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概要

本章对“公平锁”的获取锁机制进行介绍(本文的公平锁指的是互斥锁的公平锁),内容包括:
基本概念
ReentrantLock数据结构

参考代码
获取公平锁(基于JDK1.7.0_40)
  一. tryAcquire()
  二. addWaiter()
  三. acquireQueued()
  四. selfInterrupt()
“公平锁”的释放锁的机制在后面一章再做介绍,锁的使用示例请参考“Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock”。

转载请注明出处:http://www.cnblogs.com/skywang12345/p/3496147.html

 

基本概念

本章,我们会讲解“线程获取公平锁”的原理;在讲解之前,需要了解几个基本概念。后面的内容,都是基于这些概念的;这些概念可能比较枯燥,但从这些概念中,能窥见“java锁”的一些架构,这对我们了解锁是有帮助的。
1. AQS -- 指AbstractQueuedSynchronizer类。
    AQS是java中管理“锁”的抽象类,锁的许多公共方法都是在这个类中实现。AQS是独占锁(例如,ReentrantLock)和共享锁(例如,Semaphore)的公共父类。

2. AQS锁的类别 -- 分为“独占锁”和“共享锁”两种。
    (01) 独占锁 -- 锁在一个时间点只能被一个线程锁占有。根据锁的获取机制,它又划分为“公平锁”和“非公平锁”。公平锁,是按照通过CLH等待线程按照先来先得的规则,公平的获取锁;而非公平锁,则当线程要获取锁时,它会无视CLH等待队列而直接获取锁。独占锁的典型实例子是ReentrantLock,此外,ReentrantReadWriteLock.WriteLock也是独占锁。
    (02) 共享锁 -- 能被多个线程同时拥有,能被共享的锁。JUC包中的ReentrantReadWriteLock.ReadLock,CyclicBarrier, CountDownLatch和Semaphore都是共享锁。这些锁的用途和原理,在以后的章节再详细介绍。

3. CLH队列 -- Craig, Landin, and Hagersten lock queue
    CLH队列是AQS中“等待锁”的线程队列。在多线程中,为了保护竞争资源不被多个线程同时操作而起来错误,我们常常需要通过锁来保护这些资源。在独占锁中,竞争资源在一个时间点只能被一个线程锁访问;而其它线程则需要等待。CLH就是管理这些“等待锁”的线程的队列。
    CLH是一个非阻塞的 FIFO 队列。也就是说往里面插入或移除一个节点的时候,在并发条件下不会阻塞,而是通过自旋锁和 CAS 保证节点插入和移除的原子性。

4. CAS函数 -- Compare And Swap 
    CAS函数,是比较并交换函数,它是原子操作函数;即,通过CAS操作的数据都是以原子方式进行的。例如,compareAndSetHead(), compareAndSetTail(), compareAndSetNext()等函数。它们共同的特点是,这些函数所执行的动作是以原子的方式进行的。

本章是围绕“公平锁”如何获取锁而层次展开。“公平锁”涉及到的知识点比较多,但总的来说,不是特别难;如果读者能读懂AQS和ReentrantLock.java这两个类的大致意思,理解锁的原理和机制也就不成问题了。本章只是作者本人对锁的一点点理解,希望这部分知识能帮助您了解“公平锁”的获取过程,认识“锁”的框架。

 

ReentrantLock数据结构

ReentrantLock的UML类图

从图中可以看出:
(01) ReentrantLock实现了Lock接口。
(02) ReentrantLock与sync是组合关系。ReentrantLock中,包含了Sync对象;而且,Sync是AQS的子类;更重要的是,Sync有两个子类FairSync(公平锁)和NonFairSync(非公平锁)。ReentrantLock是一个独占锁,至于它到底是公平锁还是非公平锁,就取决于sync对象是"FairSync的实例"还是"NonFairSync的实例"。

 

参考代码

下面给出Java1.7.0_40版本中,ReentrantLock和AQS的源码,仅供参考!

ReentranLock.java

  1 /*
  2  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
  3  *
  4  *
  5  *
  6  *
  7  *
  8  *
  9  *
 10  *
 11  *
 12  *
 13  *
 14  *
 15  *
 16  *
 17  *
 18  *
 19  *
 20  *
 21  *
 22  *
 23  */
 24 
 25 /*
 26  *
 27  *
 28  *
 29  *
 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.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 }
View Code

 

AQS(AbstractQueuedSynchronizer.java)

   1 /*
   2  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
   3  *
   4  *
   5  *
   6  *
   7  *
   8  *
   9  *
  10  *
  11  *
  12  *
  13  *
  14  *
  15  *
  16  *
  17  *
  18  *
  19  *
  20  *
  21  *
  22  *
  23  */
  24 
  25 /*
  26  *
  27  *
  28  *
  29  *
  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.locks;
  37 import java.util.*;
  38 import java.util.concurrent.*;
  39 import java.util.concurrent.atomic.*;
  40 import sun.misc.Unsafe;
  41 
  42 /**
  43  * Provides a framework for implementing blocking locks and related
  44  * synchronizers (semaphores, events, etc) that rely on
  45  * first-in-first-out (FIFO) wait queues.  This class is designed to
  46  * be a useful basis for most kinds of synchronizers that rely on a
  47  * single atomic <tt>int</tt> value to represent state. Subclasses
  48  * must define the protected methods that change this state, and which
  49  * define what that state means in terms of this object being acquired
  50  * or released.  Given these, the other methods in this class carry
  51  * out all queuing and blocking mechanics. Subclasses can maintain
  52  * other state fields, but only the atomically updated <tt>int</tt>
  53  * value manipulated using methods {@link #getState}, {@link
  54  * #setState} and {@link #compareAndSetState} is tracked with respect
  55  * to synchronization.
  56  *
  57  * <p>Subclasses should be defined as non-public internal helper
  58  * classes that are used to implement the synchronization properties
  59  * of their enclosing class.  Class
  60  * <tt>AbstractQueuedSynchronizer</tt> does not implement any
  61  * synchronization interface.  Instead it defines methods such as
  62  * {@link #acquireInterruptibly} that can be invoked as
  63  * appropriate by concrete locks and related synchronizers to
  64  * implement their public methods.
  65  *
  66  * <p>This class supports either or both a default <em>exclusive</em>
  67  * mode and a <em>shared</em> mode. When acquired in exclusive mode,
  68  * attempted acquires by other threads cannot succeed. Shared mode
  69  * acquires by multiple threads may (but need not) succeed. This class
  70  * does not &quot;understand&quot; these differences except in the
  71  * mechanical sense that when a shared mode acquire succeeds, the next
  72  * waiting thread (if one exists) must also determine whether it can
  73  * acquire as well. Threads waiting in the different modes share the
  74  * same FIFO queue. Usually, implementation subclasses support only
  75  * one of these modes, but both can come into play for example in a
  76  * {@link ReadWriteLock}. Subclasses that support only exclusive or
  77  * only shared modes need not define the methods supporting the unused mode.
  78  *
  79  * <p>This class defines a nested {@link ConditionObject} class that
  80  * can be used as a {@link Condition} implementation by subclasses
  81  * supporting exclusive mode for which method {@link
  82  * #isHeldExclusively} reports whether synchronization is exclusively
  83  * held with respect to the current thread, method {@link #release}
  84  * invoked with the current {@link #getState} value fully releases
  85  * this object, and {@link #acquire}, given this saved state value,
  86  * eventually restores this object to its previous acquired state.  No
  87  * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a
  88  * condition, so if this constraint cannot be met, do not use it.  The
  89  * behavior of {@link ConditionObject} depends of course on the
  90  * semantics of its synchronizer implementation.
  91  *
  92  * <p>This class provides inspection, instrumentation, and monitoring
  93  * methods for the internal queue, as well as similar methods for
  94  * condition objects. These can be exported as desired into classes
  95  * using an <tt>AbstractQueuedSynchronizer</tt> for their
  96  * synchronization mechanics.
  97  *
  98  * <p>Serialization of this class stores only the underlying atomic
  99  * integer maintaining state, so deserialized objects have empty
 100  * thread queues. Typical subclasses requiring serializability will
 101  * define a <tt>readObject</tt> method that restores this to a known
 102  * initial state upon deserialization.
 103  *
 104  * <h3>Usage</h3>
 105  *
 106  * <p>To use this class as the basis of a synchronizer, redefine the
 107  * following methods, as applicable, by inspecting and/or modifying
 108  * the synchronization state using {@link #getState}, {@link
 109  * #setState} and/or {@link #compareAndSetState}:
 110  *
 111  * <ul>
 112  * <li> {@link #tryAcquire}
 113  * <li> {@link #tryRelease}
 114  * <li> {@link #tryAcquireShared}
 115  * <li> {@link #tryReleaseShared}
 116  * <li> {@link #isHeldExclusively}
 117  *</ul>
 118  *
 119  * Each of these methods by default throws {@link
 120  * UnsupportedOperationException}.  Implementations of these methods
 121  * must be internally thread-safe, and should in general be short and
 122  * not block. Defining these methods is the <em>only</em> supported
 123  * means of using this class. All other methods are declared
 124  * <tt>final</tt> because they cannot be independently varied.
 125  *
 126  * <p>You may also find the inherited methods from {@link
 127  * AbstractOwnableSynchronizer} useful to keep track of the thread
 128  * owning an exclusive synchronizer.  You are encouraged to use them
 129  * -- this enables monitoring and diagnostic tools to assist users in
 130  * determining which threads hold locks.
 131  *
 132  * <p>Even though this class is based on an internal FIFO queue, it
 133  * does not automatically enforce FIFO acquisition policies.  The core
 134  * of exclusive synchronization takes the form:
 135  *
 136  * <pre>
 137  * Acquire:
 138  *     while (!tryAcquire(arg)) {
 139  *        <em>enqueue thread if it is not already queued</em>;
 140  *        <em>possibly block current thread</em>;
 141  *     }
 142  *
 143  * Release:
 144  *     if (tryRelease(arg))
 145  *        <em>unblock the first queued thread</em>;
 146  * </pre>
 147  *
 148  * (Shared mode is similar but may involve cascading signals.)
 149  *
 150  * <p><a name="barging">Because checks in acquire are invoked before
 151  * enqueuing, a newly acquiring thread may <em>barge</em> ahead of
 152  * others that are blocked and queued.  However, you can, if desired,
 153  * define <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to
 154  * disable barging by internally invoking one or more of the inspection
 155  * methods, thereby providing a <em>fair</em> FIFO acquisition order.
 156  * In particular, most fair synchronizers can define <tt>tryAcquire</tt>
 157  * to return <tt>false</tt> if {@link #hasQueuedPredecessors} (a method
 158  * specifically designed to be used by fair synchronizers) returns
 159  * <tt>true</tt>.  Other variations are possible.
 160  *
 161  * <p>Throughput and scalability are generally highest for the
 162  * default barging (also known as <em>greedy</em>,
 163  * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
 164  * While this is not guaranteed to be fair or starvation-free, earlier
 165  * queued threads are allowed to recontend before later queued
 166  * threads, and each recontention has an unbiased chance to succeed
 167  * against incoming threads.  Also, while acquires do not
 168  * &quot;spin&quot; in the usual sense, they may perform multiple
 169  * invocations of <tt>tryAcquire</tt> interspersed with other
 170  * computations before blocking.  This gives most of the benefits of
 171  * spins when exclusive synchronization is only briefly held, without
 172  * most of the liabilities when it isn't. If so desired, you can
 173  * augment this by preceding calls to acquire methods with
 174  * "fast-path" checks, possibly prechecking {@link #hasContended}
 175  * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
 176  * is likely not to be contended.
 177  *
 178  * <p>This class provides an efficient and scalable basis for
 179  * synchronization in part by specializing its range of use to
 180  * synchronizers that can rely on <tt>int</tt> state, acquire, and
 181  * release parameters, and an internal FIFO wait queue. When this does
 182  * not suffice, you can build synchronizers from a lower level using
 183  * {@link java.util.concurrent.atomic atomic} classes, your own custom
 184  * {@link java.util.Queue} classes, and {@link LockSupport} blocking
 185  * support.
 186  *
 187  * <h3>Usage Examples</h3>
 188  *
 189  * <p>Here is a non-reentrant mutual exclusion lock class that uses
 190  * the value zero to represent the unlocked state, and one to
 191  * represent the locked state. While a non-reentrant lock
 192  * does not strictly require recording of the current owner
 193  * thread, this class does so anyway to make usage easier to monitor.
 194  * It also supports conditions and exposes
 195  * one of the instrumentation methods:
 196  *
 197  * <pre>
 198  * class Mutex implements Lock, java.io.Serializable {
 199  *
 200  *   // Our internal helper class
 201  *   private static class Sync extends AbstractQueuedSynchronizer {
 202  *     // Report whether in locked state
 203  *     protected boolean isHeldExclusively() {
 204  *       return getState() == 1;
 205  *     }
 206  *
 207  *     // Acquire the lock if state is zero
 208  *     public boolean tryAcquire(int acquires) {
 209  *       assert acquires == 1; // Otherwise unused
 210  *       if (compareAndSetState(0, 1)) {
 211  *         setExclusiveOwnerThread(Thread.currentThread());
 212  *         return true;
 213  *       }
 214  *       return false;
 215  *     }
 216  *
 217  *     // Release the lock by setting state to zero
 218  *     protected boolean tryRelease(int releases) {
 219  *       assert releases == 1; // Otherwise unused
 220  *       if (getState() == 0) throw new IllegalMonitorStateException();
 221  *       setExclusiveOwnerThread(null);
 222  *       setState(0);
 223  *       return true;
 224  *     }
 225  *
 226  *     // Provide a Condition
 227  *     Condition newCondition() { return new ConditionObject(); }
 228  *
 229  *     // Deserialize properly
 230  *     private void readObject(ObjectInputStream s)
 231  *         throws IOException, ClassNotFoundException {
 232  *       s.defaultReadObject();
 233  *       setState(0); // reset to unlocked state
 234  *     }
 235  *   }
 236  *
 237  *   // The sync object does all the hard work. We just forward to it.
 238  *   private final Sync sync = new Sync();
 239  *
 240  *   public void lock()                { sync.acquire(1); }
 241  *   public boolean tryLock()          { return sync.tryAcquire(1); }
 242  *   public void unlock()              { sync.release(1); }
 243  *   public Condition newCondition()   { return sync.newCondition(); }
 244  *   public boolean isLocked()         { return sync.isHeldExclusively(); }
 245  *   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
 246  *   public void lockInterruptibly() throws InterruptedException {
 247  *     sync.acquireInterruptibly(1);
 248  *   }
 249  *   public boolean tryLock(long timeout, TimeUnit unit)
 250  *       throws InterruptedException {
 251  *     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
 252  *   }
 253  * }
 254  * </pre>
 255  *
 256  * <p>Here is a latch class that is like a {@link CountDownLatch}
 257  * except that it only requires a single <tt>signal</tt> to
 258  * fire. Because a latch is non-exclusive, it uses the <tt>shared</tt>
 259  * acquire and release methods.
 260  *
 261  * <pre>
 262  * class BooleanLatch {
 263  *
 264  *   private static class Sync extends AbstractQueuedSynchronizer {
 265  *     boolean isSignalled() { return getState() != 0; }
 266  *
 267  *     protected int tryAcquireShared(int ignore) {
 268  *       return isSignalled() ? 1 : -1;
 269  *     }
 270  *
 271  *     protected boolean tryReleaseShared(int ignore) {
 272  *       setState(1);
 273  *       return true;
 274  *     }
 275  *   }
 276  *
 277  *   private final Sync sync = new Sync();
 278  *   public boolean isSignalled() { return sync.isSignalled(); }
 279  *   public void signal()         { sync.releaseShared(1); }
 280  *   public void await() throws InterruptedException {
 281  *     sync.acquireSharedInterruptibly(1);
 282  *   }
 283  * }
 284  * </pre>
 285  *
 286  * @since 1.5
 287  * @author Doug Lea
 288  */
 289 public abstract class AbstractQueuedSynchronizer
 290     extends AbstractOwnableSynchronizer
 291     implements java.io.Serializable {
 292 
 293     private static final long serialVersionUID = 7373984972572414691L;
 294 
 295     /**
 296      * Creates a new <tt>AbstractQueuedSynchronizer</tt> instance
 297      * with initial synchronization state of zero.
 298      */
 299     protected AbstractQueuedSynchronizer() { }
 300 
 301     /**
 302      * Wait queue node class.
 303      *
 304      * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
 305      * Hagersten) lock queue. CLH locks are normally used for
 306      * spinlocks.  We instead use them for blocking synchronizers, but
 307      * use the same basic tactic of holding some of the control
 308      * information about a thread in the predecessor of its node.  A
 309      * "status" field in each node keeps track of whether a thread
 310      * should block.  A node is signalled when its predecessor
 311      * releases.  Each node of the queue otherwise serves as a
 312      * specific-notification-style monitor holding a single waiting
 313      * thread. The status field does NOT control whether threads are
 314      * granted locks etc though.  A thread may try to acquire if it is
 315      * first in the queue. But being first does not guarantee success;
 316      * it only gives the right to contend.  So the currently released
 317      * contender thread may need to rewait.
 318      *
 319      * <p>To enqueue into a CLH lock, you atomically splice it in as new
 320      * tail. To dequeue, you just set the head field.
 321      * <pre>
 322      *      +------+  prev +-----+       +-----+
 323      * head |      | <---- |     | <---- |     |  tail
 324      *      +------+       +-----+       +-----+
 325      * </pre>
 326      *
 327      * <p>Insertion into a CLH queue requires only a single atomic
 328      * operation on "tail", so there is a simple atomic point of
 329      * demarcation from unqueued to queued. Similarly, dequeing
 330      * involves only updating the "head". However, it takes a bit
 331      * more work for nodes to determine who their successors are,
 332      * in part to deal with possible cancellation due to timeouts
 333      * and interrupts.
 334      *
 335      * <p>The "prev" links (not used in original CLH locks), are mainly
 336      * needed to handle cancellation. If a node is cancelled, its
 337      * successor is (normally) relinked to a non-cancelled
 338      * predecessor. For explanation of similar mechanics in the case
 339      * of spin locks, see the papers by Scott and Scherer at
 340      * http://www.cs.rochester.edu/u/scott/synchronization/
 341      *
 342      * <p>We also use "next" links to implement blocking mechanics.
 343      * The thread id for each node is kept in its own node, so a
 344      * predecessor signals the next node to wake up by traversing
 345      * next link to determine which thread it is.  Determination of
 346      * successor must avoid races with newly queued nodes to set
 347      * the "next" fields of their predecessors.  This is solved
 348      * when necessary by checking backwards from the atomically
 349      * updated "tail" when a node's successor appears to be null.
 350      * (Or, said differently, the next-links are an optimization
 351      * so that we don't usually need a backward scan.)
 352      *
 353      * <p>Cancellation introduces some conservatism to the basic
 354      * algorithms.  Since we must poll for cancellation of other
 355      * nodes, we can miss noticing whether a cancelled node is
 356      * ahead or behind us. This is dealt with by always unparking
 357      * successors upon cancellation, allowing them to stabilize on
 358      * a new predecessor, unless we can identify an uncancelled
 359      * predecessor who will carry this responsibility.
 360      *
 361      * <p>CLH queues need a dummy header node to get started. But
 362      * we don't create them on construction, because it would be wasted
 363      * effort if there is never contention. Instead, the node
 364      * is constructed and head and tail pointers are set upon first
 365      * contention.
 366      *
 367      * <p>Threads waiting on Conditions use the same nodes, but
 368      * use an additional link. Conditions only need to link nodes
 369      * in simple (non-concurrent) linked queues because they are
 370      * only accessed when exclusively held.  Upon await, a node is
 371      * inserted into a condition queue.  Upon signal, the node is
 372      * transferred to the main queue.  A special value of status
 373      * field is used to mark which queue a node is on.
 374      *
 375      * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
 376      * Scherer and Michael Scott, along with members of JSR-166
 377      * expert group, for helpful ideas, discussions, and critiques
 378      * on the design of this class.
 379      */
 380     static final class Node {
 381         /** Marker to indicate a node is waiting in shared mode */
 382         static final Node SHARED = new Node();
 383         /** Marker to indicate a node is waiting in exclusive mode */
 384         static final Node EXCLUSIVE = null;
 385 
 386         /** waitStatus value to indicate thread has cancelled */
 387         static final int CANCELLED =  1;
 388         /** waitStatus value to indicate successor's thread needs unparking */
 389         static final int SIGNAL    = -1;
 390         /** waitStatus value to indicate thread is waiting on condition */
 391         static final int CONDITION = -2;
 392         /**
 393          * waitStatus value to indicate the next acquireShared should
 394          * unconditionally propagate
 395          */
 396         static final int PROPAGATE = -3;
 397 
 398         /**
 399          * Status field, taking on only the values:
 400          *   SIGNAL:     The successor of this node is (or will soon be)
 401          *               blocked (via park), so the current node must
 402          *               unpark its successor when it releases or
 403          *               cancels. To avoid races, acquire methods must
 404          *               first indicate they need a signal,
 405          *               then retry the atomic acquire, and then,
 406          *               on failure, block.
 407          *   CANCELLED:  This node is cancelled due to timeout or interrupt.
 408          *               Nodes never leave this state. In particular,
 409          *               a thread with cancelled node never again blocks.
 410          *   CONDITION:  This node is currently on a condition queue.
 411          *               It will not be used as a sync queue node
 412          *               until transferred, at which time the status
 413          *               will be set to 0. (Use of this value here has
 414          *               nothing to do with the other uses of the
 415          *               field, but simplifies mechanics.)
 416          *   PROPAGATE:  A releaseShared should be propagated to other
 417          *               nodes. This is set (for head node only) in
 418          *               doReleaseShared to ensure propagation
 419          *               continues, even if other operations have
 420          *               since intervened.
 421          *   0:          None of the above
 422          *
 423          * The values are arranged numerically to simplify use.
 424          * Non-negative values mean that a node doesn't need to
 425          * signal. So, most code doesn't need to check for particular
 426          * values, just for sign.
 427          *
 428          * The field is initialized to 0 for normal sync nodes, and
 429          * CONDITION for condition nodes.  It is modified using CAS
 430          * (or when possible, unconditional volatile writes).
 431          */
 432         volatile int waitStatus;
 433 
 434         /**
 435          * Link to predecessor node that current node/thread relies on
 436          * for checking waitStatus. Assigned during enqueing, and nulled
 437          * out (for sake of GC) only upon dequeuing.  Also, upon
 438          * cancellation of a predecessor, we short-circuit while
 439          * finding a non-cancelled one, which will always exist
 440          * because the head node is never cancelled: A node becomes
 441          * head only as a result of successful acquire. A
 442          * cancelled thread never succeeds in acquiring, and a thread only
 443          * cancels itself, not any other node.
 444          */
 445         volatile Node prev;
 446 
 447         /**
 448          * Link to the successor node that the current node/thread
 449          * unparks upon release. Assigned during enqueuing, adjusted
 450          * when bypassing cancelled predecessors, and nulled out (for
 451          * sake of GC) when dequeued.  The enq operation does not
 452          * assign next field of a predecessor until after attachment,
 453          * so seeing a null next field does not necessarily mean that
 454          * node is at end of queue. However, if a next field appears
 455          * to be null, we can scan prev's from the tail to
 456          * double-check.  The next field of cancelled nodes is set to
 457          * point to the node itself instead of null, to make life
 458          * easier for isOnSyncQueue.
 459          */
 460         volatile Node next;
 461 
 462         /**
 463          * The thread that enqueued this node.  Initialized on
 464          * construction and nulled out after use.
 465          */
 466         volatile Thread thread;
 467 
 468         /**
 469          * Link to next node waiting on condition, or the special
 470          * value SHARED.  Because condition queues are accessed only
 471          * when holding in exclusive mode, we just need a simple
 472          * linked queue to hold nodes while they are waiting on
 473          * conditions. They are then transferred to the queue to
 474          * re-acquire. And because conditions can only be exclusive,
 475          * we save a field by using special value to indicate shared
 476          * mode.
 477          */
 478         Node nextWaiter;
 479 
 480         /**
 481          * Returns true if node is waiting in shared mode
 482          */
 483         final boolean isShared() {
 484             return nextWaiter == SHARED;
 485         }
 486 
 487         /**
 488          * Returns previous node, or throws NullPointerException if null.
 489          * Use when predecessor cannot be null.  The null check could
 490          * be elided, but is present to help the VM.
 491          *
 492          * @return the predecessor of this node
 493          */
 494         final Node predecessor() throws NullPointerException {
 495             Node p = prev;
 496             if (p == null)
 497                 throw new NullPointerException();
 498             else
 499                 return p;
 500         }
 501 
 502         Node() {    // Used to establish initial head or SHARED marker
 503         }
 504 
 505         Node(Thread thread, Node mode) {     // Used by addWaiter
 506             this.nextWaiter = mode;
 507             this.thread = thread;
 508         }
 509 
 510         Node(Thread thread, int waitStatus) { // Used by Condition
 511             this.waitStatus = waitStatus;
 512             this.thread = thread;
 513         }
 514     }
 515 
 516     /**
 517      * Head of the wait queue, lazily initialized.  Except for
 518      * initialization, it is modified only via method setHead.  Note:
 519      * If head exists, its waitStatus is guaranteed not to be
 520      * CANCELLED.
 521      */
 522     private transient volatile Node head;
 523 
 524     /**
 525      * Tail of the wait queue, lazily initialized.  Modified only via
 526      * method enq to add new wait node.
 527      */
 528     private transient volatile Node tail;
 529 
 530     /**
 531      * The synchronization state.
 532      */
 533     private volatile int state;
 534 
 535     /**
 536      * Returns the current value of synchronization state.
 537      * This operation has memory semantics of a <tt>volatile</tt> read.
 538      * @return current state value
 539      */
 540     protected final int getState() {
 541         return state;
 542     }
 543 
 544     /**
 545      * Sets the value of synchronization state.
 546      * This operation has memory semantics of a <tt>volatile</tt> write.
 547      * @param newState the new state value
 548      */
 549     protected final void setState(int newState) {
 550         state = newState;
 551     }
 552 
 553     /**
 554      * Atomically sets synchronization state to the given updated
 555      * value if the current state value equals the expected value.
 556      * This operation has memory semantics of a <tt>volatile</tt> read
 557      * and write.
 558      *
 559      * @param expect the expected value
 560      * @param update the new value
 561      * @return true if successful. False return indicates that the actual
 562      *         value was not equal to the expected value.
 563      */
 564     protected final boolean compareAndSetState(int expect, int update) {
 565         // See below for intrinsics setup to support this
 566         return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
 567     }
 568 
 569     // Queuing utilities
 570 
 571     /**
 572      * The number of nanoseconds for which it is faster to spin
 573      * rather than to use timed park. A rough estimate suffices
 574      * to improve responsiveness with very short timeouts.
 575      */
 576     static final long spinForTimeoutThreshold = 1000L;
 577 
 578     /**
 579      * Inserts node into queue, initializing if necessary. See picture above.
 580      * @param node the node to insert
 581      * @return node's predecessor
 582      */
 583     private Node enq(final Node node) {
 584         for (;;) {
 585             Node t = tail;
 586             if (t == null) { // Must initialize
 587                 if (compareAndSetHead(new Node()))
 588                     tail = head;
 589             } else {
 590                 node.prev = t;
 591                 if (compareAndSetTail(t, node)) {
 592                     t.next = node;
 593                     return t;
 594                 }
 595             }
 596         }
 597     }
 598 
 599     /**
 600      * Creates and enqueues node for current thread and given mode.
 601      *
 602      * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
 603      * @return the new node
 604      */
 605     private Node addWaiter(Node mode) {
 606         Node node = new Node(Thread.currentThread(), mode);
 607         // Try the fast path of enq; backup to full enq on failure
 608         Node pred = tail;
 609         if (pred != null) {
 610             node.prev = pred;
 611             if (compareAndSetTail(pred, node)) {
 612                 pred.next = node;
 613                 return node;
 614             }
 615         }
 616         enq(node);
 617         return node;
 618     }
 619 
 620     /**
 621      * Sets head of queue to be node, thus dequeuing. Called only by
 622      * acquire methods.  Also nulls out unused fields for sake of GC
 623      * and to suppress unnecessary signals and traversals.
 624      *
 625      * @param node the node
 626      */
 627     private void setHead(Node node) {
 628         head = node;
 629         node.thread = null;
 630         node.prev = null;
 631     }
 632 
 633     /**
 634      * Wakes up node's successor, if one exists.
 635      *
 636      * @param node the node
 637      */
 638     private void unparkSuccessor(Node node) {
 639         /*
 640          * If status is negative (i.e., possibly needing signal) try
 641          * to clear in anticipation of signalling.  It is OK if this
 642          * fails or if status is changed by waiting thread.
 643          */
 644         int ws = node.waitStatus;
 645         if (ws < 0)
 646             compareAndSetWaitStatus(node, ws, 0);
 647 
 648         /*
 649          * Thread to unpark is held in successor, which is normally
 650          * just the next node.  But if cancelled or apparently null,
 651          * traverse backwards from tail to find the actual
 652          * non-cancelled successor.
 653          */
 654         Node s = node.next;
 655         if (s == null || s.waitStatus > 0) {
 656             s = null;
 657             for (Node t = tail; t != null && t != node; t = t.prev)
 658                 if (t.waitStatus <= 0)
 659                     s = t;
 660         }
 661         if (s != null)
 662             LockSupport.unpark(s.thread);
 663     }
 664 
 665     /**
 666      * Release action for shared mode -- signal successor and ensure
 667      * propagation. (Note: For exclusive mode, release just amounts
 668      * to calling unparkSuccessor of head if it needs signal.)
 669      */
 670     private void doReleaseShared() {
 671         /*
 672          * Ensure that a release propagates, even if there are other
 673          * in-progress acquires/releases.  This proceeds in the usual
 674          * way of trying to unparkSuccessor of head if it needs
 675          * signal. But if it does not, status is set to PROPAGATE to
 676          * ensure that upon release, propagation continues.
 677          * Additionally, we must loop in case a new node is added
 678          * while we are doing this. Also, unlike other uses of
 679          * unparkSuccessor, we need to know if CAS to reset status
 680          * fails, if so rechecking.
 681          */
 682         for (;;) {
 683             Node h = head;
 684             if (h != null && h != tail) {
 685                 int ws = h.waitStatus;
 686                 if (ws == Node.SIGNAL) {
 687                     if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
 688                         continue;            // loop to recheck cases
 689                     unparkSuccessor(h);
 690                 }
 691                 else if (ws == 0 &&
 692                          !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
 693                     continue;                // loop on failed CAS
 694             }
 695             if (h == head)                   // loop if head changed
 696                 break;
 697         }
 698     }
 699 
 700     /**
 701      * Sets head of queue, and checks if successor may be waiting
 702      * in shared mode, if so propagating if either propagate > 0 or
 703      * PROPAGATE status was set.
 704      *
 705      * @param node the node
 706      * @param propagate the return value from a tryAcquireShared
 707      */
 708     private void setHeadAndPropagate(Node node, int propagate) {
 709         Node h = head; // Record old head for check below
 710         setHead(node);
 711         /*
 712          * Try to signal next queued node if:
 713          *   Propagation was indicated by caller,
 714          *     or was recorded (as h.waitStatus) by a previous operation
 715          *     (note: this uses sign-check of waitStatus because
 716          *      PROPAGATE status may transition to SIGNAL.)
 717          * and
 718          *   The next node is waiting in shared mode,
 719          *     or we don't know, because it appears null
 720          *
 721          * The conservatism in both of these checks may cause
 722          * unnecessary wake-ups, but only when there are multiple
 723          * racing acquires/releases, so most need signals now or soon
 724          * anyway.
 725          */
 726         if (propagate > 0 || h == null || h.waitStatus < 0) {
 727             Node s = node.next;
 728             if (s == null || s.isShared())
 729                 doReleaseShared();
 730         }
 731     }
 732 
 733     // Utilities for various versions of acquire
 734 
 735     /**
 736      * Cancels an ongoing attempt to acquire.
 737      *
 738      * @param node the node
 739      */
 740     private void cancelAcquire(Node node) {
 741         // Ignore if node doesn't exist
 742         if (node == null)
 743             return;
 744 
 745         node.thread = null;
 746 
 747         // Skip cancelled predecessors
 748         Node pred = node.prev;
 749         while (pred.waitStatus > 0)
 750             node.prev = pred = pred.prev;
 751 
 752         // predNext is the apparent node to unsplice. CASes below will
 753         // fail if not, in which case, we lost race vs another cancel
 754         // or signal, so no further action is necessary.
 755         Node predNext = pred.next;
 756 
 757         // Can use unconditional write instead of CAS here.
 758         // After this atomic step, other Nodes can skip past us.
 759         // Before, we are free of interference from other threads.
 760         node.waitStatus = Node.CANCELLED;
 761 
 762         // If we are the tail, remove ourselves.
 763         if (node == tail && compareAndSetTail(node, pred)) {
 764             compareAndSetNext(pred, predNext, null);
 765         } else {
 766             // If successor needs signal, try to set pred's next-link
 767             // so it will get one. Otherwise wake it up to propagate.
 768             int ws;
 769             if (pred != head &&
 770                 ((ws = pred.waitStatus) == Node.SIGNAL ||
 771                  (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
 772                 pred.thread != null) {
 773                 Node next = node.next;
 774                 if (next != null && next.waitStatus <= 0)
 775                     compareAndSetNext(pred, predNext, next);
 776             } else {
 777                 unparkSuccessor(node);
 778             }
 779 
 780             node.next = node; // help GC
 781         }
 782     }
 783 
 784     /**
 785      * Checks and updates status for a node that failed to acquire.
 786      * Returns true if thread should block. This is the main signal
 787      * control in all acquire loops.  Requires that pred == node.prev
 788      *
 789      * @param pred node's predecessor holding status
 790      * @param node the node
 791      * @return {@code true} if thread should block
 792      */
 793     private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
 794         int ws = pred.waitStatus;
 795         if (ws == Node.SIGNAL)
 796             /*
 797              * This node has already set status asking a release
 798              * to signal it, so it can safely park.
 799              */
 800             return true;
 801         if (ws > 0) {
 802             /*
 803              * Predecessor was cancelled. Skip over predecessors and
 804              * indicate retry.
 805              */
 806             do {
 807                 node.prev = pred = pred.prev;
 808             } while (pred.waitStatus > 0);
 809             pred.next = node;
 810         } else {
 811             /*
 812              * waitStatus must be 0 or PROPAGATE.  Indicate that we
 813              * need a signal, but don't park yet.  Caller will need to
 814              * retry to make sure it cannot acquire before parking.
 815              */
 816             compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
 817         }
 818         return false;
 819     }
 820 
 821     /**
 822      * Convenience method to interrupt current thread.
 823      */
 824     private static void selfInterrupt() {
 825         Thread.currentThread().interrupt();
 826     }
 827 
 828     /**
 829      * Convenience method to park and then check if interrupted
 830      *
 831      * @return {@code true} if interrupted
 832      */
 833     private final boolean parkAndCheckInterrupt() {
 834         LockSupport.park(this);
 835         return Thread.interrupted();
 836     }
 837 
 838     /*
 839      * Various flavors of acquire, varying in exclusive/shared and
 840      * control modes.  Each is mostly the same, but annoyingly
 841      * different.  Only a little bit of factoring is possible due to
 842      * interactions of exception mechanics (including ensuring that we
 843      * cancel if tryAcquire throws exception) and other control, at
 844      * least not without hurting performance too much.
 845      */
 846 
 847     /**
 848      * Acquires in exclusive uninterruptible mode for thread already in
 849      * queue. Used by condition wait methods as well as acquire.
 850      *
 851      * @param node the node
 852      * @param arg the acquire argument
 853      * @return {@code true} if interrupted while waiting
 854      */
 855     final boolean acquireQueued(final Node node, int arg) {
 856         boolean failed = true;
 857         try {
 858             boolean interrupted = false;
 859             for (;;) {
 860                 final Node p = node.predecessor();
 861                 if (p == head && tryAcquire(arg)) {
 862                     setHead(node);
 863                     p.next = null; // help GC
 864                     failed = false;
 865                     return interrupted;
 866                 }
 867                 if (shouldParkAfterFailedAcquire(p, node) &&
 868                     parkAndCheckInterrupt())
 869                     interrupted = true;
 870             }
 871         } finally {
 872             if (failed)
 873                 cancelAcquire(node);
 874         }
 875     }
 876 
 877     /**
 878      * Acquires in exclusive interruptible mode.
 879      * @param arg the acquire argument
 880      */
 881     private void doAcquireInterruptibly(int arg)
 882         throws InterruptedException {
 883         final Node node = addWaiter(Node.EXCLUSIVE);
 884         boolean failed = true;
 885         try {
 886             for (;;) {
 887                 final Node p = node.predecessor();
 888                 if (p == head && tryAcquire(arg)) {
 889                     setHead(node);
 890                     p.next = null; // help GC
 891                     failed = false;
 892                     return;
 893                 }
 894                 if (shouldParkAfterFailedAcquire(p, node) &&
 895                     parkAndCheckInterrupt())
 896                     throw new InterruptedException();
 897             }
 898         } finally {
 899             if (failed)
 900                 cancelAcquire(node);
 901         }
 902     }
 903 
 904     /**
 905      * Acquires in exclusive timed mode.
 906      *
 907      * @param arg the acquire argument
 908      * @param nanosTimeout max wait time
 909      * @return {@code true} if acquired
 910      */
 911     private boolean doAcquireNanos(int arg, long nanosTimeout)
 912         throws InterruptedException {
 913         long lastTime = System.nanoTime();
 914         final Node node = addWaiter(Node.EXCLUSIVE);
 915         boolean failed = true;
 916         try {
 917             for (;;) {
 918                 final Node p = node.predecessor();
 919                 if (p == head && tryAcquire(arg)) {
 920                     setHead(node);
 921                     p.next = null; // help GC
 922                     failed = false;
 923                     return true;
 924                 }
 925                 if (nanosTimeout <= 0)
 926                     return false;
 927                 if (shouldParkAfterFailedAcquire(p, node) &&
 928                     nanosTimeout > spinForTimeoutThreshold)
 929                     LockSupport.parkNanos(this, nanosTimeout);
 930                 long now = System.nanoTime();
 931                 nanosTimeout -= now - lastTime;
 932                 lastTime = now;
 933                 if (Thread.interrupted())
 934                     throw new InterruptedException();
 935             }
 936         } finally {
 937             if (failed)
 938                 cancelAcquire(node);
 939         }
 940     }
 941 
 942     /**
 943      * Acquires in shared uninterruptible mode.
 944      * @param arg the acquire argument
 945      */
 946     private void doAcquireShared(int arg) {
 947         final Node node = addWaiter(Node.SHARED);
 948         boolean failed = true;
 949         try {
 950             boolean interrupted = false;
 951             for (;;) {
 952                 final Node p = node.predecessor();
 953                 if (p == head) {
 954                     int r = tryAcquireShared(arg);
 955                     if (r >= 0) {
 956                         setHeadAndPropagate(node, r);
 957                         p.next = null; // help GC
 958                         if (interrupted)
 959                             selfInterrupt();
 960                         failed = false;
 961                         return;
 962                     }
 963                 }
 964                 if (shouldParkAfterFailedAcquire(p, node) &&
 965                     parkAndCheckInterrupt())
 966                     interrupted = true;
 967             }
 968         } finally {
 969             if (failed)
 970                 cancelAcquire(node);
 971         }
 972     }
 973 
 974     /**
 975      * Acquires in shared interruptible mode.
 976      * @param arg the acquire argument
 977      */
 978     private void doAcquireSharedInterruptibly(int arg)
 979         throws InterruptedException {
 980         final Node node = addWaiter(Node.SHARED);
 981         boolean failed = true;
 982         try {
 983             for (;;) {
 984                 final Node p = node.predecessor();
 985                 if (p == head) {
 986                     int r = tryAcquireShared(arg);
 987                     if (r >= 0) {
 988                         setHeadAndPropagate(node, r);
 989                         p.next = null; // help GC
 990                         failed = false;
 991                         return;
 992                     }
 993                 }
 994                 if (shouldParkAfterFailedAcquire(p, node) &&
 995                     parkAndCheckInterrupt())
 996                     throw new InterruptedException();
 997             }
 998         } finally {
 999             if (failed)
1000                 cancelAcquire(node);
1001         }
1002     }
1003 
1004     /**
1005      * Acquires in shared timed mode.
1006      *
1007      * @param arg the acquire argument
1008      * @param nanosTimeout max wait time
1009      * @return {@code true} if acquired
1010      */
1011     private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
1012         throws InterruptedException {
1013 
1014         long lastTime = System.nanoTime();
1015         final Node node = addWaiter(Node.SHARED);
1016         boolean failed = true;
1017         try {
1018             for (;;) {
1019                 final Node p = node.predecessor();
1020                 if (p == head) {
1021                     int r = tryAcquireShared(arg);
1022                     if (r >= 0) {
1023                         setHeadAndPropagate(node, r);
1024                         p.next = null; // help GC
1025                         failed = false;
1026                         return true;
1027                     }
1028                 }
1029                 if (nanosTimeout <= 0)
1030                     return false;
1031                 if (shouldParkAfterFailedAcquire(p, node) &&
1032                     nanosTimeout > spinForTimeoutThreshold)
1033                     LockSupport.parkNanos(this, nanosTimeout);
1034                 long now = System.nanoTime();
1035                 nanosTimeout -= now - lastTime;
1036                 lastTime = now;
1037                 if (Thread.interrupted())
1038                     throw new InterruptedException();
1039             }
1040         } finally {
1041             if (failed)
1042                 cancelAcquire(node);
1043         }
1044     }
1045 
1046     // Main exported methods
1047 
1048     /**
1049      * Attempts to acquire in exclusive mode. This method should query
1050      * if the state of the object permits it to be acquired in the
1051      * exclusive mode, and if so to acquire it.
1052      *
1053      * <p>This method is always invoked by the thread performing
1054      * acquire.  If this method reports failure, the acquire method
1055      * may queue the thread, if it is not already queued, until it is
1056      * signalled by a release from some other thread. This can be used
1057      * to implement method {@link Lock#tryLock()}.
1058      *
1059      * <p>The default
1060      * implementation throws {@link UnsupportedOperationException}.
1061      *
1062      * @param arg the acquire argument. This value is always the one
1063      *        passed to an acquire method, or is the value saved on entry
1064      *        to a condition wait.  The value is otherwise uninterpreted
1065      *        and can represent anything you like.
1066      * @return {@code true} if successful. Upon success, this object has
1067      *         been acquired.
1068      * @throws IllegalMonitorStateException if acquiring would place this
1069      *         synchronizer in an illegal state. This exception must be
1070      *         thrown in a consistent fashion for synchronization to work
1071      *         correctly.
1072      * @throws UnsupportedOperationException if exclusive mode is not supported
1073      */
1074     protected boolean tryAcquire(int arg) {
1075         throw new UnsupportedOperationException();
1076     }
1077 
1078     /**
1079      * Attempts to set the state to reflect a release in exclusive
1080      * mode.
1081      *
1082      * <p>This method is always invoked by the thread performing release.
1083      *
1084      * <p>The default implementation throws
1085      * {@link UnsupportedOperationException}.
1086      *
1087      * @param arg the release argument. This value is always the one
1088      *        passed to a release method, or the current state value upon
1089      *        entry to a condition wait.  The value is otherwise
1090      *        uninterpreted and can represent anything you like.
1091      * @return {@code true} if this object is now in a fully released
1092      *         state, so that any waiting threads may attempt to acquire;
1093      *         and {@code false} otherwise.
1094      * @throws IllegalMonitorStateException if releasing would place this
1095      *         synchronizer in an illegal state. This exception must be
1096      *         thrown in a consistent fashion for synchronization to work
1097      *         correctly.
1098      * @throws UnsupportedOperationException if exclusive mode is not supported
1099      */
1100     protected boolean tryRelease(int arg) {
1101         throw new UnsupportedOperationException();
1102     }
1103 
1104     /**
1105      * Attempts to acquire in shared mode. This method should query if
1106      * the state of the object permits it to be acquired in the shared
1107      * mode, and if so to acquire it.
1108      *
1109      * <p>This method is always invoked by the thread performing
1110      * acquire.  If this method reports failure, the acquire method
1111      * may queue the thread, if it is not already queued, until it is
1112      * signalled by a release from some other thread.
1113      *
1114      * <p>The default implementation throws {@link
1115      * UnsupportedOperationException}.
1116      *
1117      * @param arg the acquire argument. This value is always the one
1118      *        passed to an acquire method, or is the value saved on entry
1119      *        to a condition wait.  The value is otherwise uninterpreted
1120      *        and can represent anything you like.
1121      * @return a negative value on failure; zero if acquisition in shared
1122      *         mode succeeded but no subsequent shared-mode acquire can
1123      *         succeed; and a positive value if acquisition in shared
1124      *         mode succeeded and subsequent shared-mode acquires might
1125      *         also succeed, in which case a subsequent waiting thread
1126      *         must check availability. (Support for three different
1127      *         return values enables this method to be used in contexts
1128      *         where acquires only sometimes act exclusively.)  Upon
1129      *         success, this object has been acquired.
1130      * @throws IllegalMonitorStateException if acquiring would place this
1131      *         synchronizer in an illegal state. This exception must be
1132      *         thrown in a consistent fashion for synchronization to work
1133      *         correctly.
1134      * @throws UnsupportedOperationException if shared mode is not supported
1135      */
1136     protected int tryAcquireShared(int arg) {
1137         throw new UnsupportedOperationException();
1138     }
1139 
1140     /**
1141      * Attempts to set the state to reflect a release in shared mode.
1142      *
1143      * <p>This method is always invoked by the thread performing release.
1144      *
1145      * <p>The default implementation throws
1146      * {@link UnsupportedOperationException}.
1147      *
1148      * @param arg the release argument. This value is always the one
1149      *        passed to a release method, or the current state value upon
1150      *        entry to a condition wait.  The value is otherwise
1151      *        uninterpreted and can represent anything you like.
1152      * @return {@code true} if this release of shared mode may permit a
1153      *         waiting acquire (shared or exclusive) to succeed; and
1154      *         {@code false} otherwise
1155      * @throws IllegalMonitorStateException if releasing would place this
1156      *         synchronizer in an illegal state. This exception must be
1157      *         thrown in a consistent fashion for synchronization to work
1158      *         correctly.
1159      * @throws UnsupportedOperationException if shared mode is not supported
1160      */
1161     protected boolean tryReleaseShared(int arg) {
1162         throw new UnsupportedOperationException();
1163     }
1164 
1165     /**
1166      * Returns {@code true} if synchronization is held exclusively with
1167      * respect to the current (calling) thread.  This method is invoked
1168      * upon each call to a non-waiting {@link ConditionObject} method.
1169      * (Waiting methods instead invoke {@link #release}.)
1170      *
1171      * <p>The default implementation throws {@link
1172      * UnsupportedOperationException}. This method is invoked
1173      * internally only within {@link ConditionObject} methods, so need
1174      * not be defined if conditions are not used.
1175      *
1176      * @return {@code true} if synchronization is held exclusively;
1177      *         {@code false} otherwise
1178      * @throws UnsupportedOperationException if conditions are not supported
1179      */
1180     protected boolean isHeldExclusively() {
1181         throw new UnsupportedOperationException();
1182     }
1183 
1184     /**
1185      * Acquires in exclusive mode, ignoring interrupts.  Implemented
1186      * by invoking at least once {@link #tryAcquire},
1187      * returning on success.  Otherwise the thread is queued, possibly
1188      * repeatedly blocking and unblocking, invoking {@link
1189      * #tryAcquire} until success.  This method can be used
1190      * to implement method {@link Lock#lock}.
1191      *
1192      * @param arg the acquire argument.  This value is conveyed to
1193      *        {@link #tryAcquire} but is otherwise uninterpreted and
1194      *        can represent anything you like.
1195      */
1196     public final void acquire(int arg) {
1197         if (!tryAcquire(arg) &&
1198             acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
1199             selfInterrupt();
1200     }
1201 
1202     /**
1203      * Acquires in exclusive mode, aborting if interrupted.
1204      * Implemented by first checking interrupt status, then invoking
1205      * at least once {@link #tryAcquire}, returning on
1206      * success.  Otherwise the thread is queued, possibly repeatedly
1207      * blocking and unblocking, invoking {@link #tryAcquire}
1208      * until success or the thread is interrupted.  This method can be
1209      * used to implement method {@link Lock#lockInterruptibly}.
1210      *
1211      * @param arg the acquire argument.  This value is conveyed to
1212      *        {@link #tryAcquire} but is otherwise uninterpreted and
1213      *        can represent anything you like.
1214      * @throws InterruptedException if the current thread is interrupted
1215      */
1216     public final void acquireInterruptibly(int arg)
1217             throws InterruptedException {
1218         if (Thread.interrupted())
1219             throw new InterruptedException();
1220         if (!tryAcquire(arg))
1221             doAcquireInterruptibly(arg);
1222     }
1223 
1224     /**
1225      * Attempts to acquire in exclusive mode, aborting if interrupted,
1226      * and failing if the given timeout elapses.  Implemented by first
1227      * checking interrupt status, then invoking at least once {@link
1228      * #tryAcquire}, returning on success.  Otherwise, the thread is
1229      * queued, possibly repeatedly blocking and unblocking, invoking
1230      * {@link #tryAcquire} until success or the thread is interrupted
1231      * or the timeout elapses.  This method can be used to implement
1232      * method {@link Lock#tryLock(long, TimeUnit)}.
1233      *
1234      * @param arg the acquire argument.  This value is conveyed to
1235      *        {@link #tryAcquire} but is otherwise uninterpreted and
1236      *        can represent anything you like.
1237      * @param nanosTimeout the maximum number of nanoseconds to wait
1238      * @return {@code true} if acquired; {@code false} if timed out
1239      * @throws InterruptedException if the current thread is interrupted
1240      */
1241     public final boolean tryAcquireNanos(int arg, long nanosTimeout)
1242             throws InterruptedException {
1243         if (Thread.interrupted())
1244             throw new InterruptedException();
1245         return tryAcquire(arg) ||
1246             doAcquireNanos(arg, nanosTimeout);
1247     }
1248 
1249     /**
1250      * Releases in exclusive mode.  Implemented by unblocking one or
1251      * more threads if {@link #tryRelease} returns true.
1252      * This method can be used to implement method {@link Lock#unlock}.
1253      *
1254      * @param arg the release argument.  This value is conveyed to
1255      *        {@link #tryRelease} but is otherwise uninterpreted and
1256      *        can represent anything you like.
1257      * @return the value returned from {@link #tryRelease}
1258      */
1259     public final boolean release(int arg) {
1260         if (tryRelease(arg)) {
1261             Node h = head;
1262             if (h != null && h.waitStatus != 0)
1263                 unparkSuccessor(h);
1264             return true;
1265         }
1266         return false;
1267     }
1268 
1269     /**
1270      * Acquires in shared mode, ignoring interrupts.  Implemented by
1271      * first invoking at least once {@link #tryAcquireShared},
1272      * returning on success.  Otherwise the thread is queued, possibly
1273      * repeatedly blocking and unblocking, invoking {@link
1274      * #tryAcquireShared} until success.
1275      *
1276      * @param arg the acquire argument.  This value is conveyed to
1277      *        {@link #tryAcquireShared} but is otherwise uninterpreted
1278      *        and can represent anything you like.
1279      */
1280     public final void acquireShared(int arg) {
1281         if (tryAcquireShared(arg) < 0)
1282             doAcquireShared(arg);
1283     }
1284 
1285     /**
1286      * Acquires in shared mode, aborting if interrupted.  Implemented
1287      * by first checking interrupt status, then invoking at least once
1288      * {@link #tryAcquireShared}, returning on success.  Otherwise the
1289      * thread is queued, possibly repeatedly blocking and unblocking,
1290      * invoking {@link #tryAcquireShared} until success or the thread
1291      * is interrupted.
1292      * @param arg the acquire argument
1293      * This value is conveyed to {@link #tryAcquireShared} but is
1294      * otherwise uninterpreted and can represent anything
1295      * you like.
1296      * @throws InterruptedException if the current thread is interrupted
1297      */
1298     public final void acquireSharedInterruptibly(int arg)
1299             throws InterruptedException {
1300         if (Thread.interrupted())
1301             throw new InterruptedException();
1302         if (tryAcquireShared(arg) < 0)
1303             doAcquireSharedInterruptibly(arg);
1304     }
1305 
1306     /**
1307      * Attempts to acquire in shared mode, aborting if interrupted, and
1308      * failing if the given timeout elapses.  Implemented by first
1309      * checking interrupt status, then invoking at least once {@link
1310      * #tryAcquireShared}, returning on success.  Otherwise, the
1311      * thread is queued, possibly repeatedly blocking and unblocking,
1312      * invoking {@link #tryAcquireShared} until success or the thread
1313      * is interrupted or the timeout elapses.
1314      *
1315      * @param arg the acquire argument.  This value is conveyed to
1316      *        {@link #tryAcquireShared} but is otherwise uninterpreted
1317      *        and can represent anything you like.
1318      * @param nanosTimeout the maximum number of nanoseconds to wait
1319      * @return {@code true} if acquired; {@code false} if timed out
1320      * @throws InterruptedException if the current thread is interrupted
1321      */
1322     public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
1323             throws InterruptedException {
1324         if (Thread.interrupted())
1325             throw new InterruptedException();
1326         return tryAcquireShared(arg) >= 0 ||
1327             doAcquireSharedNanos(arg, nanosTimeout);
1328     }
1329 
1330     /**
1331      * Releases in shared mode.  Implemented by unblocking one or more
1332      * threads if {@link #tryReleaseShared} returns true.
1333      *
1334      * @param arg the release argument.  This value is conveyed to
1335      *        {@link #tryReleaseShared} but is otherwise uninterpreted
1336      *        and can represent anything you like.
1337      * @return the value returned from {@link #tryReleaseShared}
1338      */
1339     public final boolean releaseShared(int arg) {
1340         if (tryReleaseShared(arg)) {
1341             doReleaseShared();
1342             return true;
1343         }
1344         return false;
1345     }
1346 
1347     // Queue inspection methods
1348 
1349     /**
1350      * Queries whether any threads are waiting to acquire. Note that
1351      * because cancellations due to interrupts and timeouts may occur
1352      * at any time, a {@code true} return does not guarantee that any
1353      * other thread will ever acquire.
1354      *
1355      * <p>In this implementation, this operation returns in
1356      * constant time.
1357      *
1358      * @return {@code true} if there may be other threads waiting to acquire
1359      */
1360     public final boolean hasQueuedThreads() {
1361         return head != tail;
1362     }
1363 
1364     /**
1365      * Queries whether any threads have ever contended to acquire this
1366      * synchronizer; that is if an acquire method has ever blocked.
1367      *
1368      * <p>In this implementation, this operation returns in
1369      * constant time.
1370      *
1371      * @return {@code true} if there has ever been contention
1372      */
1373     public final boolean hasContended() {
1374         return head != null;
1375     }
1376 
1377     /**
1378      * Returns the first (longest-waiting) thread in the queue, or
1379      * {@code null} if no threads are currently queued.
1380      *
1381      * <p>In this implementation, this operation normally returns in
1382      * constant time, but may iterate upon contention if other threads are
1383      * concurrently modifying the queue.
1384      *
1385      * @return the first (longest-waiting) thread in the queue, or
1386      *         {@code null} if no threads are currently queued
1387      */
1388     public final Thread getFirstQueuedThread() {
1389         // handle only fast path, else relay
1390         return (head == tail) ? null : fullGetFirstQueuedThread();
1391     }
1392 
1393     /**
1394      * Version of getFirstQueuedThread called when fastpath fails
1395      */
1396     private Thread fullGetFirstQueuedThread() {
1397         /*
1398          * The first node is normally head.next. Try to get its
1399          * thread field, ensuring consistent reads: If thread
1400          * field is nulled out or s.prev is no longer head, then
1401          * some other thread(s) concurrently performed setHead in
1402          * between some of our reads. We try this twice before
1403          * resorting to traversal.
1404          */
1405         Node h, s;
1406         Thread st;
1407         if (((h = head) != null && (s = h.next) != null &&
1408              s.prev == head && (st = s.thread) != null) ||
1409             ((h = head) != null && (s = h.next) != null &&
1410              s.prev == head && (st = s.thread) != null))
1411             return st;
1412 
1413         /*
1414          * Head's next field might not have been set yet, or may have
1415          * been unset after setHead. So we must check to see if tail
1416          * is actually first node. If not, we continue on, safely
1417          * traversing from tail back to head to find first,
1418          * guaranteeing termination.
1419          */
1420 
1421         Node t = tail;
1422         Thread firstThread = null;
1423         while (t != null && t != head) {
1424             Thread tt = t.thread;
1425             if (tt != null)
1426                 firstThread = tt;
1427             t = t.prev;
1428         }
1429         return firstThread;
1430     }
1431 
1432     /**
1433      * Returns true if the given thread is currently queued.
1434      *
1435      * <p>This implementation traverses the queue to determine
1436      * presence of the given thread.
1437      *
1438      * @param thread the thread
1439      * @return {@code true} if the given thread is on the queue
1440      * @throws NullPointerException if the thread is null
1441      */
1442     public final boolean isQueued(Thread thread) {
1443         if (thread == null)
1444             throw new NullPointerException();
1445         for (Node p = tail; p != null; p = p.prev)
1446             if (p.thread == thread)
1447                 return true;
1448         return false;
1449     }
1450 
1451     /**
1452      * Returns {@code true} if the apparent first queued thread, if one
1453      * exists, is waiting in exclusive mode.  If this method returns
1454      * {@code true}, and the current thread is attempting to acquire in
1455      * shared mode (that is, this method is invoked from {@link
1456      * #tryAcquireShared}) then it is guaranteed that the current thread
1457      * is not the first queued thread.  Used only as a heuristic in
1458      * ReentrantReadWriteLock.
1459      */
1460     final boolean apparentlyFirstQueuedIsExclusive() {
1461         Node h, s;
1462         return (h = head) != null &&
1463             (s = h.next)  != null &&
1464             !s.isShared()         &&
1465             s.thread != null;
1466     }
1467 
1468     /**
1469      * Queries whether any threads have been waiting to acquire longer
1470      * than the current thread.
1471      *
1472      * <p>An invocation of this method is equivalent to (but may be
1473      * more efficient than):
1474      *  <pre> {@code
1475      * getFirstQueuedThread() != Thread.currentThread() &&
1476      * hasQueuedThreads()}</pre>
1477      *
1478      * <p>Note that because cancellations due to interrupts and
1479      * timeouts may occur at any time, a {@code true} return does not
1480      * guarantee that some other thread will acquire before the current
1481      * thread.  Likewise, it is possible for another thread to win a
1482      * race to enqueue after this method has returned {@code false},
1483      * due to the queue being empty.
1484      *
1485      * <p>This method is designed to be used by a fair synchronizer to
1486      * avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
1487      * Such a synchronizer's {@link #tryAcquire} method should return
1488      * {@code false}, and its {@link #tryAcquireShared} method should
1489      * return a negative value, if this method returns {@code true}
1490      * (unless this is a reentrant acquire).  For example, the {@code
1491      * tryAcquire} method for a fair, reentrant, exclusive mode
1492      * synchronizer might look like this:
1493      *
1494      *  <pre> {@code
1495      * protected boolean tryAcquire(int arg) {
1496      *   if (isHeldExclusively()) {
1497      *     // A reentrant acquire; increment hold count
1498      *     return true;
1499      *   } else if (hasQueuedPredecessors()) {
1500      *     return false;
1501      *   } else {
1502      *     // try to acquire normally
1503      *   }
1504      * }}</pre>
1505      *
1506      * @return {@code true} if there is a queued thread preceding the
1507      *         current thread, and {@code false} if the current thread
1508      *         is at the head of the queue or the queue is empty
1509      * @since 1.7
1510      */
1511     public final boolean hasQueuedPredecessors() {
1512         // The correctness of this depends on head being initialized
1513         // before tail and on head.next being accurate if the current
1514         // thread is first in queue.
1515         Node t = tail; // Read fields in reverse initialization order
1516         Node h = head;
1517         Node s;
1518         return h != t &&
1519             ((s = h.next) == null || s.thread != Thread.currentThread());
1520     }
1521 
1522 
1523     // Instrumentation and monitoring methods
1524 
1525     /**
1526      * Returns an estimate of the number of threads waiting to
1527      * acquire.  The value is only an estimate because the number of
1528      * threads may change dynamically while this method traverses
1529      * internal data structures.  This method is designed for use in
1530      * monitoring system state, not for synchronization
1531      * control.
1532      *
1533      * @return the estimated number of threads waiting to acquire
1534      */
1535     public final int getQueueLength() {
1536         int n = 0;
1537         for (Node p = tail; p != null; p = p.prev) {
1538             if (p.thread != null)
1539                 ++n;
1540         }
1541         return n;
1542     }
1543 
1544     /**
1545      * Returns a collection containing threads that may be waiting to
1546      * acquire.  Because the actual set of threads may change
1547      * dynamically while constructing this result, the returned
1548      * collection is only a best-effort estimate.  The elements of the
1549      * returned collection are in no particular order.  This method is
1550      * designed to facilitate construction of subclasses that provide
1551      * more extensive monitoring facilities.
1552      *
1553      * @return the collection of threads
1554      */
1555     public final Collection<Thread> getQueuedThreads() {
1556         ArrayList<Thread> list = new ArrayList<Thread>();
1557         for (Node p = tail; p != null; p = p.prev) {
1558             Thread t = p.thread;
1559             if (t != null)
1560                 list.add(t);
1561         }
1562         return list;
1563     }
1564 
1565     /**
1566      * Returns a collection containing threads that may be waiting to
1567      * acquire in exclusive mode. This has the same properties
1568      * as {@link #getQueuedThreads} except that it only returns
1569      * those threads waiting due to an exclusive acquire.
1570      *
1571      * @return the collection of threads
1572      */
1573     public final Collection<Thread> getExclusiveQueuedThreads() {
1574         ArrayList<Thread> list = new ArrayList<Thread>();
1575         for (Node p = tail; p != null; p = p.prev) {
1576             if (!p.isShared()) {
1577                 Thread t = p.thread;
1578                 if (t != null)
1579                     list.add(t);
1580             }
1581         }
1582         return list;
1583     }
1584 
1585     /**
1586      * Returns a collection containing threads that may be waiting to
1587      * acquire in shared mode. This has the same properties
1588      * as {@link #getQueuedThreads} except that it only returns
1589      * those threads waiting due to a shared acquire.
1590      *
1591      * @return the collection of threads
1592      */
1593     public final Collection<Thread> getSharedQueuedThreads() {
1594         ArrayList<Thread> list = new ArrayList<Thread>();
1595         for (Node p = tail; p != null; p = p.prev) {
1596             if (p.isShared()) {
1597                 Thread t = p.thread;
1598                 if (t != null)
1599                     list.add(t);
1600             }
1601         }
1602         return list;
1603     }
1604 
1605     /**
1606      * Returns a string identifying this synchronizer, as well as its state.
1607      * The state, in brackets, includes the String {@code "State ="}
1608      * followed by the current value of {@link #getState}, and either
1609      * {@code "nonempty"} or {@code "empty"} depending on whether the
1610      * queue is empty.
1611      *
1612      * @return a string identifying this synchronizer, as well as its state
1613      */
1614     public String toString() {
1615         int s = getState();
1616         String q  = hasQueuedThreads() ? "non" : "";
1617         return super.toString() +
1618             "[State = " + s + ", " + q + "empty queue]";
1619     }
1620 
1621 
1622     // Internal support methods for Conditions
1623 
1624     /**
1625      * Returns true if a node, always one that was initially placed on
1626      * a condition queue, is now waiting to reacquire on sync queue.
1627      * @param node the node
1628      * @return true if is reacquiring
1629      */
1630     final boolean isOnSyncQueue(Node node) {
1631         if (node.waitStatus == Node.CONDITION || node.prev == null)
1632             return false;
1633         if (node.next != null) // If has successor, it must be on queue
1634             return true;
1635         /*
1636          * node.prev can be non-null, but not yet on queue because
1637          * the CAS to place it on queue can fail. So we have to
1638          * traverse from tail to make sure it actually made it.  It
1639          * will always be near the tail in calls to this method, and
1640          * unless the CAS failed (which is unlikely), it will be
1641          * there, so we hardly ever traverse much.
1642          */
1643         return findNodeFromTail(node);
1644     }
1645 
1646     /**
1647      * Returns true if node is on sync queue by searching backwards from tail.
1648      * Called only when needed by isOnSyncQueue.
1649      * @return true if present
1650      */
1651     private boolean findNodeFromTail(Node node) {
1652         Node t = tail;
1653         for (;;) {
1654             if (t == node)
1655                 return true;
1656             if (t == null)
1657                 return false;
1658             t = t.prev;
1659         }
1660     }
1661 
1662     /**
1663      * Transfers a node from a condition queue onto sync queue.
1664      * Returns true if successful.
1665      * @param node the node
1666      * @return true if successfully transferred (else the node was
1667      * cancelled before signal).
1668      */
1669     final boolean transferForSignal(Node node) {
1670         /*
1671          * If cannot change waitStatus, the node has been cancelled.
1672          */
1673         if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
1674             return false;
1675 
1676         /*
1677          * Splice onto queue and try to set waitStatus of predecessor to
1678          * indicate that thread is (probably) waiting. If cancelled or
1679          * attempt to set waitStatus fails, wake up to resync (in which
1680          * case the waitStatus can be transiently and harmlessly wrong).
1681          */
1682         Node p = enq(node);
1683         int ws = p.waitStatus;
1684         if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
1685             LockSupport.unpark(node.thread);
1686         return true;
1687     }
1688 
1689     /**
1690      * Transfers node, if necessary, to sync queue after a cancelled
1691      * wait. Returns true if thread was cancelled before being
1692      * signalled.
1693      * @param current the waiting thread
1694      * @param node its node
1695      * @return true if cancelled before the node was signalled
1696      */
1697     final boolean transferAfterCancelledWait(Node node) {
1698         if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
1699             enq(node);
1700             return true;
1701         }
1702         /*
1703          * If we lost out to a signal(), then we can't proceed
1704          * until it finishes its enq().  Cancelling during an
1705          * incomplete transfer is both rare and transient, so just
1706          * spin.
1707          */
1708         while (!isOnSyncQueue(node))
1709             Thread.yield();
1710         return false;
1711     }
1712 
1713     /**
1714      * Invokes release with current state value; returns saved state.
1715      * Cancels node and throws exception on failure.
1716      * @param node the condition node for this wait
1717      * @return previous sync state
1718      */
1719     final int fullyRelease(Node node) {
1720         boolean failed = true;
1721         try {
1722             int savedState = getState();
1723             if (release(savedState)) {
1724                 failed = false;
1725                 return savedState;
1726             } else {
1727                 throw new IllegalMonitorStateException();
1728             }
1729         } finally {
1730             if (failed)
1731                 node.waitStatus = Node.CANCELLED;
1732         }
1733     }
1734 
1735     // Instrumentation methods for conditions
1736 
1737     /**
1738      * Queries whether the given ConditionObject
1739      * uses this synchronizer as its lock.
1740      *
1741      * @param condition the condition
1742      * @return <tt>true</tt> if owned
1743      * @throws NullPointerException if the condition is null
1744      */
1745     public final boolean owns(ConditionObject condition) {
1746         if (condition == null)
1747             throw new NullPointerException();
1748         return condition.isOwnedBy(this);
1749     }
1750 
1751     /**
1752      * Queries whether any threads are waiting on the given condition
1753      * associated with this synchronizer. Note that because timeouts
1754      * and interrupts may occur at any time, a <tt>true</tt> return
1755      * does not guarantee that a future <tt>signal</tt> will awaken
1756      * any threads.  This method is designed primarily for use in
1757      * monitoring of the system state.
1758      *
1759      * @param condition the condition
1760      * @return <tt>true</tt> if there are any waiting threads
1761      * @throws IllegalMonitorStateException if exclusive synchronization
1762      *         is not held
1763      * @throws IllegalArgumentException if the given condition is
1764      *         not associated with this synchronizer
1765      * @throws NullPointerException if the condition is null
1766      */
1767     public final boolean hasWaiters(ConditionObject condition) {
1768         if (!owns(condition))
1769             throw new IllegalArgumentException("Not owner");
1770         return condition.hasWaiters();
1771     }
1772 
1773     /**
1774      * Returns an estimate of the number of threads waiting on the
1775      * given condition associated with this synchronizer. Note that
1776      * because timeouts and interrupts may occur at any time, the
1777      * estimate serves only as an upper bound on the actual number of
1778      * waiters.  This method is designed for use in monitoring of the
1779      * system state, not for synchronization control.
1780      *
1781      * @param condition the condition
1782      * @return the estimated number of waiting threads
1783      * @throws IllegalMonitorStateException if exclusive synchronization
1784      *         is not held
1785      * @throws IllegalArgumentException if the given condition is
1786      *         not associated with this synchronizer
1787      * @throws NullPointerException if the condition is null
1788      */
1789     public final int getWaitQueueLength(ConditionObject condition) {
1790         if (!owns(condition))
1791             throw new IllegalArgumentException("Not owner");
1792         return condition.getWaitQueueLength();
1793     }
1794 
1795     /**
1796      * Returns a collection containing those threads that may be
1797      * waiting on the given condition associated with this
1798      * synchronizer.  Because the actual set of threads may change
1799      * dynamically while constructing this result, the returned
1800      * collection is only a best-effort estimate. The elements of the
1801      * returned collection are in no particular order.
1802      *
1803      * @param condition the condition
1804      * @return the collection of threads
1805      * @throws IllegalMonitorStateException if exclusive synchronization
1806      *         is not held
1807      * @throws IllegalArgumentException if the given condition is
1808      *         not associated with this synchronizer
1809      * @throws NullPointerException if the condition is null
1810      */
1811     public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
1812         if (!owns(condition))
1813             throw new IllegalArgumentException("Not owner");
1814         return condition.getWaitingThreads();
1815     }
1816 
1817     /**
1818      * Condition implementation for a {@link
1819      * AbstractQueuedSynchronizer} serving as the basis of a {@link
1820      * Lock} implementation.
1821      *
1822      * <p>Method documentation for this class describes mechanics,
1823      * not behavioral specifications from the point of view of Lock
1824      * and Condition users. Exported versions of this class will in
1825      * general need to be accompanied by documentation describing
1826      * condition semantics that rely on those of the associated
1827      * <tt>AbstractQueuedSynchronizer</tt>.
1828      *
1829      * <p>This class is Serializable, but all fields are transient,
1830      * so deserialized conditions have no waiters.
1831      */
1832     public class ConditionObject implements Condition, java.io.Serializable {
1833         private static final long serialVersionUID = 1173984872572414699L;
1834         /** First node of condition queue. */
1835         private transient Node firstWaiter;
1836         /** Last node of condition queue. */
1837         private transient Node lastWaiter;
1838 
1839         /**
1840          * Creates a new <tt>ConditionObject</tt> instance.
1841          */
1842         public ConditionObject() { }
1843 
1844         // Internal methods
1845 
1846         /**
1847          * Adds a new waiter to wait queue.
1848          * @return its new wait node
1849          */
1850         private Node addConditionWaiter() {
1851             Node t = lastWaiter;
1852             // If lastWaiter is cancelled, clean out.
1853             if (t != null && t.waitStatus != Node.CONDITION) {
1854                 unlinkCancelledWaiters();
1855                 t = lastWaiter;
1856             }
1857             Node node = new Node(Thread.currentThread(), Node.CONDITION);
1858             if (t == null)
1859                 firstWaiter = node;
1860             else
1861                 t.nextWaiter = node;
1862             lastWaiter = node;
1863             return node;
1864         }
1865 
1866         /**
1867          * Removes and transfers nodes until hit non-cancelled one or
1868          * null. Split out from signal in part to encourage compilers
1869          * to inline the case of no waiters.
1870          * @param first (non-null) the first node on condition queue
1871          */
1872         private void doSignal(Node first) {
1873             do {
1874                 if ( (firstWaiter = first.nextWaiter) == null)
1875                     lastWaiter = null;
1876                 first.nextWaiter = null;
1877             } while (!transferForSignal(first) &&
1878                      (first = firstWaiter) != null);
1879         }
1880 
1881         /**
1882          * Removes and transfers all nodes.
1883          * @param first (non-null) the first node on condition queue
1884          */
1885         private void doSignalAll(Node first) {
1886             lastWaiter = firstWaiter = null;
1887             do {
1888                 Node next = first.nextWaiter;
1889                 first.nextWaiter = null;
1890                 transferForSignal(first);
1891                 first = next;
1892             } while (first != null);
1893         }
1894 
1895         /**
1896          * Unlinks cancelled waiter nodes from condition queue.
1897          * Called only while holding lock. This is called when
1898          * cancellation occurred during condition wait, and upon
1899          * insertion of a new waiter when lastWaiter is seen to have
1900          * been cancelled. This method is needed to avoid garbage
1901          * retention in the absence of signals. So even though it may
1902          * require a full traversal, it comes into play only when
1903          * timeouts or cancellations occur in the absence of
1904          * signals. It traverses all nodes rather than stopping at a
1905          * particular target to unlink all pointers to garbage nodes
1906          * without requiring many re-traversals during cancellation
1907          * storms.
1908          */
1909         private void unlinkCancelledWaiters() {
1910             Node t = firstWaiter;
1911             Node trail = null;
1912             while (t != null) {
1913                 Node next = t.nextWaiter;
1914                 if (t.waitStatus != Node.CONDITION) {
1915                     t.nextWaiter = null;
1916                     if (trail == null)
1917                         firstWaiter = next;
1918                     else
1919                         trail.nextWaiter = next;
1920                     if (next == null)
1921                         lastWaiter = trail;
1922                 }
1923                 else
1924                     trail = t;
1925                 t = next;
1926             }
1927         }
1928 
1929         // public methods
1930 
1931         /**
1932          * Moves the longest-waiting thread, if one exists, from the
1933          * wait queue for this condition to the wait queue for the
1934          * owning lock.
1935          *
1936          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
1937          *         returns {@code false}
1938          */
1939         public final void signal() {
1940             if (!isHeldExclusively())
1941                 throw new IllegalMonitorStateException();
1942             Node first = firstWaiter;
1943             if (first != null)
1944                 doSignal(first);
1945         }
1946 
1947         /**
1948          * Moves all threads from the wait queue for this condition to
1949          * the wait queue for the owning lock.
1950          *
1951          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
1952          *         returns {@code false}
1953          */
1954         public final void signalAll() {
1955             if (!isHeldExclusively())
1956                 throw new IllegalMonitorStateException();
1957             Node first = firstWaiter;
1958             if (first != null)
1959                 doSignalAll(first);
1960         }
1961 
1962         /**
1963          * Implements uninterruptible condition wait.
1964          * <ol>
1965          * <li> Save lock state returned by {@link #getState}.
1966          * <li> Invoke {@link #release} with
1967          *      saved state as argument, throwing
1968          *      IllegalMonitorStateException if it fails.
1969          * <li> Block until signalled.
1970          * <li> Reacquire by invoking specialized version of
1971          *      {@link #acquire} with saved state as argument.
1972          * </ol>
1973          */
1974         public final void awaitUninterruptibly() {
1975             Node node = addConditionWaiter();
1976             int savedState = fullyRelease(node);
1977             boolean interrupted = false;
1978             while (!isOnSyncQueue(node)) {
1979                 LockSupport.park(this);
1980                 if (Thread.interrupted())
1981                     interrupted = true;
1982             }
1983             if (acquireQueued(node, savedState) || interrupted)
1984                 selfInterrupt();
1985         }
1986 
1987         /*
1988          * For interruptible waits, we need to track whether to throw
1989          * InterruptedException, if interrupted while blocked on
1990          * condition, versus reinterrupt current thread, if
1991          * interrupted while blocked waiting to re-acquire.
1992          */
1993 
1994         /** Mode meaning to reinterrupt on exit from wait */
1995         private static final int REINTERRUPT =  1;
1996         /** Mode meaning to throw InterruptedException on exit from wait */
1997         private static final int THROW_IE    = -1;
1998 
1999         /**
2000          * Checks for interrupt, returning THROW_IE if interrupted
2001          * before signalled, REINTERRUPT if after signalled, or
2002          * 0 if not interrupted.
2003          */
2004         private int checkInterruptWhileWaiting(Node node) {
2005             return Thread.interrupted() ?
2006                 (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
2007                 0;
2008         }
2009 
2010         /**
2011          * Throws InterruptedException, reinterrupts current thread, or
2012          * does nothing, depending on mode.
2013          */
2014         private void reportInterruptAfterWait(int interruptMode)
2015             throws InterruptedException {
2016             if (interruptMode == THROW_IE)
2017                 throw new InterruptedException();
2018             else if (interruptMode == REINTERRUPT)
2019                 selfInterrupt();
2020         }
2021 
2022         /**
2023          * Implements interruptible condition wait.
2024          * <ol>
2025          * <li> If current thread is interrupted, throw InterruptedException.
2026          * <li> Save lock state returned by {@link #getState}.
2027          * <li> Invoke {@link #release} with
2028          *      saved state as argument, throwing
2029          *      IllegalMonitorStateException if it fails.
2030          * <li> Block until signalled or interrupted.
2031          * <li> Reacquire by invoking specialized version of
2032          *      {@link #acquire} with saved state as argument.
2033          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2034          * </ol>
2035          */
2036         public final void await() throws InterruptedException {
2037             if (Thread.interrupted())
2038                 throw new InterruptedException();
2039             Node node = addConditionWaiter();
2040             int savedState = fullyRelease(node);
2041             int interruptMode = 0;
2042             while (!isOnSyncQueue(node)) {
2043                 LockSupport.park(this);
2044                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2045                     break;
2046             }
2047             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2048                 interruptMode = REINTERRUPT;
2049             if (node.nextWaiter != null) // clean up if cancelled
2050                 unlinkCancelledWaiters();
2051             if (interruptMode != 0)
2052                 reportInterruptAfterWait(interruptMode);
2053         }
2054 
2055         /**
2056          * Implements timed condition wait.
2057          * <ol>
2058          * <li> If current thread is interrupted, throw InterruptedException.
2059          * <li> Save lock state returned by {@link #getState}.
2060          * <li> Invoke {@link #release} with
2061          *      saved state as argument, throwing
2062          *      IllegalMonitorStateException if it fails.
2063          * <li> Block until signalled, interrupted, or timed out.
2064          * <li> Reacquire by invoking specialized version of
2065          *      {@link #acquire} with saved state as argument.
2066          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2067          * </ol>
2068          */
2069         public final long awaitNanos(long nanosTimeout)
2070                 throws InterruptedException {
2071             if (Thread.interrupted())
2072                 throw new InterruptedException();
2073             Node node = addConditionWaiter();
2074             int savedState = fullyRelease(node);
2075             long lastTime = System.nanoTime();
2076             int interruptMode = 0;
2077             while (!isOnSyncQueue(node)) {
2078                 if (nanosTimeout <= 0L) {
2079                     transferAfterCancelledWait(node);
2080                     break;
2081                 }
2082                 LockSupport.parkNanos(this, nanosTimeout);
2083                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2084                     break;
2085 
2086                 long now = System.nanoTime();
2087                 nanosTimeout -= now - lastTime;
2088                 lastTime = now;
2089             }
2090             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2091                 interruptMode = REINTERRUPT;
2092             if (node.nextWaiter != null)
2093                 unlinkCancelledWaiters();
2094             if (interruptMode != 0)
2095                 reportInterruptAfterWait(interruptMode);
2096             return nanosTimeout - (System.nanoTime() - lastTime);
2097         }
2098 
2099         /**
2100          * Implements absolute timed condition wait.
2101          * <ol>
2102          * <li> If current thread is interrupted, throw InterruptedException.
2103          * <li> Save lock state returned by {@link #getState}.
2104          * <li> Invoke {@link #release} with
2105          *      saved state as argument, throwing
2106          *      IllegalMonitorStateException if it fails.
2107          * <li> Block until signalled, interrupted, or timed out.
2108          * <li> Reacquire by invoking specialized version of
2109          *      {@link #acquire} with saved state as argument.
2110          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2111          * <li> If timed out while blocked in step 4, return false, else true.
2112          * </ol>
2113          */
2114         public final boolean awaitUntil(Date deadline)
2115                 throws InterruptedException {
2116             if (deadline == null)
2117                 throw new NullPointerException();
2118             long abstime = deadline.getTime();
2119             if (Thread.interrupted())
2120                 throw new InterruptedException();
2121             Node node = addConditionWaiter();
2122             int savedState = fullyRelease(node);
2123             boolean timedout = false;
2124             int interruptMode = 0;
2125             while (!isOnSyncQueue(node)) {
2126                 if (System.currentTimeMillis() > abstime) {
2127                     timedout = transferAfterCancelledWait(node);
2128                     break;
2129                 }
2130                 LockSupport.parkUntil(this, abstime);
2131                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2132                     break;
2133             }
2134             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2135                 interruptMode = REINTERRUPT;
2136             if (node.nextWaiter != null)
2137                 unlinkCancelledWaiters();
2138             if (interruptMode != 0)
2139                 reportInterruptAfterWait(interruptMode);
2140             return !timedout;
2141         }
2142 
2143         /**
2144          * Implements timed condition wait.
2145          * <ol>
2146          * <li> If current thread is interrupted, throw InterruptedException.
2147          * <li> Save lock state returned by {@link #getState}.
2148          * <li> Invoke {@link #release} with
2149          *      saved state as argument, throwing
2150          *      IllegalMonitorStateException if it fails.
2151          * <li> Block until signalled, interrupted, or timed out.
2152          * <li> Reacquire by invoking specialized version of
2153          *      {@link #acquire} with saved state as argument.
2154          * <li> If interrupted while blocked in step 4, throw InterruptedException.
2155          * <li> If timed out while blocked in step 4, return false, else true.
2156          * </ol>
2157          */
2158         public final boolean await(long time, TimeUnit unit)
2159                 throws InterruptedException {
2160             if (unit == null)
2161                 throw new NullPointerException();
2162             long nanosTimeout = unit.toNanos(time);
2163             if (Thread.interrupted())
2164                 throw new InterruptedException();
2165             Node node = addConditionWaiter();
2166             int savedState = fullyRelease(node);
2167             long lastTime = System.nanoTime();
2168             boolean timedout = false;
2169             int interruptMode = 0;
2170             while (!isOnSyncQueue(node)) {
2171                 if (nanosTimeout <= 0L) {
2172                     timedout = transferAfterCancelledWait(node);
2173                     break;
2174                 }
2175                 if (nanosTimeout >= spinForTimeoutThreshold)
2176                     LockSupport.parkNanos(this, nanosTimeout);
2177                 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2178                     break;
2179                 long now = System.nanoTime();
2180                 nanosTimeout -= now - lastTime;
2181                 lastTime = now;
2182             }
2183             if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2184                 interruptMode = REINTERRUPT;
2185             if (node.nextWaiter != null)
2186                 unlinkCancelledWaiters();
2187             if (interruptMode != 0)
2188                 reportInterruptAfterWait(interruptMode);
2189             return !timedout;
2190         }
2191 
2192         //  support for instrumentation
2193 
2194         /**
2195          * Returns true if this condition was created by the given
2196          * synchronization object.
2197          *
2198          * @return {@code true} if owned
2199          */
2200         final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
2201             return sync == AbstractQueuedSynchronizer.this;
2202         }
2203 
2204         /**
2205          * Queries whether any threads are waiting on this condition.
2206          * Implements {@link AbstractQueuedSynchronizer#hasWaiters}.
2207          *
2208          * @return {@code true} if there are any waiting threads
2209          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2210          *         returns {@code false}
2211          */
2212         protected final boolean hasWaiters() {
2213             if (!isHeldExclusively())
2214                 throw new IllegalMonitorStateException();
2215             for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2216                 if (w.waitStatus == Node.CONDITION)
2217                     return true;
2218             }
2219             return false;
2220         }
2221 
2222         /**
2223          * Returns an estimate of the number of threads waiting on
2224          * this condition.
2225          * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}.
2226          *
2227          * @return the estimated number of waiting threads
2228          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2229          *         returns {@code false}
2230          */
2231         protected final int getWaitQueueLength() {
2232             if (!isHeldExclusively())
2233                 throw new IllegalMonitorStateException();
2234             int n = 0;
2235             for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2236                 if (w.waitStatus == Node.CONDITION)
2237                     ++n;
2238             }
2239             return n;
2240         }
2241 
2242         /**
2243          * Returns a collection containing those threads that may be
2244          * waiting on this Condition.
2245          * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}.
2246          *
2247          * @return the collection of threads
2248          * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2249          *         returns {@code false}
2250          */
2251         protected final Collection<Thread> getWaitingThreads() {
2252             if (!isHeldExclusively())
2253                 throw new IllegalMonitorStateException();
2254             ArrayList<Thread> list = new ArrayList<Thread>();
2255             for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2256                 if (w.waitStatus == Node.CONDITION) {
2257                     Thread t = w.thread;
2258                     if (t != null)
2259                         list.add(t);
2260                 }
2261             }
2262             return list;
2263         }
2264     }
2265 
2266     /**
2267      * Setup to support compareAndSet. We need to natively implement
2268      * this here: For the sake of permitting future enhancements, we
2269      * cannot explicitly subclass AtomicInteger, which would be
2270      * efficient and useful otherwise. So, as the lesser of evils, we
2271      * natively implement using hotspot intrinsics API. And while we
2272      * are at it, we do the same for other CASable fields (which could
2273      * otherwise be done with atomic field updaters).
2274      */
2275     private static final Unsafe unsafe = Unsafe.getUnsafe();
2276     private static final long stateOffset;
2277     private static final long headOffset;
2278     private static final long tailOffset;
2279     private static final long waitStatusOffset;
2280     private static final long nextOffset;
2281 
2282     static {
2283         try {
2284             stateOffset = unsafe.objectFieldOffset
2285                 (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
2286             headOffset = unsafe.objectFieldOffset
2287                 (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
2288             tailOffset = unsafe.objectFieldOffset
2289                 (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
2290             waitStatusOffset = unsafe.objectFieldOffset
2291                 (Node.class.getDeclaredField("waitStatus"));
2292             nextOffset = unsafe.objectFieldOffset
2293                 (Node.class.getDeclaredField("next"));
2294 
2295         } catch (Exception ex) { throw new Error(ex); }
2296     }
2297 
2298     /**
2299      * CAS head field. Used only by enq.
2300      */
2301     private final boolean compareAndSetHead(Node update) {
2302         return unsafe.compareAndSwapObject(this, headOffset, null, update);
2303     }
2304 
2305     /**
2306      * CAS tail field. Used only by enq.
2307      */
2308     private final boolean compareAndSetTail(Node expect, Node update) {
2309         return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
2310     }
2311 
2312     /**
2313      * CAS waitStatus field of a node.
2314      */
2315     private static final boolean compareAndSetWaitStatus(Node node,
2316                                                          int expect,
2317                                                          int update) {
2318         return unsafe.compareAndSwapInt(node, waitStatusOffset,
2319                                         expect, update);
2320     }
2321 
2322     /**
2323      * CAS next field of a node.
2324      */
2325     private static final boolean compareAndSetNext(Node node,
2326                                                    Node expect,
2327                                                    Node update) {
2328         return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
2329     }
2330 }
View Code

 

获取公平锁(基于JDK1.7.0_40)

通过前面“Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock”的“示例1”,我们知道,获取锁是通过lock()函数。下面,我们以lock()对获取公平锁的过程进行展开。

1. lock()

lock()在ReentrantLock.java的FairSync类中实现,它的源码如下:

final void lock() {
    acquire(1);
}

说明:“当前线程”实际上是通过acquire(1)获取锁的。
        这里说明一下“1”的含义,它是设置“锁的状态”的参数。对于“独占锁”而言,锁处于可获取状态时,它的状态值是0;锁被线程初次获取到了,它的状态值就变成了1。
        由于ReentrantLock(公平锁/非公平锁)是可重入锁,所以“独占锁”可以被单个线程多此获取,每获取1次就将锁的状态+1。也就是说,初次获取锁时,通过acquire(1)将锁的状态值设为1;再次获取锁时,将锁的状态值设为2;依次类推...这就是为什么获取锁时,传入的参数是1的原因了。
        可重入就是指锁可以被单个线程多次获取。

 

2. acquire()

acquire()在AQS中实现的,它的源码如下:

public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}

(01) “当前线程”首先通过tryAcquire()尝试获取锁。获取成功的话,直接返回;尝试失败的话,进入到等待队列排序等待(前面还有可能有需要线程在等待该锁)。
(02) “当前线程”尝试失败的情况下,先通过addWaiter(Node.EXCLUSIVE)来将“当前线程”加入到"CLH队列(非阻塞的FIFO队列)"末尾。CLH队列就是线程等待队列。
(03) 再执行完addWaiter(Node.EXCLUSIVE)之后,会调用acquireQueued()来获取锁。由于此时ReentrantLock是公平锁,它会根据公平性原则来获取锁。
(04) “当前线程”在执行acquireQueued()时,会
进入到CLH队列中休眠等待,直到获取锁了才返回!如果“当前线程”在休眠等待过程中被中断过,acquireQueued会返回true,此时"当前线程"会调用selfInterrupt()来自己给自己产生一个中断。至于为什么要自己给自己产生一个中断,后面再介绍。

上面是对acquire()的概括性说明。下面,我们将该函数分为4部分来逐步解析。
一. tryAcquire()
二. addWaiter()
三. acquireQueued()
四. selfInterrupt()

 

一. tryAcquire()

1. tryAcquire()

公平锁的tryAcquire()在ReentrantLock.java的FairSync类中实现,源码如下:

protected final boolean tryAcquire(int acquires) {
    // 获取“当前线程”
    final Thread current = Thread.currentThread();
    // 获取“独占锁”的状态
    int c = getState();
    // c=0意味着“锁没有被任何线程锁拥有”,
    if (c == 0) {
        // 若“锁没有被任何线程锁拥有”,
        // 则判断“当前线程”是不是CLH队列中的第一个线程线程,
        // 若是的话,则获取该锁,设置锁的状态,并切设置锁的拥有者为“当前线程”。
        if (!hasQueuedPredecessors() &&
            compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {
        // 如果“独占锁”的拥有者已经为“当前线程”,
        // 则将更新锁的状态。
        int nextc = c + acquires;
        if (nextc < 0)
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

说明根据代码,我们可以分析出,tryAcquire()的作用就是尝试去获取锁。注意,这里只是尝试!
         尝试成功的话,返回true;尝试失败的话,返回false,后续再通过其它办法来获取该锁。后面我们会说明,在尝试失败的情况下,是如何一步步获取锁的。

 

2. hasQueuedPredecessors()

hasQueuedPredecessors()在AQS中实现,源码如下:

public final boolean hasQueuedPredecessors() {
    Node t = tail; 
    Node h = head;
    Node s;
    return h != t &&
        ((s = h.next) == null || s.thread != Thread.currentThread());
}

说明: 通过代码,能分析出,hasQueuedPredecessors() 是通过判断"当前线程"是不是在CLH队列的队首,来返回AQS中是不是有比“当前线程”等待更久的线程。下面对head、tail和Node进行说明。

 

3. Node的源码

Node就是CLH队列的节点。Node在AQS中实现,它的数据结构如下:

private transient volatile Node head;    // CLH队列的队首
private transient volatile Node tail;    // CLH队列的队尾

// CLH队列的节点
static final class Node {
    static final Node SHARED = new Node();
    static final Node EXCLUSIVE = null;

    // 线程已被取消,对应的waitStatus的值
    static final int CANCELLED =  1;
    // “当前线程的后继线程需要被unpark(唤醒)”,对应的waitStatus的值。
    // 一般发生情况是:当前线程的后继线程处于阻塞状态,而当前线程被release或cancel掉,因此需要唤醒当前线程的后继线程。
    static final int SIGNAL    = -1;
    // 线程(处在Condition休眠状态)在等待Condition唤醒,对应的waitStatus的值
    static final int CONDITION = -2;
    // (共享锁)其它线程获取到“共享锁”,对应的waitStatus的值
    static final int PROPAGATE = -3;

    // waitStatus为“CANCELLED, SIGNAL, CONDITION, PROPAGATE”时分别表示不同状态,
    // 若waitStatus=0,则意味着当前线程不属于上面的任何一种状态。
    volatile int waitStatus;

    // 前一节点
    volatile Node prev;

    // 后一节点
    volatile Node next;

    // 节点所对应的线程
    volatile Thread thread;

    // nextWaiter是“区别当前CLH队列是 ‘独占锁’队列 还是 ‘共享锁’队列 的标记”
    // 若nextWaiter=SHARED,则CLH队列是“独占锁”队列;
    // 若nextWaiter=EXCLUSIVE,(即nextWaiter=null),则CLH队列是“共享锁”队列。
    Node nextWaiter;

    // “共享锁”则返回true,“独占锁”则返回false。
    final boolean isShared() {
        return nextWaiter == SHARED;
    }

    // 返回前一节点
    final Node predecessor() throws NullPointerException {
        Node p = prev;
        if (p == null)
            throw new NullPointerException();
        else
            return p;
    }

    Node() {    // Used to establish initial head or SHARED marker
    }

    // 构造函数。thread是节点所对应的线程,mode是用来表示thread的锁是“独占锁”还是“共享锁”。
    Node(Thread thread, Node mode) {     // Used by addWaiter
        this.nextWaiter = mode;
        this.thread = thread;
    }

    // 构造函数。thread是节点所对应的线程,waitStatus是线程的等待状态。
    Node(Thread thread, int waitStatus) { // Used by Condition
        this.waitStatus = waitStatus;
        this.thread = thread;
    }
}

说明
Node是CLH队列的节点,代表“等待锁的线程队列”。
(01) 每个Node都会一个线程对应。
(02) 每个Node会通过prev和next分别指向上一个节点和下一个节点,这分别代表上一个等待线程和下一个等待线程。
(03) Node通过waitStatus保存线程的等待状态。
(04) Node通过nextWaiter来区分线程是“独占锁”线程还是“共享锁”线程。如果是“独占锁”线程,则nextWaiter的值为EXCLUSIVE;如果是“共享锁”线程,则nextWaiter的值是SHARED。

 

4. compareAndSetState()

compareAndSetState()在AQS中实现。它的源码如下:

protected final boolean compareAndSetState(int expect, int update) {
    return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}

说明: compareAndSwapInt() 是sun.misc.Unsafe类中的一个本地方法。对此,我们需要了解的是 compareAndSetState(expect, update) 是以原子的方式操作当前线程;若当前线程的状态为expect,则设置它的状态为update。

 

5. setExclusiveOwnerThread()

setExclusiveOwnerThread()在AbstractOwnableSynchronizer.java中实现,它的源码如下:

// exclusiveOwnerThread是当前拥有“独占锁”的线程
private transient Thread exclusiveOwnerThread;
protected final void setExclusiveOwnerThread(Thread t) {
    exclusiveOwnerThread = t;
}

说明:setExclusiveOwnerThread()的作用就是,设置线程t为当前拥有“独占锁”的线程。

 

6. getState(), setState()

getState()和setState()都在AQS中实现,源码如下:

// 锁的状态
private volatile int state;
// 设置锁的状态
protected final void setState(int newState) {
    state = newState;
}
// 获取锁的状态
protected final int getState() {
    return state;
}

说明:state表示锁的状态,对于“独占锁”而已,state=0表示锁是可获取状态(即,锁没有被任何线程锁持有)。由于java中的独占锁是可重入的,state的值可以>1。

 

小结tryAcquire()的作用就是让“当前线程”尝试获取锁。获取成功返回true,失败则返回false。

 

二. addWaiter(Node.EXCLUSIVE)

addWaiter(Node.EXCLUSIVE)的作用是,创建“当前线程”的Node节点,且Node中记录“当前线程”对应的锁是“独占锁”类型,并且将该节点添加到CLH队列的末尾。

1.addWaiter()

addWaiter()在AQS中实现,源码如下:

private Node addWaiter(Node mode) {
    // 新建一个Node节点,节点对应的线程是“当前线程”,“当前线程”的锁的模型是mode。
    Node node = new Node(Thread.currentThread(), mode);
    Node pred = tail;
    // 若CLH队列不为空,则将“当前线程”添加到CLH队列末尾
    if (pred != null) {
        node.prev = pred;
        if (compareAndSetTail(pred, node)) {
            pred.next = node;
            return node;
        }
    }
    // 若CLH队列为空,则调用enq()新建CLH队列,然后再将“当前线程”添加到CLH队列中。
    enq(node);
    return node;
}

说明对于“公平锁”而言,addWaiter(Node.EXCLUSIVE)会首先创建一个Node节点,节点的类型是“独占锁”(Node.EXCLUSIVE)类型。然后,再将该节点添加到CLH队列的末尾。

 

2. compareAndSetTail()

compareAndSetTail()在AQS中实现,源码如下:

private final boolean compareAndSetTail(Node expect, Node update) {
    return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}

说明compareAndSetTail也属于CAS函数,也是通过“本地方法”实现的。compareAndSetTail(expect, update)会以原子的方式进行操作,它的作用是判断CLH队列的队尾是不是为expect,是的话,就将队尾设为update。

 

3. enq()

enq()在AQS中实现,源码如下:

private Node enq(final Node node) {
    for (;;) {
        Node t = tail;
        if (t == null) { // Must initialize
            if (compareAndSetHead(new Node()))
                tail = head;
        } else {
            node.prev = t;
            if (compareAndSetTail(t, node)) {
                t.next = node;
                return t;
            }
        }
    }
}

说明: enq()的作用很简单。如果CLH队列为空,则新建一个CLH表头;然后将node添加到CLH末尾。否则,直接将node添加到CLH末尾。

 

小结:addWaiter()的作用,就是将当前线程添加到CLH队列中。这就意味着将当前线程添加到等待获取“锁”的等待线程队列中了。

 

三. acquireQueued()

前面,我们已经将当前线程添加到CLH队列中了。而acquireQueued()的作用就是逐步的去执行CLH队列的线程,如果当前线程获取到了锁,则返回;否则,当前线程进行休眠,直到唤醒并重新获取锁了才返回。下面,我们看看acquireQueued()的具体流程。

 

1. acquireQueued()

acquireQueued()在AQS中实现,源码如下:

final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        // interrupted表示在CLH队列的调度中,
        // “当前线程”在休眠时,有没有被中断过。
        boolean interrupted = false;
        for (;;) {
            // 获取上一个节点。
            // node是“当前线程”对应的节点,这里就意味着“获取上一个等待锁的线程”。
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

说明:acquireQueued()的目的是从队列中获取锁。

 

2. shouldParkAfterFailedAcquire()

shouldParkAfterFailedAcquire()在AQS中实现,源码如下:

// 返回“当前线程是否应该阻塞”
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
    // 前继节点的状态
    int ws = pred.waitStatus;
    // 如果前继节点是SIGNAL状态,则意味这当前线程需要被unpark唤醒。此时,返回true。
    if (ws == Node.SIGNAL)
        return true;
    // 如果前继节点是“取消”状态,则设置 “当前节点”的 “当前前继节点”  为  “‘原前继节点’的前继节点”。
    if (ws > 0) {
        do {
            node.prev = pred = pred.prev;
        } while (pred.waitStatus > 0);
        pred.next = node;
    } else {
        // 如果前继节点为“0”或者“共享锁”状态,则设置前继节点为SIGNAL状态。
        compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
    }
    return false;
}

说明
(01) 关于waitStatus请参考下表(中扩号内为waitStatus的值),更多关于waitStatus的内容,可以参考前面的Node类的介绍。

CANCELLED[1]  -- 当前线程已被取消
SIGNAL[-1]    -- “当前线程的后继线程需要被unpark(唤醒)”。一般发生情况是:当前线程的后继线程处于阻塞状态,而当前线程被release或cancel掉,因此需要唤醒当前线程的后继线程。
CONDITION[-2] -- 当前线程(处在Condition休眠状态)在等待Condition唤醒
PROPAGATE[-3] -- (共享锁)其它线程获取到“共享锁”
[0]           -- 当前线程不属于上面的任何一种状态。

(02) shouldParkAfterFailedAcquire()通过以下规则,判断“当前线程”是否需要被阻塞。

规则1:如果前继节点状态为SIGNAL,表明当前节点需要被unpark(唤醒),此时则返回true。
规则2:如果前继节点状态为CANCELLED(ws>0),说明前继节点已经被取消,则通过先前回溯找到一个有效(非CANCELLED状态)的节点,并返回false。
规则3:如果前继节点状态为非SIGNAL、非CANCELLED,则设置前继的状态为SIGNAL,并返回false。

如果“规则1”发生,即“前继节点是SIGNAL”状态,则意味着“当前线程”需要被阻塞。接下来会调用parkAndCheckInterrupt()阻塞当前线程,直到当前先被唤醒才从parkAndCheckInterrupt()中返回。

 

3. parkAndCheckInterrupt())

parkAndCheckInterrupt()在AQS中实现,源码如下:

private final boolean parkAndCheckInterrupt() {
    // 通过LockSupport的park()阻塞“当前线程”。
    LockSupport.park(this);
    // 返回线程的中断状态。
    return Thread.interrupted();
}

说明parkAndCheckInterrupt()的作用是阻塞当前线程,并且返回“线程被唤醒之后”的中断状态。
它会先通过LockSupport.park()阻塞“当前线程”,然后通过Thread.interrupted()返回线程的中断状态。

这里介绍一下线程被阻塞之后如何唤醒。一般有2种情况:
第1种情况:unpark()唤醒。“前继节点对应的线程”使用完锁之后,通过unpark()方式唤醒当前线程。
第2种情况:中断唤醒。其它线程通过interrupt()中断当前线程。

补充:LockSupport()中的park(),unpark()的作用 和 Object中的wait(),notify()作用类似,是阻塞/唤醒。
它们的用法不同,park(),unpark()是轻量级的,而wait(),notify()是必须先通过Synchronized获取同步锁。
关于LockSupport,我们会在之后的章节再专门进行介绍!

 

4. 再次tryAcquire()

了解了shouldParkAfterFailedAcquire()和parkAndCheckInterrupt()函数之后。我们接着分析acquireQueued()的for循环部分。

final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
    setHead(node);
    p.next = null; // help GC
    failed = false;
    return interrupted;
}

说明
(01) 通过node.predecessor()获取前继节点。predecessor()就是返回node的前继节点,若对此有疑惑可以查看下面关于Node类的介绍。
(02) p == head && tryAcquire(arg)
       首先,判断“前继节点”是不是CHL表头。如果是的话,则通过tryAcquire()尝试获取锁。
       其实,这样做的目的是为了“让当前线程获取锁”,但是为什么需要先判断p==head呢?理解这个对理解“公平锁”的机制很重要,因为这么做的原因就是为了保证公平性!
       (a) 前面,我们在shouldParkAfterFailedAcquire()我们判断“当前线程”是否需要阻塞;
       (b) 接着,“当前线程”阻塞的话,会调用parkAndCheckInterrupt()来阻塞线程。当线程被解除阻塞的时候,我们会返回线程的中断状态。而线程被解决阻塞,可能是由于“线程被中断”,也可能是由于“其它线程调用了该线程的unpark()函数”。
       (c) 再回到p==head这里。如果当前线程是因为其它线程调用了unpark()函数而被唤醒,那么唤醒它的线程,应该是它的前继节点所对应的线程(关于这一点,后面在“释放锁”的过程中会看到)。 OK,是前继节点调用unpark()唤醒了当前线程!
            此时,再来理解p==head就很简单了:当前继节点是CLH队列的头节点,并且它释放锁之后;就轮到当前节点获取锁了。然后,当前节点通过tryAcquire()获取锁;获取成功的话,通过setHead(node)设置当前节点为头节点,并返回。
       总之,如果“前继节点调用unpark()唤醒了当前线程”并且“前继节点是CLH表头”,此时就是满足p==head,也就是符合公平性原则的。否则,如果当前线程是因为“线程被中断”而唤醒,那么显然就不是公平了。这就是为什么说p==head就是保证公平性!


小结acquireQueued()的作用就是“当前线程”会根据公平性原则进行阻塞等待,直到获取锁为止;并且返回当前线程在等待过程中有没有并中断过。

 

四. selfInterrupt()

selfInterrupt()是AQS中实现,源码如下:

private static void selfInterrupt() {
    Thread.currentThread().interrupt();
}

说明selfInterrupt()的代码很简单,就是“当前线程”自己产生一个中断。但是,为什么需要这么做呢?
这必须结合acquireQueued()进行分析。如果在acquireQueued()中,当前线程被中断过,则执行selfInterrupt();否则不会执行。

在acquireQueued()中,即使是线程在阻塞状态被中断唤醒而获取到cpu执行权利;但是,如果该线程的前面还有其它等待锁的线程,根据公平性原则,该线程依然无法获取到锁。它会再次阻塞! 该线程再次阻塞,直到该线程被它的前面等待锁的线程锁唤醒;线程才会获取锁,然后“真正执行起来”!
也就是说,在该线程“成功获取锁并真正执行起来”之前,它的中断会被忽略并且中断标记会被清除! 因为在parkAndCheckInterrupt()中,我们线程的中断状态时调用了Thread.interrupted()。该函数不同于Thread的isInterrupted()函数,isInterrupted()仅仅返回中断状态,而interrupted()在返回当前中断状态之后,还会清除中断状态。 正因为之前的中断状态被清除了,所以这里需要调用selfInterrupt()重新产生一个中断!

 

小结selfInterrupt()的作用就是当前线程自己产生一个中断。

 

总结

再回过头看看acquire()函数,它最终的目的是获取锁!

public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}

(01) 先是通过tryAcquire()尝试获取锁。获取成功的话,直接返回;尝试失败的话,再通过acquireQueued()获取锁。
(02) 尝试失败的情况下,会先通过addWaiter()来将“当前线程”加入到"CLH队列"末尾;然后调用acquireQueued(),在CLH队列中排序等待获取锁,在此过程中,线程处于休眠状态。直到获取锁了才返回。 如果在休眠等待过程中被中断过,则调用selfInterrupt()来自己产生一个中断。

 


更多内容

1. Java多线程系列--“JUC锁”01之 框架

2. Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock

3. Java多线程系列目录(共xx篇)

posted on 2014-01-20 22:05 如果天空不死 阅读(...) 评论(...) 编辑 收藏