If the lock is held by another thread then the
 394      * current thread becomes disabled for thread scheduling
 395      * purposes and lies dormant until one of three things happens:
 396      *
 397      * <ul>
 398      *
 399      * <li>The lock is acquired by the current thread; or
 400      *
 401      * <li>Some other thread {@linkplain Thread#interrupt interrupts}
 402      * the current thread; or
 403      *
 404      * <li>The specified waiting time elapses
 405      *
 406      * </ul>
 407      *
 408      * <p>If the lock is acquired then the value {@code true} is returned and
 409      * the lock hold count is set to one.
 410      *
 411      * <p>If the current thread:
 412      *
 413      * <ul>
 414      *
 415      * <li>has its interrupted status set on entry to this method; or
 416      *
 417      * <li>is {@linkplain Thread#interrupt interrupted} while
 418      * acquiring the lock,
 419      *
 420      * </ul>
 421      * then {@link InterruptedException} is thrown and the current thread's
 422      * interrupted status is cleared.
 423      *
 424      * <p>If the specified waiting time elapses then the value {@code false}
 425      * is returned.  If the time is less than or equal to zero, the method
 426      * will not wait at all.
 427      *
 428      * <p>In this implementation, as this method is an explicit
 429      * interruption point, preference is given to responding to the
 430      * interrupt over normal or reentrant acquisition of the lock, and
 431      * over reporting the elapse of the waiting time.
 432      *
 433      * @param timeout the time to wait for the lock
 434      * @param unit the time unit of the timeout argument
 435      * @return {@code true} if the lock was free and was acquired by the
 436      *         current thread, or the lock was already held by the current
 437      *         thread; and {@code false} if the waiting time elapsed before
 438      *         the lock could be acquired
 439      * @throws InterruptedException if the current thread is interrupted
 440      * @throws NullPointerException if the time unit is null
 441      *
 442      */
 443     public boolean tryLock(long timeout, TimeUnit unit)
 444             throws InterruptedException {
 445         return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 446     }
 447 
 448     /**
 449      * Attempts to release this lock.
 450      *
 451      * <p>If the current thread is the holder of this lock then the hold
 452      * count is decremented.  If the hold count is now zero then the lock
 453      * is released.  If the current thread is not the holder of this
 454      * lock then {@link IllegalMonitorStateException} is thrown.
 455      *
 456      * @throws IllegalMonitorStateException if the current thread does not
 457      *         hold this lock
 458      */
 459     public void unlock() {
 460         sync.release(1);
 461     }
 462 
 463     /**
 464      * Returns a {@link Condition} instance for use with this
 465      * {@link Lock} instance.
 466      *
 467      * <p>The returned {@link Condition} instance supports the same
 468      * usages as do the {@link Object} monitor methods ({@link
 469      * Object#wait() wait}, {@link Object#notify notify}, and {@link
 470      * Object#notifyAll notifyAll}) when used with the built-in
 471      * monitor lock.
 472      *
 473      * <ul>
 474      *
 475      * <li>If this lock is not held when any of the {@link Condition}
 476      * {@linkplain Condition#await() waiting} or {@linkplain
 477      * Condition#signal signalling} methods are called, then an {@link
 478      * IllegalMonitorStateException} is thrown.
 479      *
 480      * <li>When the condition {@linkplain Condition#await() waiting}
 481      * methods are called the lock is released and, before they
 482      * return, the lock is reacquired and the lock hold count restored
 483      * to what it was when the method was called.
 484      *
 485      * <li>If a thread is {@linkplain Thread#interrupt interrupted}
 486      * while waiting then the wait will terminate, an {@link
 487      * InterruptedException} will be thrown, and the thread's
 488      * interrupted status will be cleared.
 489      *
 490      * <li> Waiting threads are signalled in FIFO order.
 491      *
 492      * <li>The ordering of lock reacquisition for threads returning
 493      * from waiting methods is the same as for threads initially
 494      * acquiring the lock, which is in the default case not specified,
 495      * but for <em>fair</em> locks favors those threads that have been
 496      * waiting the longest.
 497      *
 498      * </ul>
 499      *
 500      * @return the Condition object
 501      */
 502     public Condition newCondition() {
 503         return sync.newCondition();
 504     }
 505 
 506     /**
 507      * Queries the number of holds on this lock by the current thread.
 508      *
 509      * <p>A thread has a hold on a lock for each lock action that is not
 510      * matched by an unlock action.
 511      *
 512      * <p>The hold count information is typically only used for testing and
 513      * debugging purposes. For example, if a certain section of code should
 514      * not be entered with the lock already held then we can assert that
 515      * fact:
 516      *
 517      * <pre>
 518      * class X {
 519      *   ReentrantLock lock = new ReentrantLock();
 520      *   // ...
 521      *   public void m() {
 522      *     assert lock.getHoldCount() == 0;
 523      *     lock.lock();
 524      *     try {
 525      *       // ... method body
 526      *     } finally {
 527      *       lock.unlock();
 528      *     }
 529      *   }
 530      * }
 531      * </pre>
 532      *
 533      * @return the number of holds on this lock by the current thread,
 534      *         or zero if this lock is not held by the current thread
 535      */
 536     public int getHoldCount() {
 537         return sync.getHoldCount();
 538     }
 539 
 540     /**
 541      * Queries if this lock is held by the current thread.
 542      *
 543      * <p>Analogous to the {@link Thread#holdsLock} method for built-in
 544      * monitor locks, this method is typically used for debugging and
 545      * testing. For example, a method that should only be called while
 546      * a lock is held can assert that this is the case:
 547      *
 548      * <pre>
 549      * class X {
 550      *   ReentrantLock lock = new ReentrantLock();
 551      *   // ...
 552      *
 553      *   public void m() {
 554      *       assert lock.isHeldByCurrentThread();
 555      *       // ... method body
 556      *   }
 557      * }
 558      * </pre>
 559      *
 560      * <p>It can also be used to ensure that a reentrant lock is used
 561      * in a non-reentrant manner, for example:
 562      *
 563      * <pre>
 564      * class X {
 565      *   ReentrantLock lock = new ReentrantLock();
 566      *   // ...
 567      *
 568      *   public void m() {
 569      *       assert !lock.isHeldByCurrentThread();
 570      *       lock.lock();
 571      *       try {
 572      *           // ... method body
 573      *       } finally {
 574      *           lock.unlock();
 575      *       }
 576      *   }
 577      * }
 578      * </pre>
 579      *
 580      * @return {@code true} if current thread holds this lock and
 581      *         {@code false} otherwise
 582      */
 583     public boolean isHeldByCurrentThread() {
 584         return sync.isHeldExclusively();
 585     }
 586 
 587     /**
 588      * Queries if this lock is held by any thread. This method is
 589      * designed for use in monitoring of the system state,
 590      * not for synchronization control.
 591      *
 592      * @return {@code true} if any thread holds this lock and
 593      *         {@code false} otherwise
 594      */
 595     public boolean isLocked() {
 596         return sync.isLocked();
 597     }
 598 
 599     /**
 600      * Returns {@code true} if this lock has fairness set true.
 601      *
 602      * @return {@code true} if this lock has fairness set true
 603      */
 604     public final boolean isFair() {
 605         return sync instanceof FairSync;
 606     }
 607 
 608     /**
 609      * Returns the thread that currently owns this lock, or
 610      * {@code null} if not owned. When this method is called by a
 611      * thread that is not the owner, the return value reflects a
 612      * best-effort approximation of current lock status. For example,
 613      * the owner may be momentarily {@code null} even if there are
 614      * threads trying to acquire the lock but have not yet done so.
 615      * This method is designed to facilitate construction of
 616      * subclasses that provide more extensive lock monitoring
 617      * facilities.
 618      *
 619      * @return the owner, or {@code null} if not owned
 620      */
 621     protected Thread getOwner() {
 622         return sync.getOwner();
 623     }
 624 
 625     /**
 626      * Queries whether any threads are waiting to acquire this lock. Note that
 627      * because cancellations may occur at any time, a {@code true}
 628      * return does not guarantee that any other thread will ever
 629      * acquire this lock.  This method is designed primarily for use in
 630      * monitoring of the system state.
 631      *
 632      * @return {@code true} if there may be other threads waiting to
 633      *         acquire the lock
 634      */
 635     public final boolean hasQueuedThreads() {
 636         return sync.hasQueuedThreads();
 637     }
 638 
 639 
 640     /**
 641      * Queries whether the given thread is waiting to acquire this
 642      * lock. Note that because cancellations may occur at any time, a
 643      * {@code true} return does not guarantee that this thread
 644      * will ever acquire this lock.  This method is designed primarily for use
 645      * in monitoring of the system state.
 646      *
 647      * @param thread the thread
 648      * @return {@code true} if the given thread is queued waiting for this lock
 649      * @throws NullPointerException if the thread is null
 650      */
 651     public final boolean hasQueuedThread(Thread thread) {
 652         return sync.isQueued(thread);
 653     }
 654 
 655 
 656     /**
 657      * Returns an estimate of the number of threads waiting to
 658      * acquire this lock.  The value is only an estimate because the number of
 659      * threads may change dynamically while this method traverses
 660      * internal data structures.  This method is designed for use in
 661      * monitoring of the system state, not for synchronization
 662      * control.
 663      *
 664      * @return the estimated number of threads waiting for this lock
 665      */
 666     public final int getQueueLength() {
 667         return sync.getQueueLength();
 668     }
 669 
 670     /**
 671      * Returns a collection containing threads that may be waiting to
 672      * acquire this lock.  Because the actual set of threads may change
 673      * dynamically while constructing this result, the returned
 674      * collection is only a best-effort estimate.  The elements of the
 675      * returned collection are in no particular order.  This method is
 676      * designed to facilitate construction of subclasses that provide
 677      * more extensive monitoring facilities.
 678      *
 679      * @return the collection of threads
 680      */
 681     protected Collection<Thread> getQueuedThreads() {
 682         return sync.getQueuedThreads();
 683     }
 684 
 685     /**
 686      * Queries whether any threads are waiting on the given condition
 687      * associated with this lock. Note that because timeouts and
 688      * interrupts may occur at any time, a {@code true} return does
 689      * not guarantee that a future {@code signal} will awaken any
 690      * threads.  This method is designed primarily for use in
 691      * monitoring of the system state.
 692      *
 693      * @param condition the condition
 694      * @return {@code true} if there are any waiting threads
 695      * @throws IllegalMonitorStateException if this lock is not held
 696      * @throws IllegalArgumentException if the given condition is
 697      *         not associated with this lock
 698      * @throws NullPointerException if the condition is null
 699      */
 700     public boolean hasWaiters(Condition condition) {
 701         if (condition == null)
 702             throw new NullPointerException();
 703         if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
 704             throw new IllegalArgumentException("not owner");
 705         return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
 706     }
 707 
 708     /**
 709      * Returns an estimate of the number of threads waiting on the
 710      * given condition associated with this lock. Note that because
 711      * timeouts and interrupts may occur at any time, the estimate
 712      * serves only as an upper bound on the actual number of waiters.
 713      * This method is designed for use in monitoring of the system
 714      * state, not for synchronization control.
 715      *
 716      * @param condition the condition
 717      * @return the estimated number of waiting threads
 718      * @throws IllegalMonitorStateException if this lock is not held
 719      * @throws IllegalArgumentException if the given condition is
 720      *         not associated with this lock
 721      * @throws NullPointerException if the condition is null
 722      */
 723     public int getWaitQueueLength(Condition condition) {
 724         if (condition == null)
 725             throw new NullPointerException();
 726         if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
 727             throw new IllegalArgumentException("not owner");
 728         return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
 729     }
 730 
 731     /**
 732      * Returns a collection containing those threads that may be
 733      * waiting on the given condition associated with this lock.
 734      * Because the actual set of threads may change dynamically while
 735      * constructing this result, the returned collection is only a
 736      * best-effort estimate. The elements of the returned collection
 737      * are in no particular order.  This method is designed to
 738      * facilitate construction of subclasses that provide more
 739      * extensive condition monitoring facilities.
 740      *
 741      * @param condition the condition
 742      * @return the collection of threads
 743      * @throws IllegalMonitorStateException if this lock is not held
 744      * @throws IllegalArgumentException if the given condition is
 745      *         not associated with this lock
 746      * @throws NullPointerException if the condition is null
 747      */
 748     protected Collection<Thread> getWaitingThreads(Condition condition) {
 749         if (condition == null)
 750             throw new NullPointerException();
 751         if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
 752             throw new IllegalArgumentException("not owner");
 753         return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
 754     }
 755 
 756     /**
 757      * Returns a string identifying this lock, as well as its lock state.
 758      * The state, in brackets, includes either the String {@code "Unlocked"}
 759      * or the String {@code "Locked by"} followed by the
 760      * {@linkplain Thread#getName name} of the owning thread.
 761      *
 762      * @return a string identifying this lock, as well as its lock state
 763      */
 764     public String toString() {
 765         Thread o = sync.getOwner();
 766         return super.toString() + ((o == null) ?
 767                                    "[Unlocked]" :
 768                                    "[Locked by thread " + o.getName() + "]");
 769     }
 770 }