JavaAQS源码解读-AbstractQueuedSynchronizer

莫张周刘王 发表于 2023-5-25 13:21:32

AQS源码解读----AbstractQueuedSynchronizer

36 package cn.com.pep;
37 import java.util.concurrent.TimeUnit;
38 import java.util.concurrent.locks.AbstractOwnableSynchronizer;
39 import java.util.concurrent.locks.Condition;
40 import java.util.concurrent.locks.LockSupport;
41 import java.util.ArrayList;
42 import java.util.Collection;
43 import java.util.Date;
44
45 import sun.misc.Unsafe;
46
47 /**
48* Provides a framework for implementing blocking locks and related
49* synchronizers (semaphores, events, etc) that rely on
50* first-in-first-out (FIFO) wait queues.This class is designed to
51* be a useful basis for most kinds of synchronizers that rely on a
52* single atomic {@code int} value to represent state. Subclasses
53* must define the protected methods that change this state, and which
54* define what that state means in terms of this object being acquired
55* or released.Given these, the other methods in this class carry
56* out all queuing and blocking mechanics. Subclasses can maintain
57* other state fields, but only the atomically updated {@code int}
58* value manipulated using methods {@link #getState}, {@link
59* #setState} and {@link #compareAndSetState} is tracked with respect
60* to synchronization.
61*
62* Subclasses should be defined as non-public internal helper
63* classes that are used to implement the synchronization properties
64* of their enclosing class.Class
65* {@code AbstractQueuedSynchronizer} does not implement any
66* synchronization interface.Instead it defines methods such as
67* {@link #acquireInterruptibly} that can be invoked as
68* appropriate by concrete locks and related synchronizers to
69* implement their public methods.
70*
71* This class supports either or both a default exclusive
72* mode and a shared mode. When acquired in exclusive mode,
73* attempted acquires by other threads cannot succeed. Shared mode
74* acquires by multiple threads may (but need not) succeed. This class
75* does not "understand" these differences except in the
76* mechanical sense that when a shared mode acquire succeeds, the next
77* waiting thread (if one exists) must also determine whether it can
78* acquire as well. Threads waiting in the different modes share the
79* same FIFO queue. Usually, implementation subclasses support only
80* one of these modes, but both can come into play for example in a
81* {@link ReadWriteLock}. Subclasses that support only exclusive or
82* only shared modes need not define the methods supporting the unused mode.
83*
84* This class defines a nested {@link ConditionObject} class that
85* can be used as a {@link Condition} implementation by subclasses
86* supporting exclusive mode for which method {@link
87* #isHeldExclusively} reports whether synchronization is exclusively
88* held with respect to the current thread, method {@link #release}
89* invoked with the current {@link #getState} value fully releases
90* this object, and {@link #acquire}, given this saved state value,
91* eventually restores this object to its previous acquired state.No
92* {@code AbstractQueuedSynchronizer} method otherwise creates such a
93* condition, so if this constraint cannot be met, do not use it.The
94* behavior of {@link ConditionObject} depends of course on the
95* semantics of its synchronizer implementation.
96*
97* This class provides inspection, instrumentation, and monitoring
98* methods for the internal queue, as well as similar methods for
99* condition objects. These can be exported as desired into classes
100* using an {@code AbstractQueuedSynchronizer} for their
101* synchronization mechanics.
102*
103* Serialization of this class stores only the underlying atomic
104* integer maintaining state, so deserialized objects have empty
105* thread queues. Typical subclasses requiring serializability will
106* define a {@code readObject} method that restores this to a known
107* initial state upon deserialization.
108*
109* <h3>Usage</h3>
110*
111* To use this class as the basis of a synchronizer, redefine the
112* following methods, as applicable, by inspecting and/or modifying
113* the synchronization state using {@link #getState}, {@link
114* #setState} and/or {@link #compareAndSetState}:
115*
116* <ul>
117* <li> {@link #tryAcquire}
118* <li> {@link #tryRelease}
119* <li> {@link #tryAcquireShared}
120* <li> {@link #tryReleaseShared}
121* <li> {@link #isHeldExclusively}
122* </ul>
123*
124* Each of these methods by default throws {@link
125* UnsupportedOperationException}.Implementations of these methods
126* must be internally thread-safe, and should in general be short and
127* not block. Defining these methods is the only supported
128* means of using this class. All other methods are declared
129* {@code final} because they cannot be independently varied.
130*
131* You may also find the inherited methods from {@link
132* AbstractOwnableSynchronizer} useful to keep track of the thread
133* owning an exclusive synchronizer.You are encouraged to use them
134* -- this enables monitoring and diagnostic tools to assist users in
135* determining which threads hold locks.
136*
137* Even though this class is based on an internal FIFO queue, it
138* does not automatically enforce FIFO acquisition policies.The core
139* of exclusive synchronization takes the form:
140*
141* <pre>
142* Acquire:
143* while (!tryAcquire(arg)) {
144* enqueue thread if it is not already queued;
145* possibly block current thread;
146* }
147*
148* Release:
149* if (tryRelease(arg))
150* unblock the first queued thread;
151* </pre>
152*
153* (Shared mode is similar but may involve cascading signals.)
154*
155* Because checks in acquire are invoked before
156* enqueuing, a newly acquiring thread may barge ahead of
157* others that are blocked and queued.However, you can, if desired,
158* define {@code tryAcquire} and/or {@code tryAcquireShared} to
159* disable barging by internally invoking one or more of the inspection
160* methods, thereby providing a fair FIFO acquisition order.
161* In particular, most fair synchronizers can define {@code tryAcquire}
162* to return {@code false} if {@link #hasQueuedPredecessors} (a method
163* specifically designed to be used by fair synchronizers) returns
164* {@code true}.Other variations are possible.
165*
166* Throughput and scalability are generally highest for the
167* default barging (also known as greedy,
168* renouncement, and convoy-avoidance) strategy.
169* While this is not guaranteed to be fair or starvation-free, earlier
170* queued threads are allowed to recontend before later queued
171* threads, and each recontention has an unbiased chance to succeed
172* against incoming threads.Also, while acquires do not
173* "spin" in the usual sense, they may perform multiple
174* invocations of {@code tryAcquire} interspersed with other
175* computations before blocking.This gives most of the benefits of
176* spins when exclusive synchronization is only briefly held, without
177* most of the liabilities when it isn't. If so desired, you can
178* augment this by preceding calls to acquire methods with
179* "fast-path" checks, possibly prechecking {@link #hasContended}
180* and/or {@link #hasQueuedThreads} to only do so if the synchronizer
181* is likely not to be contended.
182*
183* This class provides an efficient and scalable basis for
184* synchronization in part by specializing its range of use to
185* synchronizers that can rely on {@code int} state, acquire, and
186* release parameters, and an internal FIFO wait queue. When this does
187* not suffice, you can build synchronizers from a lower level using
188* {@link java.util.concurrent.atomic atomic} classes, your own custom
189* {@link java.util.Queue} classes, and {@link LockSupport} blocking
190* support.
191*
192* <h3>Usage Examples</h3>
193*
194* Here is a non-reentrant mutual exclusion lock class that uses
195* the value zero to represent the unlocked state, and one to
196* represent the locked state. While a non-reentrant lock
197* does not strictly require recording of the current owner
198* thread, this class does so anyway to make usage easier to monitor.
199* It also supports conditions and exposes
200* one of the instrumentation methods:
201*
202*<pre> {@code
203* class Mutex implements Lock, java.io.Serializable {
204*
205* // Our internal helper class
206* private static class Sync extends AbstractQueuedSynchronizer {
207* // Reports whether in locked state
208* protected boolean isHeldExclusively() {
209* return getState() == 1;
210* }
211*
212* // Acquires the lock if state is zero
213* public boolean tryAcquire(int acquires) {
214* assert acquires == 1; // Otherwise unused
215* if (compareAndSetState(0, 1)) {
216* setExclusiveOwnerThread(Thread.currentThread());
217* return true;
218* }
219* return false;
220* }
221*
222* // Releases the lock by setting state to zero
223* protected boolean tryRelease(int releases) {
224* assert releases == 1; // Otherwise unused
225* if (getState() == 0) throw new IllegalMonitorStateException();
226* setExclusiveOwnerThread(null);
227* setState(0);
228* return true;
229* }
230*
231* // Provides a Condition
232* Condition newCondition() { return new ConditionObject(); }
233*
234* // Deserializes properly
235* private void readObject(ObjectInputStream s)
236* throws IOException, ClassNotFoundException {
237* s.defaultReadObject();
238* setState(0); // reset to unlocked state
239* }
240* }
241*
242* // The sync object does all the hard work. We just forward to it.
243* private final Sync sync = new Sync();
244*
245* public void lock() { sync.acquire(1); }
246* public boolean tryLock() { return sync.tryAcquire(1); }
247* public void unlock() { sync.release(1); }
248* public Condition newCondition() { return sync.newCondition(); }
249* public boolean isLocked() { return sync.isHeldExclusively(); }
250* public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
251* public void lockInterruptibly() throws InterruptedException {
252* sync.acquireInterruptibly(1);
253* }
254* public boolean tryLock(long timeout, TimeUnit unit)
255* throws InterruptedException {
256* return sync.tryAcquireNanos(1, unit.toNanos(timeout));
257* }
258* }}</pre>
259*
260* Here is a latch class that is like a
261* {@link java.util.concurrent.CountDownLatch CountDownLatch}
262* except that it only requires a single {@code signal} to
263* fire. Because a latch is non-exclusive, it uses the {@code shared}
264* acquire and release methods.
265*
266*<pre> {@code
267* class BooleanLatch {
268*
269* private static class Sync extends AbstractQueuedSynchronizer {
270* boolean isSignalled() { return getState() != 0; }
271*
272* protected int tryAcquireShared(int ignore) {
273* return isSignalled() ? 1 : -1;
274* }
275*
276* protected boolean tryReleaseShared(int ignore) {
277* setState(1);
278* return true;
279* }
280* }
281*
282* private final Sync sync = new Sync();
283* public boolean isSignalled() { return sync.isSignalled(); }
284* public void signal() { sync.releaseShared(1); }
285* public void await() throws InterruptedException {
286* sync.acquireSharedInterruptibly(1);
287* }
288* }}</pre>
289*
290* @since 1.5
291* @author Doug Lea
292*/
293 public abstract class AbstractQueuedSynchronizer
294 extends AbstractOwnableSynchronizer
295 implements java.io.Serializable {
296
297 private static final long serialVersionUID = 7373984972572414691L;
298
299 /**
300 * Creates a new {@code AbstractQueuedSynchronizer} instance
301 * with initial synchronization state of zero.
302 */
303 protected AbstractQueuedSynchronizer() { }
304
305 /**
306 * Wait queue node class.
307 *
308 * The wait queue is a variant of a "CLH" (Craig, Landin, and
309 * Hagersten) lock queue. CLH locks are normally used for
310 * spinlocks.We instead use them for blocking synchronizers, but
311 * use the same basic tactic of holding some of the control
312 * information about a thread in the predecessor of its node.A
313 * "status" field in each node keeps track of whether a thread
314 * should block.A node is signalled when its predecessor
315 * releases.Each node of the queue otherwise serves as a
316 * specific-notification-style monitor holding a single waiting
317 * thread. The status field does NOT control whether threads are
318 * granted locks etc though.A thread may try to acquire if it is
319 * first in the queue. But being first does not guarantee success;
320 * it only gives the right to contend.So the currently released
321 * contender thread may need to rewait.
322 *
323 * To enqueue into a CLH lock, you atomically splice it in as new
324 * tail. To dequeue, you just set the head field.
325 * <pre>
326 * +------+prev +-----+ +-----+
327 * head | | <---- | | <---- | |tail
328 * +------+ +-----+ +-----+
329 * </pre>
330 *
331 * Insertion into a CLH queue requires only a single atomic
332 * operation on "tail", so there is a simple atomic point of
333 * demarcation from unqueued to queued. Similarly, dequeuing
334 * involves only updating the "head". However, it takes a bit
335 * more work for nodes to determine who their successors are,
336 * in part to deal with possible cancellation due to timeouts
337 * and interrupts.
338 *
339 * The "prev" links (not used in original CLH locks), are mainly
340 * needed to handle cancellation. If a node is cancelled, its
341 * successor is (normally) relinked to a non-cancelled
342 * predecessor. For explanation of similar mechanics in the case
343 * of spin locks, see the papers by Scott and Scherer at
344 * http://www.cs.rochester.edu/u/scott/synchronization/
345 *
346 * We also use "next" links to implement blocking mechanics.
347 * The thread id for each node is kept in its own node, so a
348 * predecessor signals the next node to wake up by traversing
349 * next link to determine which thread it is.Determination of
350 * successor must avoid races with newly queued nodes to set
351 * the "next" fields of their predecessors.This is solved
352 * when necessary by checking backwards from the atomically
353 * updated "tail" when a node's successor appears to be null.
354 * (Or, said differently, the next-links are an optimization
355 * so that we don't usually need a backward scan.)
356 *
357 * Cancellation introduces some conservatism to the basic
358 * algorithms.Since we must poll for cancellation of other
359 * nodes, we can miss noticing whether a cancelled node is
360 * ahead or behind us. This is dealt with by always unparking
361 * successors upon cancellation, allowing them to stabilize on
362 * a new predecessor, unless we can identify an uncancelled
363 * predecessor who will carry this responsibility.
364 *
365 * CLH queues need a dummy header node to get started. But
366 * we don't create them on construction, because it would be wasted
367 * effort if there is never contention. Instead, the node
368 * is constructed and head and tail pointers are set upon first
369 * contention.
370 *
371 * Threads waiting on Conditions use the same nodes, but
372 * use an additional link. Conditions only need to link nodes
373 * in simple (non-concurrent) linked queues because they are
374 * only accessed when exclusively held.Upon await, a node is
375 * inserted into a condition queue.Upon signal, the node is
376 * transferred to the main queue.A special value of status
377 * field is used to mark which queue a node is on.
378 *
379 * Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
380 * Scherer and Michael Scott, along with members of JSR-166
381 * expert group, for helpful ideas, discussions, and critiques
382 * on the design of this class.
383 */
384 static final class Node {
385 /** Marker to indicate a node is waiting in shared mode */
386 static final Node SHARED = new Node();
387 /** Marker to indicate a node is waiting in exclusive mode */
388 static final Node EXCLUSIVE = null;
389
390 /** waitStatus value to indicate thread has cancelled */
391 static final int CANCELLED =1;
392 /** waitStatus value to indicate successor's thread needs unparking */
393 static final int SIGNAL = -1;
394 /** waitStatus value to indicate thread is waiting on condition */
395 static final int CONDITION = -2;
396 /**
397 * waitStatus value to indicate the next acquireShared should
398 * unconditionally propagate
399 */
400 static final int PROPAGATE = -3;
401
402 /**
403 * Status field, taking on only the values:
404 * SIGNAL: The successor of this node is (or will soon be)
405 * blocked (via park), so the current node must
406 * unpark its successor when it releases or
407 * cancels. To avoid races, acquire methods must
408 * first indicate they need a signal,
409 * then retry the atomic acquire, and then,
410 * on failure, block.
411 * CANCELLED:This node is cancelled due to timeout or interrupt.
412 * Nodes never leave this state. In particular,
413 * a thread with cancelled node never again blocks.
414 * CONDITION:This node is currently on a condition queue.
415 * It will not be used as a sync queue node
416 * until transferred, at which time the status
417 * will be set to 0. (Use of this value here has
418 * nothing to do with the other uses of the
419 * field, but simplifies mechanics.)
420 * PROPAGATE:A releaseShared should be propagated to other
421 * nodes. This is set (for head node only) in
422 * doReleaseShared to ensure propagation
423 * continues, even if other operations have
424 * since intervened.
425 * 0: None of the above
426 *
427 * The values are arranged numerically to simplify use.
428 * Non-negative values mean that a node doesn't need to
429 * signal. So, most code doesn't need to check for particular
430 * values, just for sign.
431 *
432 * The field is initialized to 0 for normal sync nodes, and
433 * CONDITION for condition nodes.It is modified using CAS
434 * (or when possible, unconditional volatile writes).
435 */
436 volatile int waitStatus;
437
438 /**
439 * Link to predecessor node that current node/thread relies on
440 * for checking waitStatus. Assigned during enqueuing, and nulled
441 * out (for sake of GC) only upon dequeuing.Also, upon
442 * cancellation of a predecessor, we short-circuit while
443 * finding a non-cancelled one, which will always exist
444 * because the head node is never cancelled: A node becomes
445 * head only as a result of successful acquire. A
446 * cancelled thread never succeeds in acquiring, and a thread only
447 * cancels itself, not any other node.
448 */
449 volatile Node prev;
450
451 /**
452 * Link to the successor node that the current node/thread
453 * unparks upon release. Assigned during enqueuing, adjusted
454 * when bypassing cancelled predecessors, and nulled out (for
455 * sake of GC) when dequeued.The enq operation does not
456 * assign next field of a predecessor until after attachment,
457 * so seeing a null next field does not necessarily mean that
458 * node is at end of queue. However, if a next field appears
459 * to be null, we can scan prev's from the tail to
460 * double-check.The next field of cancelled nodes is set to
461 * point to the node itself instead of null, to make life
462 * easier for isOnSyncQueue.
463 */
464 volatile Node next;
465
466 /**
467 * The thread that enqueued this node.Initialized on
468 * construction and nulled out after use.
469 */
470 volatile Thread thread;
471
472 /**
473 * Link to next node waiting on condition, or the special
474 * value SHARED.Because condition queues are accessed only
475 * when holding in exclusive mode, we just need a simple
476 * linked queue to hold nodes while they are waiting on
477 * conditions. They are then transferred to the queue to
478 * re-acquire. And because conditions can only be exclusive,
479 * we save a field by using special value to indicate shared
480 * mode.
481 */
482 Node nextWaiter;
483
484 /**
485 * Returns true if node is waiting in shared mode.
486 */
487 final boolean isShared() {
488 return nextWaiter == SHARED;
489 }
490
491 /**
492 * Returns previous node, or throws NullPointerException if null.
493 * Use when predecessor cannot be null.The null check could
494 * be elided, but is present to help the VM.
495 *
496 * @return the predecessor of this node
497 */
498 final Node predecessor() throws NullPointerException {
499 Node p = prev;
500 if (p == null)
501 throw new NullPointerException();
502 else
503 return p;
504 }
505
506 Node() { // Used to establish initial head or SHARED marker
507 }
508
509 Node(Thread thread, Node mode) { // Used by addWaiter
510 this.nextWaiter = mode;
511 this.thread = thread;
512 }
513
514 Node(Thread thread, int waitStatus) { // Used by Condition
515 this.waitStatus = waitStatus;
516 this.thread = thread;
517 }
518 }
519
520 /**
521 * Head of the wait queue, lazily initialized.Except for
522 * initialization, it is modified only via method setHead.Note:
523 * If head exists, its waitStatus is guaranteed not to be
524 * CANCELLED.
525 */
526 private transient volatile Node head;
527
528 /**
529 * Tail of the wait queue, lazily initialized.Modified only via
530 * method enq to add new wait node.
531 */
532 private transient volatile Node tail;
533
534 /**
535 * The synchronization state.
536 */
537 private volatile int state;
538
539 /**
540 * Returns the current value of synchronization state.
541 * This operation has memory semantics of a {@code volatile} read.
542 * @return current state value
543 */
544 protected final int getState() {
545 return state;
546 }
547
548 /**
549 * Sets the value of synchronization state.
550 * This operation has memory semantics of a {@code volatile} write.
551 * @param newState the new state value
552 */
553 protected final void setState(int newState) {
554 state = newState;
555 }
556
557 /**
558 * Atomically sets synchronization state to the given updated
559 * value if the current state value equals the expected value.
560 * This operation has memory semantics of a {@code volatile} read
561 * and write.
562 *
563 * @param expect the expected value
564 * @param update the new value
565 * @return {@code true} if successful. False return indicates that the actual
566 * value was not equal to the expected value.
567 */
568 protected final boolean compareAndSetState(int expect, int update) {
569 // See below for intrinsics setup to support this
570 return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
571 }
572
573 // Queuing utilities
574
575 /**
576 * The number of nanoseconds for which it is faster to spin
577 * rather than to use timed park. A rough estimate suffices
578 * to improve responsiveness with very short timeouts.
579 */
580 static final long spinForTimeoutThreshold = 1000L;
581
582 /**
583 * Inserts node into queue, initializing if necessary. See picture above.
584 * @param node the node to insert
585 * @return node's predecessor
586 */
587 private Node enq(final Node node) {
588 /*"自旋"，将给定的节点插入到同步队列的尾部*/
589 for (;;) {
590 Node t = tail;
591 /*同步队列为空，则dummy哑节点作为同步队列的头结点head，并且将尾节点tail也指向头结点head*/
592 if (t == null) {
593 /*CAS操作，设置同步队列的头结点*/
594 if (compareAndSetHead(new Node())) {
595 /*将尾节点设置为头结点，进入下次"自旋"*/
596 tail = head;
597 }
598 }else {
599 /*尾部节点不为空，则进行正常添加动作*/
600 node.prev = t;
601 /*CAS操作，设置同步队列的头结点*/
602 if (compareAndSetTail(t, node)) {
603 t.next = node;
604 return t;
605 }
606 }
607 }
608 }
609
610 /**
611 * Creates and enqueues node for current thread and given mode.
612 * 以给定的模式包装当前线程节点，将当前节点加入到阻塞队列的队尾
613 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
614 * @return the new node
615 */
616 private Node addWaiter(Node mode) {
617 /*以给定的模式将当前线程包装成node节点*/
618 Node node = new Node(Thread.currentThread(), mode);
619
620 /*快速采用尾插法，将当前节点插入到同步队列的队尾*/
621 Node predNode = tail;
622 if (predNode != null) {
623 /*preNode <-- node*/
624 node.prev = predNode;
625 /*采用CAS将node设置阻塞队列的尾节点，设置成功，说明没有并发*/
626 if (compareAndSetTail(tail, node)) {
627 predNode.next = node;
628 /*尾插法插入成功，则直接返回当前节点*/
629 return node;
630 }
631 }
632 /*"自旋"将节点加入到队列的尾部，直到成功为止*/
633 enq(node);
634 return node;
635 }
636
637 /**
638 * Sets head of queue to be node, thus dequeuing. Called only by
639 * acquire methods.Also nulls out unused fields for sake of GC
640 * and to suppress unnecessary signals and traversals.
641 *
642 * @param node the node
643 */
644 private void setHead(Node node) {
645 head = node;
646 node.thread = null;
647 node.prev = null;
648 }
649
650 /**
651 * Wakes up node's successor, if one exists.
652 *
653 * @param node the node
654 */
655 private void unparkSuccessor(Node node) {
656 /*
657 * If status is negative (i.e., possibly needing signal) try
658 * to clear in anticipation of signalling.It is OK if this
659 * fails or if status is changed by waiting thread.
660 */
661 //在这里，这个节点其实是同步队列的头结点，头结点唤醒后继节点之后，使命就完成了，所以应该将其状态置为0
662 int ws = node.waitStatus;
663 if (ws < 0)
664 compareAndSetWaitStatus(node, ws, 0);
665
666 /*
667 * Thread to unpark is held in successor, which is normally
668 * just the next node.But if cancelled or apparently null,
669 * traverse backwards from tail to find the actual
670 * non-cancelled successor.
671 */
672 Node s = node.next;
673 //因为s.next相当于从同步队列的头部遍历所以可能会出现s == null的情况，上面分析过原因，不再赘述了。
674 if (s == null || s.waitStatus > 0) {
675 s = null;
676 //从同步队列的尾部向前遍历，找到当前node节点(头结点)的最近的有效后继节点
677 for (Node t = tail; t != null && t != node; t = t.prev)
678 if (t.waitStatus <= 0)
679 s = t;
680 }
681
682 /**
683 * 找到最近的有效后继节点，则唤醒后继节点中的线程在parkAndCheckInterrupt()方法上的阻塞，去尝试竞争共享资源，
684 * 这就体现了线程之间的协作，而在这个竞争的过程中也会忽略这个Node.CANCELLED状态的节点，这当前node节点也就放弃了竞争共享资源的机会，相当于出队了。
685 */
686 if (s != null)
687 LockSupport.unpark(s.thread);
688 }
689
690 /**
691 * Release action for shared mode -- signals successor and ensures
692 * propagation. (Note: For exclusive mode, release just amounts
693 * to calling unparkSuccessor of head if it needs signal.)
694 */
695 private void doReleaseShared() {
696 /*
697 * Ensure that a release propagates, even if there are other
698 * in-progress acquires/releases.This proceeds in the usual
699 * way of trying to unparkSuccessor of head if it needs
700 * signal. But if it does not, status is set to PROPAGATE to
701 * ensure that upon release, propagation continues.
702 * Additionally, we must loop in case a new node is added
703 * while we are doing this. Also, unlike other uses of
704 * unparkSuccessor, we need to know if CAS to reset status
705 * fails, if so rechecking.
706 */
707 for (;;) {
708 Node h = head;
709 if (h != null && h != tail) {
710 int ws = h.waitStatus;
711 if (ws == Node.SIGNAL) {
712 if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
713 continue; // loop to recheck cases
714 unparkSuccessor(h);
715 }
716 else if (ws == 0 &&
717 !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
718 continue; // loop on failed CAS
719 }
720 if (h == head) // loop if head changed
721 break;
722 }
723 }
724
725 /**
726 * Sets head of queue, and checks if successor may be waiting
727 * in shared mode, if so propagating if either propagate > 0 or
728 * PROPAGATE status was set.
729 *
730 * @param node the node
731 * @param propagate the return value from a tryAcquireShared
732 */
733 private void setHeadAndPropagate(Node node, int propagate) {
734 //后继节点成功获取了共享锁，队列的"旧head"还没有改变，将其保存下来，锁定到方法的局部变量做后序的判断使用；
735 Node h = head; // Record old head for check below
736 /**
737 * 将这个获取共享锁成功的后继节点设置为同步队列的“新head”,此时同步队列的head发生变化，此线程还未唤起任何线程。
738 */
739 setHead(node);
740 /**
741 * 1、h == null这个条件什么时候成立呢？仔细翻了下AQS中的源码发现：
742 * 这个setHeadAndPropagate()方法只在共享锁模式下，同步队列head的后继节点成功获取了共享锁才会调用。
743 * 获取到共享锁的当前线程是同步队列的头结点的后继节点，"旧head"有后继节点，说明同步队列不为空,那么"旧head"也必定不为空，
744 * 此方法中第一行通过h == head,在执行setHead(node)方法之前将"旧head"保存了下来，所以h == null必定不会成立,
745 * 至于为什么这么写呢? 查阅了下资料网上说"发现这个是防止空指针异常发生的标准写法（既如果要取一个对象的某个属性进行判断的时候，首先对这个对象进行null判断）。"
746 * 这说的过去吧?
747 *
748 * 2、(h = head) == null这个条件什么时候成立呢？
749 * 这个条件也是不可能成立的，下面这种情况应该是最常见的：
750 * （1）、例如有个Semaphore实例s初始化了2个许可，线程A首先调用s.acquire(2)申请了两个许可，成功申请到了许可；
751 * （2）、线程B调用了s.acquire()方法申请一个许可，申请失败，加入到同步队列；
752 * （3）、线程C调用了s.acquire()方法申请一个许可，申请失败，加入到同步队列；
753 *（4）、线程A调用了s.releaseShared(2)方法释放了两个许可，再调用doReleaseShared()方法，进行同步队列唤醒；
754 *（5）、首先唤醒了同步队列中的线程B，B线程获取到共享锁：
755 * a)、如果此时线程B还未setHead(Node)方法，还未改变同步队列的head头结点，那么线程A的唤醒工作就结束，也仅仅只是唤醒了同步队列中的线程B，
756 * 则必定有（h = head） == Node(C) ！= null成立，线程C的唤醒工作仍然需要线程B去执行；
757 * b)、如果此时线程B执行了SetHead(Node)方法，改变了同步队列的head头结点，那么线程A同时也会唤醒线程C，相当于线程A同时唤醒了线程B和线程C：
758 * 1）、如果线程C中的setHeadAndPropagate()在线程B前调用完毕（即线程C执行了setHead()方法改变了同步队列的head），那么 (h = head) == Node(C);
759 * 2）、如果线程C中的setHeadAndPropagate()在线程B之后才调用(即线程C此时还未执行setHead()方法，未改变同步队列的head)，那么 (h = head) == Node(B)
760 *所以综上所述，只要执行过addWaiter()方法，向同步队列中添加过线程，那么（h = head）== null必定不成立。只能理解为“防止空指针的标准写法”。
761 */
762 if (propagate > 0 || h == null || h.waitStatus < 0 ||
763 (h = head) == null || h.waitStatus < 0) {
764 Node s = node.next;
765 /**
766 * s == null这种情况是可能存在的，如果当前唤醒的这个node节点是同步队列的尾节点就可能出现node.next == null;
767 * s.isShared()指定是共享锁模式，当前线程获取共享锁之后，是需要尝试唤醒同步队列中的其它线程的。
768 */
769 if (s == null || s.isShared())
770 doReleaseShared();
771 }
772 }
773
774 // Utilities for various versions of acquire
775
776 /**
777 * Cancels an ongoing attempt to acquire.
778 * @param node the node
779 */
780 private void cancelAcquire(Node node) {
781 //当前节点为空，则说明当前线程永远不会被调度到了，所以直接返回
782 if (node == null) {
783 return;
784 }
785
786 /**
787 * 接下来将点前Node节点从同步队列出队，主要做以下几件事：
788 * 1、将当前节点不与任何线程绑定，设置当前节点为Node.CANCELLED状态；
789 * 2、将当前取消节点的前置非取消节点和后置非取消节点"链接"起来；
790 * 3、如果前置节点释放了锁，那么当前取消节点承担起后续节点的唤醒职责。
791 */
792
793 //1、取消当前节点与线程的绑定
794 node.thread = null;
795
796 //2、找到当前节点的有效前继节点pred
797 Node pred = node.prev;
798 while (pred.waitStatus > 0) {
799 //为什么双向链表从后往前遍历呢？而不是从前往后遍历呢？
800 node.prev = pred = pred.prev;
801 }
802
803 //用作CAS操作时候的条件判断需要使用的值
804 Node predNext = pred.next;
805
806 //3、将当前节点设置为取消状态
807 node.waitStatus = Node.CANCELLED;
808
809 /**
810 * 接下来就需要将当前取消节点的前后两个有效节点"链接"起来了，"达成让当前node节点出队的目的"。
811 * 这里按照node节点在同步队列中的不同位置分了三种情况：
812 * 1、node节点是同步队列的尾节点tail;
813 * 2、node节点既不是同步队列头结点head的后继节点，也不是尾节点tail;
814 * 3、node节点是同步队列头结点head的后继节点；
815 */
816
817 //1、node是尾节点，并且执行过程中没有并发，直接将pred设置为同步队列的tail
818 if (node == tail && compareAndSetTail(node, pred)) {
819 /*
820 * 此时pred已经设置为同步队列的tail，需要通过CAS操作，将pred的next指向null,没有节点再引用node，就完成了node节点的出队
821 * 可以看出出队操作会破坏这个同步队列的next指针，这应该“向链表从后往前遍历呢？而不是从前往后遍历呢”的原因吧？
822 */
823 compareAndSetNext(pred, predNext, null);
824 }else {
825 /*
826 * 2、node不是尾节点，也不是头结点head的后继节点,那么当前节点node出队以后，node的有效前继结点pred，
827 *就有义务在它自身释放资源的时候，唤醒node的有效后继节点successor,即将pred的状态设置为Node.SIGNAL;
828 */
829 int ws;
830 //能执行到这里，说明当前node节点不是head的后继节点，也不是同步队列tail节点
831 if (pred != head &&
832 ((ws = pred.waitStatus) == Node.SIGNAL ||
833 //前继节点状态虽然有效但不是SIGNAL,采用CAS操作设置为SIGNAL确保后继有效节点可以被唤醒
834 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
835 pred.thread != null) {
836 Node next = node.next;
837 //只负责唤醒有效后继节点
838 if (next != null && next.waitStatus <= 0) {
839 /**
840 * 下面这段代码相当于将pred-->next，我们提到这个同步队列是个双向队列，那么pred<--next这是谁执行的呢？
841 * 答案是其他线程：其它线程在获取共享资源在同步队列中阻塞的时候，调用shouldParkAfterFailedAcquire()方法，
842 * 从后向前遍历队列，寻找能唤醒它的有效前继节点，当找到node的时候，因为它的状态已经是Node.CANCELLED,所以会忽略node节点，
843 * 直到遍历到有效前继节点pred,将next.prev执行pred,即next--->pred，没有节点再引用node节点，所以node节点至此才完成出队。
844 */
845 compareAndSetNext(pred, predNext, next);
846 }
847 }else {
848 //3、说明node节点是同步队列head的后继节点，调用unparkSuccessor(Node)"出队"。
849 unparkSuccessor(node);
850 }
851
852 node.next = node;//help GC
853 }
854 }
855
856 /**
857 * Checks and updates status for a node that failed to acquire.
858 * Returns true if thread should block. This is the main signal
859 * control in all acquire loops.Requires that pred == node.prev.
860 *
861 * @param pred node's predecessor holding status
862 * @param node the node
863 * @return {@code true} if thread should block
864 */
865 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
866 int ws = pred.waitStatus;
867 /*判断前驱结点的状态，只有前驱结点的状态为SIGNAL,后继节点才能被唤醒，所以其可以安心地挂起来了*/
868 if (ws == node.waitStatus) {
869 return true;
870 }
871
872 /*ws>0表示前驱结点中的线程已经被取消调度了，则认为其是无效节点，继续向前查找，直至找到有效状态的节点*/
873 if (ws > 0) {
874 do {
875 //前驱结点已经被取消，则将前驱结点设置为pred = pred.prev
876 node.prev = pred = pred.prev;
877 //不断遍历，直到找到第一个不是取消状态的节点
878 } while (pred.waitStatus > 0);
879 //
880 pred.next = node;
881 }else {
882 /*前驱结点状态正常，将前驱结点状态设置为SIGNAL,则前驱结点释放资源的时候，就可以尝试唤醒它的后继节点了*/
883 compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
884 }
885 return false;
886 }
887
888 /**
889 * Convenience method to interrupt current thread.
890 */
891 static void selfInterrupt() {
892 Thread.currentThread().interrupt();
893 }
894
895 /**
896 * Convenience method to park and then check if interrupted
897 *
898 * @return {@code true} if interrupted
899 */
900 private final boolean parkAndCheckInterrupt() {
901 //将当前线程挂起
902 LockSupport.park(this);
903 //检测当前线程的中断状态，并且会清除线程的中断标记
904 return Thread.interrupted();
905 }
906
907 /*
908 * Various flavors of acquire, varying in exclusive/shared and
909 * control modes.Each is mostly the same, but annoyingly
910 * different.Only a little bit of factoring is possible due to
911 * interactions of exception mechanics (including ensuring that we
912 * cancel if tryAcquire throws exception) and other control, at
913 * least not without hurting performance too much.
914 */
915
916 /**
917 * Acquires in exclusive uninterruptible mode for thread already in
918 * queue. Used by condition wait methods as well as acquire.
919 * 使线程阻塞在同步队列中等待获取资源，直到获取资源成功才返回，此过程中线程发生了中断就返回true,否则就返回false
920 * @param node the node
921 * @param arg the acquire argument
922 * @return {@code true} if interrupted while waiting
923 */
924 final boolean acquireQueued(final Node node, int arg) {
925 /*标记等待过程中是否发生了异常*/
926 boolean failed = true;
927
928 try {
929 /*标记线程阻塞的过程中是否发生了中断*/
930 boolean interrupted = false;
931 /*线程自旋阻塞*/
932 for(;;){
933 /*获取当前节点的前驱结点*/
934 final Node p = node.predecessor();
935 /*前驱结点是头结点，则说明当前线程有资格获取共享资源，尝试获取，获取成功，将当前节点设置为头结点*/
936 if (p == head && tryAcquire(arg)) {
937 /*将当前节点设置为头结点*/
938 setHead(node);
939 p.next = null; //help GC
940 failed = false;
941 return interrupted;
942 }
943
944 /*判断当前线程是否可以挂起*/
945 if (shouldParkAfterFailedAcquire(p, node)
946 /*当前线程可以挂起，则挂起线程，并且线程被unpark()唤醒，检查线程的状态*/
947 && parkAndCheckInterrupt()) {
948 interrupted = true;
949 }
950 }
951 }finally{
952 if (failed) {
953 /*阻塞获取同步资源的时候，发生了异常，取消当前线程的在同步队列中的排队*/
954 cancelAcquire(node);
955 }
956 }
957 }
958
959 /**
960 * Acquires in exclusive interruptible mode.
961 * @param arg the acquire argument
962 */
963 //尝试获取锁，并响应中断
964 private void doAcquireInterruptibly(int arg)
965 throws InterruptedException {
966 //将当前线程包装成Node节点，加入到同步队列sync queue中
967 final Node node = addWaiter(Node.EXCLUSIVE);
968 //标记是否获取锁的过程中是否发生了异常
969 boolean failed = true;
970 try {
971 //自旋
972 for (;;) {
973 //获取当前节点的前驱结点
974 final Node p = node.predecessor();
975 //如果当前节点的前驱结点是头结点head，尝试获取锁，当前线程才有资格获取排他锁（头结点head是个哑结点，不代表任何线程）
976 if (p == head && tryAcquire(arg)) {
977 //当前节点获取锁成功，将当前节点设置为头结点head
978 setHead(node);
979 //将原来的头结点从同步队列sync queue中移除
980 p.next = null; // help GC
981 failed = false;
982 return;
983 }
984 //尝试获取锁失败，判断是否可以将当前线程安全地挂起，只有当前线程有效的前驱结点状态为Node.SIGNAL，当前线程才可以安全地被挂起，后续也会被及时地唤醒
985 if (shouldParkAfterFailedAcquire(p, node) &&
986 //将当前线程挂起，并等他唤醒的时候判断是否发生了线程中断，发生了线程中断，则进入finally块中，取消当前线程对锁的尝试获取
987 parkAndCheckInterrupt())
988 throw new InterruptedException();
989 }
990 } finally {
991 if (failed)
992 cancelAcquire(node);
993 }
994 }
995
996 /**
997 * Acquires in exclusive timed mode.
998 *
999 * @param arg the acquire argument
1000 * @param nanosTimeout max wait time
1001 * @return {@code true} if acquired
1002 */
1003 private boolean doAcquireNanos(int arg, long nanosTimeout)
1004 throws InterruptedException {
1005 //指定的时间<=0则直接返回false
1006 if (nanosTimeout <= 0L)
1007 return false;
1008 //计算当前线程阻塞的截至时间
1009 final long deadline = System.nanoTime() + nanosTimeout;
1010 //将当前线程包装成Node节点，加入到同步队列sync queue中
1011 final Node node = addWaiter(Node.EXCLUSIVE);
1012 boolean failed = true;
1013 try {
1014 //自旋尝试获取锁
1015 for (;;) {
1016 //获取当前节点的前驱结点
1017 final Node p = node.predecessor();
1018 //前驱结点是头结点，则当前节点有资格尝试获取锁，则尝试获取锁
1019 if (p == head && tryAcquire(arg)) {
1020 //获取锁成功，则将当前节点设置为头结点head
1021 setHead(node);
1022 //将同步队列sync queue中原来的头结点移除队列
1023 p.next = null; // help GC
1024 //表示获取锁的过程没有发生异常
1025 failed = false;
1026 return true;
1027 }
1028 //计算剩余的等待时间
1029 nanosTimeout = deadline - System.nanoTime();
1030 //剩余的等待时间<=0，直接返回false
1031 if (nanosTimeout <= 0L)
1032 return false;
1033 //当前线程获取锁失败，判断是否可以将当前线程安全地挂起
1034 if (shouldParkAfterFailedAcquire(p, node) &&
1035 //如果剩余的等待时间<= spinForTimeoutThreshold，则不用将当前线程挂起，进行自旋即可
1036 nanosTimeout > spinForTimeoutThreshold)
1037 LockSupport.parkNanos(this, nanosTimeout);
1038 //获取锁的过程中发生了中断，则直接抛出InterruptedException异常
1039 if (Thread.interrupted())
1040 throw new InterruptedException();
1041 }
1042 } finally {
1043 //获取锁的过程中发生了异常，则将当前线程取消
1044 if (failed)
1045 cancelAcquire(node);
1046 }
1047 }
1048
1049 /**
1050 * Acquires in shared uninterruptible mode.
1051 * @param arg the acquire argument
1052 */
1053 private void doAcquireShared(int arg) {
1054 //将竞争共享资源失败的线程加入到同步队列中，并标记为共享模式
1055 final Node node = addWaiter(Node.SHARED);
1056 //标记阻塞获取资源的过程中是否发生了异常
1057 boolean failed = true;
1058
1059 try {
1060 //标记阻塞获取资源的过程中，是否发生了线程中断请求
1061 boolean interrupted = false;
1062 //线程阻塞等待获取资源，被有效前继节点唤醒后，尝试竞争共享资源
1063 for(;;){
1064 /**
1065 * 当前线程被唤醒之后，什么时候有资格竞争共享资源呢？
1066 * 之后当它的前继节点是头结点（头结点是当前持有共享资源的线程），在唤醒后继节点的过程中，可能释放了资源，所以后继节点尝试获取一次共享资源。
1067 */
1068 final Node p = node.predecessor();
1069 if (p == head) {
1070 int r = tryAcquireShared(arg);
1071 if (r >= 0) {
1072 setHeadAndPropagate(node, r);
1073 p.next = null;//help GC
1074 if (interrupted) {
1075 selfInterrupt();
1076 }
1077 failed = false;
1078 return;
1079 }
1080 }
1081
1082 if (shouldParkAfterFailedAcquire(p, node) &&
1083 parkAndCheckInterrupt()) {
1084 interrupted = true;
1085 }
1086 }
1087 } finally{
1088 if (failed) {
1089 cancelAcquire(node);
1090 }
1091 }
1092 }
1093
1094 /**
1095 * Acquires in shared interruptible mode.
1096 * @param arg the acquire argument
1097 */
1098 private void doAcquireSharedInterruptibly(int arg)
1099 throws InterruptedException {
1100 final Node node = addWaiter(Node.SHARED);
1101 boolean failed = true;
1102 try {
1103 for (;;) {
1104 final Node p = node.predecessor();
1105 if (p == head) {
1106 int r = tryAcquireShared(arg);
1107 if (r >= 0) {
1108 setHeadAndPropagate(node, r);
1109 p.next = null; // help GC
1110 failed = false;
1111 return;
1112 }
1113 }
1114 if (shouldParkAfterFailedAcquire(p, node) &&
1115 parkAndCheckInterrupt())
1116 throw new InterruptedException();
1117 }
1118 } finally {
1119 if (failed)
1120 cancelAcquire(node);
1121 }
1122 }
1123
1124 /**
1125 * Acquires in shared timed mode.
1126 *
1127 * @param arg the acquire argument
1128 * @param nanosTimeout max wait time
1129 * @return {@code true} if acquired
1130 */
1131 private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
1132 throws InterruptedException {
1133 if (nanosTimeout <= 0L)
1134 return false;
1135 final long deadline = System.nanoTime() + nanosTimeout;
1136 final Node node = addWaiter(Node.SHARED);
1137 boolean failed = true;
1138 try {
1139 for (;;) {
1140 final Node p = node.predecessor();
1141 if (p == head) {
1142 int r = tryAcquireShared(arg);
1143 if (r >= 0) {
1144 setHeadAndPropagate(node, r);
1145 p.next = null; // help GC
1146 failed = false;
1147 return true;
1148 }
1149 }
1150 nanosTimeout = deadline - System.nanoTime();
1151 if (nanosTimeout <= 0L)
1152 return false;
1153 if (shouldParkAfterFailedAcquire(p, node) &&
1154 nanosTimeout > spinForTimeoutThreshold)
1155 LockSupport.parkNanos(this, nanosTimeout);
1156 if (Thread.interrupted())
1157 throw new InterruptedException();
1158 }
1159 } finally {
1160 if (failed)
1161 cancelAcquire(node);
1162 }
1163 }
1164
1165 // Main exported methods
1166
1167 /**
1168 * Attempts to acquire in exclusive mode. This method should query
1169 * if the state of the object permits it to be acquired in the
1170 * exclusive mode, and if so to acquire it.
1171 *
1172 * This method is always invoked by the thread performing
1173 * acquire.If this method reports failure, the acquire method
1174 * may queue the thread, if it is not already queued, until it is
1175 * signalled by a release from some other thread. This can be used
1176 * to implement method {@link Lock#tryLock()}.
1177 *
1178 * The default
1179 * implementation throws {@link UnsupportedOperationException}.
1180 *
1181 * @param arg the acquire argument. This value is always the one
1182 * passed to an acquire method, or is the value saved on entry
1183 * to a condition wait.The value is otherwise uninterpreted
1184 * and can represent anything you like.
1185 * @return {@code true} if successful. Upon success, this object has
1186 * been acquired.
1187 * @throws IllegalMonitorStateException if acquiring would place this
1188 * synchronizer in an illegal state. This exception must be
1189 * thrown in a consistent fashion for synchronization to work
1190 * correctly.
1191 * @throws UnsupportedOperationException if exclusive mode is not supported
1192 */
1193 protected boolean tryAcquire(int arg) {
1194 throw new UnsupportedOperationException();
1195 }
1196
1197 /**
1198 * Attempts to set the state to reflect a release in exclusive
1199 * mode.
1200 *
1201 * This method is always invoked by the thread performing release.
1202 *
1203 * The default implementation throws
1204 * {@link UnsupportedOperationException}.
1205 *
1206 * @param arg the release argument. This value is always the one
1207 * passed to a release method, or the current state value upon
1208 * entry to a condition wait.The value is otherwise
1209 * uninterpreted and can represent anything you like.
1210 * @return {@code true} if this object is now in a fully released
1211 * state, so that any waiting threads may attempt to acquire;
1212 * and {@code false} otherwise.
1213 * @throws IllegalMonitorStateException if releasing would place this
1214 * synchronizer in an illegal state. This exception must be
1215 * thrown in a consistent fashion for synchronization to work
1216 * correctly.
1217 * @throws UnsupportedOperationException if exclusive mode is not supported
1218 */
1219 protected boolean tryRelease(int arg) {
1220 throw new UnsupportedOperationException();
1221 }
1222
1223 /**
1224 * Attempts to acquire in shared mode. This method should query if
1225 * the state of the object permits it to be acquired in the shared
1226 * mode, and if so to acquire it.
1227 *
1228 * This method is always invoked by the thread performing
1229 * acquire.If this method reports failure, the acquire method
1230 * may queue the thread, if it is not already queued, until it is
1231 * signalled by a release from some other thread.
1232 *
1233 * The default implementation throws {@link
1234 * UnsupportedOperationException}.
1235 *
1236 * @param arg the acquire argument. This value is always the one
1237 * passed to an acquire method, or is the value saved on entry
1238 * to a condition wait.The value is otherwise uninterpreted
1239 * and can represent anything you like.
1240 * @return a negative value on failure; zero if acquisition in shared
1241 * mode succeeded but no subsequent shared-mode acquire can
1242 * succeed; and a positive value if acquisition in shared
1243 * mode succeeded and subsequent shared-mode acquires might
1244 * also succeed, in which case a subsequent waiting thread
1245 * must check availability. (Support for three different
1246 * return values enables this method to be used in contexts
1247 * where acquires only sometimes act exclusively.)Upon
1248 * success, this object has been acquired.
1249 * @throws IllegalMonitorStateException if acquiring would place this
1250 * synchronizer in an illegal state. This exception must be
1251 * thrown in a consistent fashion for synchronization to work
1252 * correctly.
1253 * @throws UnsupportedOperationException if shared mode is not supported
1254 */
1255 protected int tryAcquireShared(int arg) {
1256 for(;;){
1257 //计算可用资源量
1258 int available = getState();
1259 //计算剩余资源量
1260 int remaining = available - arg;
1261 //计算资源数量，不够直接放回，加入CAS是为了保证state状态一定能够更新成功
1262 if (remaining < 0 ||
1263 compareAndSetState(available, remaining)) {
1264 return remaining;
1265 }
1266 }
1267
1268 //throw new UnsupportedOperationException();
1269 }
1270
1271 /**
1272 * Attempts to set the state to reflect a release in shared mode.
1273 *
1274 * This method is always invoked by the thread performing release.
1275 *
1276 * The default implementation throws
1277 * {@link UnsupportedOperationException}.
1278 *
1279 * @param arg the release argument. This value is always the one
1280 * passed to a release method, or the current state value upon
1281 * entry to a condition wait.The value is otherwise
1282 * uninterpreted and can represent anything you like.
1283 * @return {@code true} if this release of shared mode may permit a
1284 * waiting acquire (shared or exclusive) to succeed; and
1285 * {@code false} otherwise
1286 * @throws IllegalMonitorStateException if releasing would place this
1287 * synchronizer in an illegal state. This exception must be
1288 * thrown in a consistent fashion for synchronization to work
1289 * correctly.
1290 * @throws UnsupportedOperationException if shared mode is not supported
1291 */
1292 protected boolean tryReleaseShared(int arg) {
1293 throw new UnsupportedOperationException();
1294 }
1295
1296 /**
1297 * Returns {@code true} if synchronization is held exclusively with
1298 * respect to the current (calling) thread.This method is invoked
1299 * upon each call to a non-waiting {@link ConditionObject} method.
1300 * (Waiting methods instead invoke {@link #release}.)
1301 *
1302 * The default implementation throws {@link
1303 * UnsupportedOperationException}. This method is invoked
1304 * internally only within {@link ConditionObject} methods, so need
1305 * not be defined if conditions are not used.
1306 *
1307 * @return {@code true} if synchronization is held exclusively;
1308 * {@code false} otherwise
1309 * @throws UnsupportedOperationException if conditions are not supported
1310 */
1311 protected boolean isHeldExclusively() {
1312 throw new UnsupportedOperationException();
1313 }
1314
1315 /**
1316 * Acquires in exclusive mode, ignoring interrupts.Implemented
1317 * by invoking at least once {@link #tryAcquire},
1318 * returning on success.Otherwise the thread is queued, possibly
1319 * repeatedly blocking and unblocking, invoking {@link
1320 * #tryAcquire} until success.This method can be used
1321 * to implement method {@link Lock#lock}.
1322 *
1323 * @param arg the acquire argument.This value is conveyed to
1324 * {@link #tryAcquire} but is otherwise uninterpreted and
1325 * can represent anything you like.
1326 */
1327 public final void acquire(int arg) {
1328 /*加塞抢占共享资源（因为同步队列中可能还有其它节点等待），获取成功则直接返回*/
1329 if (!tryAcquire(arg)
1330 /* 1、抢占共享资源失败，则将当前节点放入到同步队列的尾部，并标记为独占模式；
1331 * 2、使线程阻塞在同步队列中获取资源，直到获取成功才返回；如果整个过程中被中断过就返回true，否则就返回false；*/
1332 && acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) {
1333 /*阻塞获取资源的过程中是不响应线程中断的，内部进行了中断检测，所以这块进行线程中断*/
1334 selfInterrupt();
1335 }
1336 }
1337
1338 /**
1339 * Acquires in exclusive mode, aborting if interrupted.
1340 * Implemented by first checking interrupt status, then invoking
1341 * at least once {@link #tryAcquire}, returning on
1342 * success.Otherwise the thread is queued, possibly repeatedly
1343 * blocking and unblocking, invoking {@link #tryAcquire}
1344 * until success or the thread is interrupted.This method can be
1345 * used to implement method {@link Lock#lockInterruptibly}.
1346 *
1347 * @param arg the acquire argument.This value is conveyed to
1348 * {@link #tryAcquire} but is otherwise uninterpreted and
1349 * can represent anything you like.
1350 * @throws InterruptedException if the current thread is interrupted
1351 */
1352 public final void acquireInterruptibly(int arg)
1353 throws InterruptedException {
1354 if (Thread.interrupted())
1355 throw new InterruptedException();
1356 if (!tryAcquire(arg))
1357 doAcquireInterruptibly(arg);
1358 }
1359
1360 /**
1361 * Attempts to acquire in exclusive mode, aborting if interrupted,
1362 * and failing if the given timeout elapses.Implemented by first
1363 * checking interrupt status, then invoking at least once {@link
1364 * #tryAcquire}, returning on success.Otherwise, the thread is
1365 * queued, possibly repeatedly blocking and unblocking, invoking
1366 * {@link #tryAcquire} until success or the thread is interrupted
1367 * or the timeout elapses.This method can be used to implement
1368 * method {@link Lock#tryLock(long, TimeUnit)}.
1369 *
1370 * @param arg the acquire argument.This value is conveyed to
1371 * {@link #tryAcquire} but is otherwise uninterpreted and
1372 * can represent anything you like.
1373 * @param nanosTimeout the maximum number of nanoseconds to wait
1374 * @return {@code true} if acquired; {@code false} if timed out
1375 * @throws InterruptedException if the current thread is interrupted
1376 */
1377 public final boolean tryAcquireNanos(int arg, long nanosTimeout)
1378 throws InterruptedException {
1379 //当前线程被中断，直接抛出异常
1380 if (Thread.interrupted())
1381 throw new InterruptedException();
1382 //尝试直接获取锁，获取成功就直接返回
1383 return tryAcquire(arg) ||
1384 //否则就“阻塞”指定时间，再尝试获取锁
1385 doAcquireNanos(arg, nanosTimeout);
1386 }
1387
1388 /**
1389 * Releases in exclusive mode.Implemented by unblocking one or
1390 * more threads if {@link #tryRelease} returns true.
1391 * This method can be used to implement method {@link Lock#unlock}.
1392 *
1393 * @param arg the release argument.This value is conveyed to
1394 * {@link #tryRelease} but is otherwise uninterpreted and
1395 * can represent anything you like.
1396 * @return the value returned from {@link #tryRelease}
1397 */
1398 public final boolean release(int arg) {
1399 if (tryRelease(arg)) {
1400 Node h = head;
1401 //当前线程已经释放锁，但是sync queue同步队列不为空，并且头结点head未被取消
1402 if (h != null && h.waitStatus != 0)
1403 //说明还有其它线程等待竞争锁，则尝试唤醒sync queue队列中等待锁的线程
1404 unparkSuccessor(h);
1405 return true;
1406 }
1407 return false;
1408 }
1409
1410 /**
1411 * Acquires in shared mode, ignoring interrupts.Implemented by
1412 * first invoking at least once {@link #tryAcquireShared},
1413 * returning on success.Otherwise the thread is queued, possibly
1414 * repeatedly blocking and unblocking, invoking {@link
1415 * #tryAcquireShared} until success.
1416 *
1417 * @param arg the acquire argument.This value is conveyed to
1418 * {@link #tryAcquireShared} but is otherwise uninterpreted
1419 * and can represent anything you like.
1420 */
1421 public final void acquireShared(int arg) {
1422 if (tryAcquireShared(arg) < 0)
1423 doAcquireShared(arg);
1424 }
1425
1426 /**
1427 * Acquires in shared mode, aborting if interrupted.Implemented
1428 * by first checking interrupt status, then invoking at least once
1429 * {@link #tryAcquireShared}, returning on success.Otherwise the
1430 * thread is queued, possibly repeatedly blocking and unblocking,
1431 * invoking {@link #tryAcquireShared} until success or the thread
1432 * is interrupted.
1433 * @param arg the acquire argument.
1434 * This value is conveyed to {@link #tryAcquireShared} but is
1435 * otherwise uninterpreted and can represent anything
1436 * you like.
1437 * @throws InterruptedException if the current thread is interrupted
1438 */
1439 public final void acquireSharedInterruptibly(int arg)
1440 throws InterruptedException {
1441 //当前线程已经被中断，则直接抛出InterruptedException异常
1442 if (Thread.interrupted())
1443 throw new InterruptedException();
1444 if (tryAcquireShared(arg) < 0)
1445 doAcquireSharedInterruptibly(arg);
1446 }
1447
1448 /**
1449 * Attempts to acquire in shared mode, aborting if interrupted, and
1450 * failing if the given timeout elapses.Implemented by first
1451 * checking interrupt status, then invoking at least once {@link
1452 * #tryAcquireShared}, returning on success.Otherwise, the
1453 * thread is queued, possibly repeatedly blocking and unblocking,
1454 * invoking {@link #tryAcquireShared} until success or the thread
1455 * is interrupted or the timeout elapses.
1456 *
1457 * @param arg the acquire argument.This value is conveyed to
1458 * {@link #tryAcquireShared} but is otherwise uninterpreted
1459 * and can represent anything you like.
1460 * @param nanosTimeout the maximum number of nanoseconds to wait
1461 * @return {@code true} if acquired; {@code false} if timed out
1462 * @throws InterruptedException if the current thread is interrupted
1463 */
1464 public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
1465 throws InterruptedException {
1466 if (Thread.interrupted())
1467 throw new InterruptedException();
1468 return tryAcquireShared(arg) >= 0 ||
1469 doAcquireSharedNanos(arg, nanosTimeout);
1470 }
1471
1472 /**
1473 * Releases in shared mode.Implemented by unblocking one or more
1474 * threads if {@link #tryReleaseShared} returns true.
1475 *
1476 * @param arg the release argument.This value is conveyed to
1477 * {@link #tryReleaseShared} but is otherwise uninterpreted
1478 * and can represent anything you like.
1479 * @return the value returned from {@link #tryReleaseShared}
1480 */
1481 public final boolean releaseShared(int arg) {
1482 //尝试释放锁，释放成功，则返回true
1483 if (tryReleaseShared(arg)) {
1484 //唤醒同步队列中，等待获取锁的线程
1485 doReleaseShared();
1486 return true;
1487 }
1488 return false;
1489 }
1490
1491 // Queue inspection methods
1492
1493 /**
1494 * Queries whether any threads are waiting to acquire. Note that
1495 * because cancellations due to interrupts and timeouts may occur
1496 * at any time, a {@code true} return does not guarantee that any
1497 * other thread will ever acquire.
1498 *
1499 * In this implementation, this operation returns in
1500 * constant time.
1501 *
1502 * @return {@code true} if there may be other threads waiting to acquire
1503 */
1504 public final boolean hasQueuedThreads() {
1505 return head != tail;
1506 }
1507
1508 /**
1509 * Queries whether any threads have ever contended to acquire this
1510 * synchronizer; that is if an acquire method has ever blocked.
1511 *
1512 * In this implementation, this operation returns in
1513 * constant time.
1514 *
1515 * @return {@code true} if there has ever been contention
1516 */
1517 public final boolean hasContended() {
1518 return head != null;
1519 }
1520
1521 /**
1522 * Returns the first (longest-waiting) thread in the queue, or
1523 * {@code null} if no threads are currently queued.
1524 *
1525 * In this implementation, this operation normally returns in
1526 * constant time, but may iterate upon contention if other threads are
1527 * concurrently modifying the queue.
1528 *
1529 * @return the first (longest-waiting) thread in the queue, or
1530 * {@code null} if no threads are currently queued
1531 */
1532 public final Thread getFirstQueuedThread() {
1533 // handle only fast path, else relay
1534 return (head == tail) ? null : fullGetFirstQueuedThread();
1535 }
1536
1537 /**
1538 * Version of getFirstQueuedThread called when fastpath fails
1539 */
1540 private Thread fullGetFirstQueuedThread() {
1541 /*
1542 * The first node is normally head.next. Try to get its
1543 * thread field, ensuring consistent reads: If thread
1544 * field is nulled out or s.prev is no longer head, then
1545 * some other thread(s) concurrently performed setHead in
1546 * between some of our reads. We try this twice before
1547 * resorting to traversal.
1548 */
1549 Node h, s;
1550 Thread st;
1551 if (((h = head) != null && (s = h.next) != null &&
1552 s.prev == head && (st = s.thread) != null) ||
1553 ((h = head) != null && (s = h.next) != null &&
1554 s.prev == head && (st = s.thread) != null))
1555 return st;
1556
1557 /*
1558 * Head's next field might not have been set yet, or may have
1559 * been unset after setHead. So we must check to see if tail
1560 * is actually first node. If not, we continue on, safely
1561 * traversing from tail back to head to find first,
1562 * guaranteeing termination.
1563 */
1564
1565 Node t = tail;
1566 Thread firstThread = null;
1567 while (t != null && t != head) {
1568 Thread tt = t.thread;
1569 if (tt != null)
1570 firstThread = tt;
1571 t = t.prev;
1572 }
1573 return firstThread;
1574 }
1575
1576 /**
1577 * Returns true if the given thread is currently queued.
1578 *
1579 * This implementation traverses the queue to determine
1580 * presence of the given thread.
1581 *
1582 * @param thread the thread
1583 * @return {@code true} if the given thread is on the queue
1584 * @throws NullPointerException if the thread is null
1585 */
1586 public final boolean isQueued(Thread thread) {
1587 if (thread == null)
1588 throw new NullPointerException();
1589 for (Node p = tail; p != null; p = p.prev)
1590 if (p.thread == thread)
1591 return true;
1592 return false;
1593 }
1594
1595 /**
1596 * Returns {@code true} if the apparent first queued thread, if one
1597 * exists, is waiting in exclusive mode.If this method returns
1598 * {@code true}, and the current thread is attempting to acquire in
1599 * shared mode (that is, this method is invoked from {@link
1600 * #tryAcquireShared}) then it is guaranteed that the current thread
1601 * is not the first queued thread.Used only as a heuristic in
1602 * ReentrantReadWriteLock.
1603 */
1604 final boolean apparentlyFirstQueuedIsExclusive() {
1605 Node h, s;
1606 return (h = head) != null &&
1607 (s = h.next)!= null &&
1608 !s.isShared() &&
1609 s.thread != null;
1610 }
1611
1612 /**
1613 * Queries whether any threads have been waiting to acquire longer
1614 * than the current thread.
1615 *
1616 * An invocation of this method is equivalent to (but may be
1617 * more efficient than):
1618 *<pre> {@code
1619 * getFirstQueuedThread() != Thread.currentThread() &&
1620 * hasQueuedThreads()}</pre>
1621 *
1622 * Note that because cancellations due to interrupts and
1623 * timeouts may occur at any time, a {@code true} return does not
1624 * guarantee that some other thread will acquire before the current
1625 * thread.Likewise, it is possible for another thread to win a
1626 * race to enqueue after this method has returned {@code false},
1627 * due to the queue being empty.
1628 *
1629 * This method is designed to be used by a fair synchronizer to
1630 * avoid <a target="_blank" href="https://www.cnblogs.com/AbstractQueuedSynchronizer#barging">barging</a>.
1631 * Such a synchronizer's {@link #tryAcquire} method should return
1632 * {@code false}, and its {@link #tryAcquireShared} method should
1633 * return a negative value, if this method returns {@code true}
1634 * (unless this is a reentrant acquire).For example, the {@code
1635 * tryAcquire} method for a fair, reentrant, exclusive mode
1636 * synchronizer might look like this:
1637 *
1638 *<pre> {@code
1639 * protected boolean tryAcquire(int arg) {
1640 * if (isHeldExclusively()) {
1641 * // A reentrant acquire; increment hold count
1642 * return true;
1643 * } else if (hasQueuedPredecessors()) {
1644 * return false;
1645 * } else {
1646 * // try to acquire normally
1647 * }
1648 * }}</pre>
1649 *
1650 * @return {@code true} if there is a queued thread preceding the
1651 * current thread, and {@code false} if the current thread
1652 * is at the head of the queue or the queue is empty
1653 * @since 1.7
1654 */
1655 public final boolean hasQueuedPredecessors() {
1656 // The correctness of this depends on head being initialized
1657 // before tail and on head.next being accurate if the current
1658 // thread is first in queue.
1659 Node t = tail; // Read fields in reverse initialization order
1660 Node h = head;
1661 Node s;
1662 return h != t &&
1663 ((s = h.next) == null || s.thread != Thread.currentThread());
1664 }
1665
1666
1667 // Instrumentation and monitoring methods
1668
1669 /**
1670 * Returns an estimate of the number of threads waiting to
1671 * acquire.The value is only an estimate because the number of
1672 * threads may change dynamically while this method traverses
1673 * internal data structures.This method is designed for use in
1674 * monitoring system state, not for synchronization
1675 * control.
1676 *
1677 * @return the estimated number of threads waiting to acquire
1678 */
1679 public final int getQueueLength() {
1680 int n = 0;
1681 for (Node p = tail; p != null; p = p.prev) {
1682 if (p.thread != null)
1683 ++n;
1684 }
1685 return n;
1686 }
1687
1688 /**
1689 * Returns a collection containing threads that may be waiting to
1690 * acquire.Because the actual set of threads may change
1691 * dynamically while constructing this result, the returned
1692 * collection is only a best-effort estimate.The elements of the
1693 * returned collection are in no particular order.This method is
1694 * designed to facilitate construction of subclasses that provide
1695 * more extensive monitoring facilities.
1696 *
1697 * @return the collection of threads
1698 */
1699 public final Collection<Thread> getQueuedThreads() {
1700 ArrayList<Thread> list = new ArrayList<Thread>();
1701 for (Node p = tail; p != null; p = p.prev) {
1702 Thread t = p.thread;
1703 if (t != null)
1704 list.add(t);
1705 }
1706 return list;
1707 }
1708
1709 /**
1710 * Returns a collection containing threads that may be waiting to
1711 * acquire in exclusive mode. This has the same properties
1712 * as {@link #getQueuedThreads} except that it only returns
1713 * those threads waiting due to an exclusive acquire.
1714 *
1715 * @return the collection of threads
1716 */
1717 public final Collection<Thread> getExclusiveQueuedThreads() {
1718 ArrayList<Thread> list = new ArrayList<Thread>();
1719 for (Node p = tail; p != null; p = p.prev) {
1720 if (!p.isShared()) {
1721 Thread t = p.thread;
1722 if (t != null)
1723 list.add(t);
1724 }
1725 }
1726 return list;
1727 }
1728
1729 /**
1730 * Returns a collection containing threads that may be waiting to
1731 * acquire in shared mode. This has the same properties
1732 * as {@link #getQueuedThreads} except that it only returns
1733 * those threads waiting due to a shared acquire.
1734 *
1735 * @return the collection of threads
1736 */
1737 public final Collection<Thread> getSharedQueuedThreads() {
1738 ArrayList<Thread> list = new ArrayList<Thread>();
1739 for (Node p = tail; p != null; p = p.prev) {
1740 if (p.isShared()) {
1741 Thread t = p.thread;
1742 if (t != null)
1743 list.add(t);
1744 }
1745 }
1746 return list;
1747 }
1748
1749 /**
1750 * Returns a string identifying this synchronizer, as well as its state.
1751 * The state, in brackets, includes the String {@code "State ="}
1752 * followed by the current value of {@link #getState}, and either
1753 * {@code "nonempty"} or {@code "empty"} depending on whether the
1754 * queue is empty.
1755 *
1756 * @return a string identifying this synchronizer, as well as its state
1757 */
1758 public String toString() {
1759 int s = getState();
1760 String q= hasQueuedThreads() ? "non" : "";
1761 return super.toString() +
1762 "";
1763 }
1764
1765
1766 // Internal support methods for Conditions
1767
1768 /**
1769 * Returns true if a node, always one that was initially placed on
1770 * a condition queue, is now waiting to reacquire on sync queue.
1771 * @param node the node
1772 * @return true if is reacquiring
1773 */
1774 //判断当前线程是否在sync queue同步队列中。
1775 final boolean isOnSyncQueue(Node node) {
1776 /**
1777 * 1、当前节点node的状态waitStatus为Node.CONDITION，则其必定还在条件队列condition中；
1778 * 2、如果当前节点node的node.prev == null，说明当前节点还未被加入到sync queue队列中，因为node节点要加入到sync queue中（无论本次加入是否成功），则本次尝试必有node.prev != null；
1779 */
1780 if (node.waitStatus == Node.CONDITION || node.prev == null)
1781 return false;
1782 //如果当前节点node.next != null，说明其已经有后继节点了，则其必定在sync queue同步队列中
1783 if (node.next != null) // If has successor, it must be on queue
1784 return true;
1785 /*
1786 * node.prev can be non-null, but not yet on queue because
1787 * the CAS to place it on queue can fail. So we have to
1788 * traverse from tail to make sure it actually made it.It
1789 * will always be near the tail in calls to this method, and
1790 * unless the CAS failed (which is unlikely), it will be
1791 * there, so we hardly ever traverse much.
1792 */
1793 /**
1794 * 能走到这里来，说明当前节点node.waitStatus != Node.CONDITION && node.prev != null && node.next == null
1795 * ，说明已经开始尝试将当前节点node加入到sync queue同步队列中，已经完成了node.prev = tail,但是进行接下来的CAS操作设置sync queue的尾节点，有可能失败
1796 * 导致当前节点入队失败，所以从sync queue的尾节点开始遍历，进一步确认当前节点是否成功加入到了sync queue中。
1797 */
1798 return findNodeFromTail(node);
1799 }
1800
1801 /**
1802 * Returns true if node is on sync queue by searching backwards from tail.
1803 * Called only when needed by isOnSyncQueue.
1804 * @return true if present
1805 */
1806 //判断指定的node节点是否在sync queue同步队列中
1807 private boolean findNodeFromTail(Node node) {
1808 Node t = tail;
1809 //从链表的尾部开始遍历
1810 for (;;) {
1811 //遍历到的当前节点等于指定的节点，则返回true
1812 if (t == node)
1813 return true;
1814 //遍历到的当前节点已经为空，说明链表已经遍历结束，未找到指定的节点，则返回false
1815 if (t == null)
1816 return false;
1817 //移动指针，进入下一次自旋遍历
1818 t = t.prev;
1819 }
1820 }
1821
1822 /**
1823 * Transfers a node from a condition queue onto sync queue.
1824 * Returns true if successful.
1825 * @param node the node
1826 * @return true if successfully transferred (else the node was
1827 * cancelled before signal)
1828 */
1829 final boolean transferForSignal(Node node) {
1830 /*
1831 * If cannot change waitStatus, the node has been cancelled.
1832 */
1833 //调用signal方法的时候，当前node节点已经取消了等待，则忽略这个节点
1834 if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
1835 return false;
1836
1837 /*
1838 * Splice onto queue and try to set waitStatus of predecessor to
1839 * indicate that thread is (probably) waiting. If cancelled or
1840 * attempt to set waitStatus fails, wake up to resync (in which
1841 * case the waitStatus can be transiently and harmlessly wrong).
1842 */
1843 //如果这个节点node在条件队列condition中正常等待，则将node加入到sync queue同步队列中，并返回这个新结点的前驱结点
1844 Node p = enq(node);
1845 int ws = p.waitStatus;
1846 /**
1847 * 同步队列sync queue中的节点是通过前驱结点来唤醒的，前驱结点已经被取消，或者前驱结点的waitStatus设置Node.SIGNAL失败，
1848 * 后继节点不能通过前驱结点正常唤醒，则直接唤醒这个刚刚加入到同步队列sync queue中的节点
1849 */
1850 if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
1851 LockSupport.unpark(node.thread);
1852 return true;
1853 }
1854
1855 /**
1856 * Transfers node, if necessary, to sync queue after a cancelled wait.
1857 * Returns true if thread was cancelled before being signalled.
1858 *
1859 * @param node the node
1860 * @return true if cancelled before the node was signalled
1861 */
1862 final boolean transferAfterCancelledWait(Node node) {
1863 /**
1864 * 判断一个node是否被signal()过，最简单有效的方式就是是否离开了condition条件队列，进入到了sync queue同步队列中
1865 */
1866 //如果一个节点的状态还是Node.CONDITION，说明它还未被signal过，是因为中断导致了它被唤醒（从condition条件队列转移到sync queue同步队列中）
1867 if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
1868 //将当前被唤醒的节点加入到同步队列sync queue中，准备竞争锁，但是还没有将这个节点从条件队列中移除，所以后面的处理一下
1869 enq(node);
1870 return true;
1871 }
1872 /*
1873 * If we lost out to a signal(), then we can't proceed
1874 * until it finishes its enq().Cancelling during an
1875 * incomplete transfer is both rare and transient, so just
1876 * spin.
1877 */
1878 while (!isOnSyncQueue(node))
1879 Thread.yield();
1880 return false;
1881 }
1882
1883 /**
1884 * Invokes release with current state value; returns saved state.
1885 * Cancels node and throws exception on failure.
1886 * @param node the condition node for this wait
1887 * @return previous sync state
1888 */
1889 //完全释放线程所占用的锁
1890 final int fullyRelease(Node node) {
1891 boolean failed = true;
1892 try {
1893 //获取当前锁状态
1894 int savedState = getState();
1895 //完全释放锁，对于重入锁而言，无论重入了几次都一次释放；同时持有锁的线程不是当前线程时候，也会直接抛出IllegalMonitorStateException异常，直接进入finally块
1896 if (release(savedState)) {
1897 failed = false;
1898 return savedState;
1899 } else {
1900 throw new IllegalMonitorStateException();
1901 }
1902 } finally {
1903 /**
1904 * 如果释放锁失败，则将刚才新加入到条件队列尾节点的node节点的waitStatus设置为Node.CANCELLED，
1905 * 后序其它线程被包装成Node节点加入到条件队列中时候，会将这些已经被取消的节点移除
1906 */
1907 if (failed)
1908 node.waitStatus = Node.CANCELLED;
1909 }
1910 }
1911
1912 // Instrumentation methods for conditions
1913
1914 /**
1915 * Queries whether the given ConditionObject
1916 * uses this synchronizer as its lock.
1917 *
1918 * @param condition the condition
1919 * @return {@code true} if owned
1920 * @throws NullPointerException if the condition is null
1921 */
1922 public final boolean owns(ConditionObject condition) {
1923 return condition.isOwnedBy(this);
1924 }
1925
1926 /**
1927 * Queries whether any threads are waiting on the given condition
1928 * associated with this synchronizer. Note that because timeouts
1929 * and interrupts may occur at any time, a {@code true} return
1930 * does not guarantee that a future {@code signal} will awaken
1931 * any threads.This method is designed primarily for use in
1932 * monitoring of the system state.
1933 *
1934 * @param condition the condition
1935 * @return {@code true} if there are any waiting threads
1936 * @throws IllegalMonitorStateException if exclusive synchronization
1937 * is not held
1938 * @throws IllegalArgumentException if the given condition is
1939 * not associated with this synchronizer
1940 * @throws NullPointerException if the condition is null
1941 */
1942 public final boolean hasWaiters(ConditionObject condition) {
1943 if (!owns(condition))
1944 throw new IllegalArgumentException("Not owner");
1945 return condition.hasWaiters();
1946 }
1947
1948 /**
1949 * Returns an estimate of the number of threads waiting on the
1950 * given condition associated with this synchronizer. Note that
1951 * because timeouts and interrupts may occur at any time, the
1952 * estimate serves only as an upper bound on the actual number of
1953 * waiters.This method is designed for use in monitoring of the
1954 * system state, not for synchronization control.
1955 *
1956 * @param condition the condition
1957 * @return the estimated number of waiting threads
1958 * @throws IllegalMonitorStateException if exclusive synchronization
1959 * is not held
1960 * @throws IllegalArgumentException if the given condition is
1961 * not associated with this synchronizer
1962 * @throws NullPointerException if the condition is null
1963 */
1964 public final int getWaitQueueLength(ConditionObject condition) {
1965 if (!owns(condition))
1966 throw new IllegalArgumentException("Not owner");
1967 return condition.getWaitQueueLength();
1968 }
1969
1970 /**
1971 * Returns a collection containing those threads that may be
1972 * waiting on the given condition associated with this
1973 * synchronizer.Because the actual set of threads may change
1974 * dynamically while constructing this result, the returned
1975 * collection is only a best-effort estimate. The elements of the
1976 * returned collection are in no particular order.
1977 *
1978 * @param condition the condition
1979 * @return the collection of threads
1980 * @throws IllegalMonitorStateException if exclusive synchronization
1981 * is not held
1982 * @throws IllegalArgumentException if the given condition is
1983 * not associated with this synchronizer
1984 * @throws NullPointerException if the condition is null
1985 */
1986 public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
1987 if (!owns(condition))
1988 throw new IllegalArgumentException("Not owner");
1989 return condition.getWaitingThreads();
1990 }
1991
1992 /**
1993 * Condition implementation for a {@link
1994 * AbstractQueuedSynchronizer} serving as the basis of a {@link
1995 * Lock} implementation.
1996 *
1997 * Method documentation for this class describes mechanics,
1998 * not behavioral specifications from the point of view of Lock
1999 * and Condition users. Exported versions of this class will in
2000 * general need to be accompanied by documentation describing
2001 * condition semantics that rely on those of the associated
2002 * {@code AbstractQueuedSynchronizer}.
2003 *
2004 * This class is Serializable, but all fields are transient,
2005 * so deserialized conditions have no waiters.
2006 */
2007 public class ConditionObject implements Condition, java.io.Serializable {
2008 private static final long serialVersionUID = 1173984872572414699L;
2009 /** First node of condition queue. */
2010 private transient Node firstWaiter;
2011 /** Last node of condition queue. */
2012 private transient Node lastWaiter;
2013
2014 /**
2015 * Creates a new {@code ConditionObject} instance.
2016 */
2017 public ConditionObject() { }
2018
2019 // Internal methods
2020
2021 /**
2022 * Adds a new waiter to wait queue.
2023 * @return its new wait node
2024 */
2025 //将当前线程包装成Node节点，加入到条件队列中
2026 private Node addConditionWaiter() {
2027 //条件队列的尾节点
2028 Node t = lastWaiter;
2029 // If lastWaiter is cancelled, clean out.
2030 //条件尾节点不为空，说明条件队列不为空，但是它的尾节点的waitStatus已经不是Node.CONDITION了，说明其已经取消了，则清除掉条件队列中已经取消的节点
2031 if (t != null && t.waitStatus != Node.CONDITION) {
2032 //清除掉条件队列中已经取消的节点
2033 unlinkCancelledWaiters();
2034 t = lastWaiter;
2035 }
2036 //将当前线程包装成Node节点，并将新的node节点的状态设置为Node.CONDITION
2037 Node node = new Node(Thread.currentThread(), Node.CONDITION);
2038 //如果条件队列为空，则将条件队列的头结点firstWaiter指向刚创建出来的node节点
2039 if (t == null)
2040 firstWaiter = node;
2041 else
2042 //否则，将条件队列旧的尾节点的nextWaiter指向新创建的node节点
2043 t.nextWaiter = node;
2044 //将新创建的node节点设置为条件队列的尾节点
2045 lastWaiter = node;
2046 return node;
2047 }
2048
2049 /**
2050 * Removes and transfers nodes until hit non-cancelled one or
2051 * null. Split out from signal in part to encourage compilers
2052 * to inline the case of no waiters.
2053 * @param first (non-null) the first node on condition queue
2054 */
2055 //唤醒条件队列中第一个未被取消的节点
2056 private void doSignal(Node first) {
2057 do {
2058 //如果条件队列中只有一个节点，则将条件队列清空
2059 if ( (firstWaiter = first.nextWaiter) == null)
2060 //则将条件队列的尾节点lastWaiter也置为null
2061 lastWaiter = null;
2062 //将当前要被唤醒的节点从条件队列中移除
2063 first.nextWaiter = null;
2064 //当前节点已经被取消了，并且条件队列中还有节点，则进行下一次循环
2065 } while (!transferForSignal(first) &&
2066 (first = firstWaiter) != null);
2067 }
2068
2069 /**
2070 * Removes and transfers all nodes.
2071 * @param first (non-null) the first node on condition queue
2072 */
2073 private void doSignalAll(Node first) {
2074 //清空条件队列
2075 lastWaiter = firstWaiter = null;
2076 do {
2077 //获取条件队列头结点的下一个结点
2078 Node next = first.nextWaiter;
2079 //将条件队列的头结点从链表上移除
2080 first.nextWaiter = null;
2081 //将这个刚从条件队列condition中移除的节点转移到sync queue同步队列中
2082 transferForSignal(first);
2083 //修改指针，准备继续下一次的遍历
2084 first = next;
2085 //first ！= null表示链表还未遍历完成，继续下次遍历
2086 } while (first != null);
2087 }
2088
2089 /**
2090 * Unlinks cancelled waiter nodes from condition queue.
2091 * Called only while holding lock. This is called when
2092 * cancellation occurred during condition wait, and upon
2093 * insertion of a new waiter when lastWaiter is seen to have
2094 * been cancelled. This method is needed to avoid garbage
2095 * retention in the absence of signals. So even though it may
2096 * require a full traversal, it comes into play only when
2097 * timeouts or cancellations occur in the absence of
2098 * signals. It traverses all nodes rather than stopping at a
2099 * particular target to unlink all pointers to garbage nodes
2100 * without requiring many re-traversals during cancellation
2101 * storms.
2102 */
2103 private void unlinkCancelledWaiters() {
2104 //当前节点,从表示条件队列链表的头结点firstWaiter开始遍历
2105 Node t = firstWaiter;
2106 //表示条件队列中最后一个waitStatus不为Node.CANCELLED的节点
2107 Node trail = null;
2108 //条件队列不为空
2109 while (t != null) {
2110 //当前节点的后继节点
2111 Node next = t.nextWaiter;
2112 //如果当前节点t已经取消了正常等待，则将节点t从条件队列中移除
2113 if (t.waitStatus != Node.CONDITION) {
2114 //将当前节点从条件队列移除
2115 t.nextWaiter = null;
2116 //trail == null表示条件队列当前节点之前的都被取消了，所以将条件队列的头结点firstWaiter重置指向next
2117 if (trail == null)
2118 firstWaiter = next;
2119 else
2120 //当前节点已经被取消，trail != null,则需要将trail与当前节点断开，指向next;
2121 trail.nextWaiter = next;
2122 //如果next == null说明队列已经完成遍历，将原来的条件队列中最后一个正常等待的节点赋值给lastWaiter；
2123 if (next == null)
2124 lastWaiter = trail;
2125 }
2126 else{
2127 //如果当前节点未被取消，就将当前节点记为队列中最后一个未被取消的节点，令trail = t;
2128 trail = t;
2129 }
2130 //修改当前节点的指针，指向下一个节点next
2131 t = next;
2132 }
2133 }
2134
2135 // public methods
2136
2137 /**
2138 * Moves the longest-waiting thread, if one exists, from the
2139 * wait queue for this condition to the wait queue for the
2140 * owning lock.
2141 *
2142 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2143 * returns {@code false}
2144 */
2145 public final void signal() {
2146 //判断当前线程是否是持有锁的线程，如果不是则直接抛出IllegalMonitorStateException异常
2147 if (!isHeldExclusively())
2148 throw new IllegalMonitorStateException();
2149 Node first = firstWaiter;
2150 //条件队列condition不为空
2151 if (first != null)
2152 //唤醒条件队列condition中的头结点
2153 doSignal(first);
2154 }
2155
2156 /**
2157 * Moves all threads from the wait queue for this condition to
2158 * the wait queue for the owning lock.
2159 *
2160 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2161 * returns {@code false}
2162 */
2163 public final void signalAll() {
2164 //判断当前线程是否是持有锁的线程，如果不是，则直接抛出IllegalMonitorStateException异常
2165 if (!isHeldExclusively())
2166 throw new IllegalMonitorStateException();
2167 Node first = firstWaiter;
2168 //判断条件队列是否为空
2169 if (first != null)
2170 //唤醒条件队列中进行等待的节点
2171 doSignalAll(first);
2172 }
2173
2174 /**
2175 * Implements uninterruptible condition wait.
2176 * <ol>
2177 * <li> Save lock state returned by {@link #getState}.
2178 * <li> Invoke {@link #release} with saved state as argument,
2179 * throwing IllegalMonitorStateException if it fails.
2180 * <li> Block until signalled.
2181 * <li> Reacquire by invoking specialized version of
2182 * {@link #acquire} with saved state as argument.
2183 * </ol>
2184 */
2185 /**
2186 * await()方法中，中断和signal()起到了相同的效果，将当前线程唤醒，但是中断一个正常等待的线程，线程被唤醒后，
2187 * 抢到了锁，但是发现等待的条件还未满足，线程会重新被挂起，所以我们希望有一个方法，在等待的时候，不要响应线程中断，
2188 * 所以下面awaitUnitertuptibly()方法就实现了这个功能。
2189 */
2190 public final void awaitUninterruptibly() {
2191 //将当前线程包装成Node节点，加入到条件队列condition中
2192 Node node = addConditionWaiter();
2193 //释放当前线程所持有的锁
2194 int savedState = fullyRelease(node);
2195 boolean interrupted = false;
2196 //判断当前线程是否已经在sync queue同步队列
2197 while (!isOnSyncQueue(node)) {
2198 //当前线程未在sync queue同步队列中，说明其还没有资格竞争独占锁，将当前线程挂起
2199 LockSupport.park(this);
2200 /**
2201 * 判断当前线程是否是由于被其他线程中断唤醒的，如果是则只是简单记录下其中断标记即可，
2202 * 当前线程要跳出此while循环则满足的唯一条件是，被其他线程signal()，进入到同步队列
2203 */
2204 if (Thread.interrupted())
2205 interrupted = true;
2206 }
2207
2208 /**
2209 * 这也是一个阻塞操作，当前线程已经加入到同步队列中，尝试竞争锁，竞争失败，找一个合适的地方，
2210 * 将当前线程挂起，等待其他线程signal()，如果在竞争锁的过程中，发生了异常，则需要记录一下
2211 */
2212 if (acquireQueued(node, savedState) || interrupted)
2213 selfInterrupt();
2214 }
2215
2216 /*
2217 * For interruptible waits, we need to track whether to throw
2218 * InterruptedException, if interrupted while blocked on
2219 * condition, versus reinterrupt current thread, if
2220 * interrupted while blocked waiting to re-acquire.
2221 */
2222
2223 /** Mode meaning to reinterrupt on exit from wait */
2224 private static final int REINTERRUPT =1;
2225 /** Mode meaning to throw InterruptedException on exit from wait */
2226 private static final int THROW_IE = -1;
2227
2228 /**
2229 * Checks for interrupt, returning THROW_IE if interrupted
2230 * before signalled, REINTERRUPT if after signalled, or
2231 * 0 if not interrupted.
2232 */
2233 private int checkInterruptWhileWaiting(Node node) {
2234 //判断在等待的过程中，线程是否发生了中断
2235 return Thread.interrupted() ?
2236 //判断线程中断时发生在signal()之前，还是signal()之后，THROW_IE表示中断发生在signal()之前，REINTERRUPT表示中断发生在signal()之后，中断来得太晚了
2237 (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
2238 0;
2239 }
2240
2241 /**
2242 * Throws InterruptedException, reinterrupts current thread, or
2243 * does nothing, depending on mode.
2244 */
2245 private void reportInterruptAfterWait(int interruptMode)
2246 throws InterruptedException {
2247 //表示还未被其他线程signal()就发生了中断，当前线程是由于中断被唤醒的，所以抛出InterruptedException异常
2248 if (interruptMode == THROW_IE)
2249 throw new InterruptedException();
2250 //表示被其它线程signal()过了之后才发生了线程中断，中断来得太迟了，所以补一下中断标记就行了
2251 else if (interruptMode == REINTERRUPT)
2252 selfInterrupt();
2253 }
2254
2255 /**
2256 * Implements interruptible condition wait.
2257 * <ol>
2258 * <li> If current thread is interrupted, throw InterruptedException.
2259 * <li> Save lock state returned by {@link #getState}.
2260 * <li> Invoke {@link #release} with saved state as argument,
2261 * throwing IllegalMonitorStateException if it fails.
2262 * <li> Block until signalled or interrupted.
2263 * <li> Reacquire by invoking specialized version of
2264 * {@link #acquire} with saved state as argument.
2265 * <li> If interrupted while blocked in step 4, throw InterruptedException.
2266 * </ol>
2267 */
2268 public final void await() throws InterruptedException {
2269 //调用await()方法时，当前线程已经被中断，则直接抛出中断异常
2270 if (Thread.interrupted())
2271 throw new InterruptedException();
2272 //将当前线程包装成Node节点，加入到条件队列中
2273 Node node = addConditionWaiter();
2274 //释放当前线程所占用的锁
2275 int savedState = fullyRelease(node);
2276 int interruptMode = 0;
2277 //判断代表当前线程节点是否在同步队列中，如果不在，说明还没有其它线程调用signal()方法，将当前线程从条件队列Conditon种转移到同步队列中
2278 while (!isOnSyncQueue(node)) {
2279 //当前线程还在条件队列中，没有资格竞争锁，则直接将当前线程挂起
2280 LockSupport.park(this);
2281 //能走到这里，说明当前线程被其他线程signal()或者被其它线程中断了
2282 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2283 break;
2284 }
2285 //interruptMode != THROW_IE表示当前线程不是由于线程中断而被唤醒的
2286 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2287 interruptMode = REINTERRUPT;
2288 //上面将从条件队列condition中唤醒的节点加入到sync queue条件队列中时候，并没有从条件队列中移除，这里需要清理一下
2289 if (node.nextWaiter != null) // clean up if cancelled
2290 unlinkCancelledWaiters();
2291 //当前节点在等待的过程中发生了中断
2292 if (interruptMode != 0)
2293 reportInterruptAfterWait(interruptMode);
2294 }
2295
2296 /**
2297 * Implements timed condition wait.
2298 * <ol>
2299 * <li> If current thread is interrupted, throw InterruptedException.
2300 * <li> Save lock state returned by {@link #getState}.
2301 * <li> Invoke {@link #release} with saved state as argument,
2302 * throwing IllegalMonitorStateException if it fails.
2303 * <li> Block until signalled, interrupted, or timed out.
2304 * <li> Reacquire by invoking specialized version of
2305 * {@link #acquire} with saved state as argument.
2306 * <li> If interrupted while blocked in step 4, throw InterruptedException.
2307 * </ol>
2308 */
2309 public final long awaitNanos(long nanosTimeout)
2310 throws InterruptedException {
2311 if (Thread.interrupted())
2312 throw new InterruptedException();
2313 Node node = addConditionWaiter();
2314 int savedState = fullyRelease(node);
2315 final long deadline = System.nanoTime() + nanosTimeout;
2316 int interruptMode = 0;
2317 while (!isOnSyncQueue(node)) {
2318 if (nanosTimeout <= 0L) {
2319 transferAfterCancelledWait(node);
2320 break;
2321 }
2322 if (nanosTimeout >= spinForTimeoutThreshold)
2323 LockSupport.parkNanos(this, nanosTimeout);
2324 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2325 break;
2326 nanosTimeout = deadline - System.nanoTime();
2327 }
2328 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2329 interruptMode = REINTERRUPT;
2330 if (node.nextWaiter != null)
2331 unlinkCancelledWaiters();
2332 if (interruptMode != 0)
2333 reportInterruptAfterWait(interruptMode);
2334 return deadline - System.nanoTime();
2335 }
2336
2337 /**
2338 * Implements absolute timed condition wait.
2339 * <ol>
2340 * <li> If current thread is interrupted, throw InterruptedException.
2341 * <li> Save lock state returned by {@link #getState}.
2342 * <li> Invoke {@link #release} with saved state as argument,
2343 * throwing IllegalMonitorStateException if it fails.
2344 * <li> Block until signalled, interrupted, or timed out.
2345 * <li> Reacquire by invoking specialized version of
2346 * {@link #acquire} with saved state as argument.
2347 * <li> If interrupted while blocked in step 4, throw InterruptedException.
2348 * <li> If timed out while blocked in step 4, return false, else true.
2349 * </ol>
2350 */
2351 public final boolean awaitUntil(Date deadline)
2352 throws InterruptedException {
2353 long abstime = deadline.getTime();
2354 if (Thread.interrupted())
2355 throw new InterruptedException();
2356 Node node = addConditionWaiter();
2357 int savedState = fullyRelease(node);
2358 boolean timedout = false;
2359 int interruptMode = 0;
2360 while (!isOnSyncQueue(node)) {
2361 if (System.currentTimeMillis() > abstime) {
2362 timedout = transferAfterCancelledWait(node);
2363 break;
2364 }
2365 LockSupport.parkUntil(this, abstime);
2366 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2367 break;
2368 }
2369 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2370 interruptMode = REINTERRUPT;
2371 if (node.nextWaiter != null)
2372 unlinkCancelledWaiters();
2373 if (interruptMode != 0)
2374 reportInterruptAfterWait(interruptMode);
2375 return !timedout;
2376 }
2377
2378 /**
2379 * Implements timed condition wait.
2380 * <ol>
2381 * <li> If current thread is interrupted, throw InterruptedException.
2382 * <li> Save lock state returned by {@link #getState}.
2383 * <li> Invoke {@link #release} with saved state as argument,
2384 * throwing IllegalMonitorStateException if it fails.
2385 * <li> Block until signalled, interrupted, or timed out.
2386 * <li> Reacquire by invoking specialized version of
2387 * {@link #acquire} with saved state as argument.
2388 * <li> If interrupted while blocked in step 4, throw InterruptedException.
2389 * <li> If timed out while blocked in step 4, return false, else true.
2390 * </ol>
2391 */
2392 public final boolean await(long time, TimeUnit unit)
2393 throws InterruptedException {
2394 long nanosTimeout = unit.toNanos(time);
2395 if (Thread.interrupted())
2396 throw new InterruptedException();
2397 Node node = addConditionWaiter();
2398 int savedState = fullyRelease(node);
2399 final long deadline = System.nanoTime() + nanosTimeout;
2400 boolean timedout = false;
2401 int interruptMode = 0;
2402 while (!isOnSyncQueue(node)) {
2403 if (nanosTimeout <= 0L) {
2404 timedout = transferAfterCancelledWait(node);
2405 break;
2406 }
2407 if (nanosTimeout >= spinForTimeoutThreshold)
2408 LockSupport.parkNanos(this, nanosTimeout);
2409 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
2410 break;
2411 nanosTimeout = deadline - System.nanoTime();
2412 }
2413 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
2414 interruptMode = REINTERRUPT;
2415 if (node.nextWaiter != null)
2416 unlinkCancelledWaiters();
2417 if (interruptMode != 0)
2418 reportInterruptAfterWait(interruptMode);
2419 return !timedout;
2420 }
2421
2422 //support for instrumentation
2423
2424 /**
2425 * Returns true if this condition was created by the given
2426 * synchronization object.
2427 *
2428 * @return {@code true} if owned
2429 */
2430 final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
2431 return sync == AbstractQueuedSynchronizer.this;
2432 }
2433
2434 /**
2435 * Queries whether any threads are waiting on this condition.
2436 * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
2437 *
2438 * @return {@code true} if there are any waiting threads
2439 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2440 * returns {@code false}
2441 */
2442 protected final boolean hasWaiters() {
2443 if (!isHeldExclusively())
2444 throw new IllegalMonitorStateException();
2445 for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2446 if (w.waitStatus == Node.CONDITION)
2447 return true;
2448 }
2449 return false;
2450 }
2451
2452 /**
2453 * Returns an estimate of the number of threads waiting on
2454 * this condition.
2455 * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
2456 *
2457 * @return the estimated number of waiting threads
2458 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2459 * returns {@code false}
2460 */
2461 protected final int getWaitQueueLength() {
2462 if (!isHeldExclusively())
2463 throw new IllegalMonitorStateException();
2464 int n = 0;
2465 for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2466 if (w.waitStatus == Node.CONDITION)
2467 ++n;
2468 }
2469 return n;
2470 }
2471
2472 /**
2473 * Returns a collection containing those threads that may be
2474 * waiting on this Condition.
2475 * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
2476 *
2477 * @return the collection of threads
2478 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
2479 * returns {@code false}
2480 */
2481 protected final Collection<Thread> getWaitingThreads() {
2482 if (!isHeldExclusively())
2483 throw new IllegalMonitorStateException();
2484 ArrayList<Thread> list = new ArrayList<Thread>();
2485 for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
2486 if (w.waitStatus == Node.CONDITION) {
2487 Thread t = w.thread;
2488 if (t != null)
2489 list.add(t);
2490 }
2491 }
2492 return list;
2493 }
2494 }
2495
2496 /**
2497 * Setup to support compareAndSet. We need to natively implement
2498 * this here: For the sake of permitting future enhancements, we
2499 * cannot explicitly subclass AtomicInteger, which would be
2500 * efficient and useful otherwise. So, as the lesser of evils, we
2501 * natively implement using hotspot intrinsics API. And while we
2502 * are at it, we do the same for other CASable fields (which could
2503 * otherwise be done with atomic field updaters).
2504 */
2505 private static final Unsafe unsafe = Unsafe.getUnsafe();
2506 private static final long stateOffset;
2507 private static final long headOffset;
2508 private static final long tailOffset;
2509 private static final long waitStatusOffset;
2510 private static final long nextOffset;
2511
2512 static {
2513 try {
2514 stateOffset = unsafe.objectFieldOffset
2515 (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
2516 headOffset = unsafe.objectFieldOffset
2517 (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
2518 tailOffset = unsafe.objectFieldOffset
2519 (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
2520 waitStatusOffset = unsafe.objectFieldOffset
2521 (Node.class.getDeclaredField("waitStatus"));
2522 nextOffset = unsafe.objectFieldOffset
2523 (Node.class.getDeclaredField("next"));
2524
2525 } catch (Exception ex) { throw new Error(ex); }
2526 }
2527
2528 /**
2529 * CAS head field. Used only by enq.
2530 */
2531 private final boolean compareAndSetHead(Node update) {
2532 return unsafe.compareAndSwapObject(this, headOffset, null, update);
2533 }
2534
2535 /**
2536 * CAS tail field. Used only by enq.
2537 */
2538 private final boolean compareAndSetTail(Node expect, Node update) {
2539 return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
2540 }
2541
2542 /**
2543 * CAS waitStatus field of a node.
2544 */
2545 private static final boolean compareAndSetWaitStatus(Node node,
2546 int expect,
2547 int update) {
2548 return unsafe.compareAndSwapInt(node, waitStatusOffset,
2549 expect, update);
2550 }
2551
2552 /**
2553 * CAS next field of a node.
2554 */
2555 private static final boolean compareAndSetNext(Node node,
2556 Node expect,
2557 Node update) {
2558 return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
2559 }
2560 }

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AQS源码解读----AbstractQueuedSynchronizer