前言：AQS框架在J.U.C中的地位不言而喻，可以说没有AQS就没有J.U.C包，可见其重要性，因此有必要对其原理进行详细深入的理解。
1.AQS是什么
在深入AQS之前，首先我们要搞清楚什么是AQS。AQS全称是AbstractQueuedSynchronizer，我们直接查看AQS源码的注释。
大致意思就是说：AQS提供了实现阻塞锁和相关同步器并依赖先进先出（FIFO）等待队列的框架。
AQS依赖一个原子数值作为锁的状态，子类可以有多个状态值，只能通过原子方法区操作该值，从而保证同步。
通过第一段的注释大致总结下AQS是什么：
①AQS是一个同步的基础框架，基于一个先进先出的队列。
②锁机制依赖一个原子值的状态。
③AQS的子类负责定义与操作这个状态值，但必须通过AQS提供的原子操作。
④AQS剩余的方法就是围绕队列，与线程阻塞唤醒等功能。
2.重要成员变量
AQS中有两个重要的成员变量：Node和ConditionObject。
①Node的作用是存储获取锁失败的线程，并且维护一个CLH FIFO队列，该队列是会被多线程操作的，所以Node中大部分变量都是被volatile修饰，并且通过自旋和CAS进行原子性的操作。CLH的数据结构如下：
Node有一个模式的属性：独占模式和共享模式，独占模式下资源是线程独占的，共享模式下，资源是可以被多个线程占用的。
Node源码如下：
1 static final class Node {
2 /** Marker to indicate a node is waiting in shared mode */
3 static final Node SHARED = new Node(); // 共享模式
4 /** Marker to indicate a node is waiting in exclusive mode */
5 static final Node EXCLUSIVE = null; // 独占模式
6
7 /** waitStatus value to indicate thread has cancelled */
8 static final int CANCELLED = 1; // 表明线程已处于结束状态（被取消）
9 /** waitStatus value to indicate successor's thread needs unparking */
10 static final int SIGNAL = -1; // 表明线程需要被唤醒
11 /** waitStatus value to indicate thread is waiting on condition */
12 static final int CONDITION = -2; // 表明线程正处于条件队列上，等待某一条件
13 /**
14 * waitStatus value to indicate the next acquireShared should
15 * unconditionally propagate
16 */
17 static final int PROPAGATE = -3; // 共享模式下同步状态会被传播
18
19 /**
20 * Status field, taking on only the values:
21 * SIGNAL: The successor of this node is (or will soon be)
22 * blocked (via park), so the current node must
23 * unpark its successor when it releases or
24 * cancels. To avoid races, acquire methods must
25 * first indicate they need a signal,
26 * then retry the atomic acquire, and then,
27 * on failure, block.
28 * CANCELLED: This node is cancelled due to timeout or interrupt.
29 * Nodes never leave this state. In particular,
30 * a thread with cancelled node never again blocks.
31 * CONDITION: This node is currently on a condition queue.
32 * It will not be used as a sync queue node
33 * until transferred, at which time the status
34 * will be set to 0. (Use of this value here has
35 * nothing to do with the other uses of the
36 * field, but simplifies mechanics.)
37 * PROPAGATE: A releaseShared should be propagated to other
38 * nodes. This is set (for head node only) in
39 * doReleaseShared to ensure propagation
40 * continues, even if other operations have
41 * since intervened.
42 * 0: None of the above
43 *
44 * The values are arranged numerically to simplify use.
45 * Non-negative values mean that a node doesn't need to
46 * signal. So, most code doesn't need to check for particular
47 * values, just for sign.
48 *
49 * The field is initialized to 0 for normal sync nodes, and
50 * CONDITION for condition nodes. It is modified using CAS
51 * (or when possible, unconditional volatile writes).
52 */
53 volatile int waitStatus;
54
55 /**
56 * Link to predecessor node that current node/thread relies on
57 * for checking waitStatus. Assigned during enqueuing, and nulled
58 * out (for sake of GC) only upon dequeuing. Also, upon
59 * cancellation of a predecessor, we short-circuit while
60 * finding a non-cancelled one, which will always exist
61 * because the head node is never cancelled: A node becomes
62 * head only as a result of successful acquire. A
63 * cancelled thread never succeeds in acquiring, and a thread only
64 * cancels itself, not any other node.
65 */
66 volatile Node prev;
67
68 /**
69 * Link to the successor node that the current node/thread
70 * unparks upon release. Assigned during enqueuing, adjusted
71 * when bypassing cancelled predecessors, and nulled out (for
72 * sake of GC) when dequeued. The enq operation does not
73 * assign next field of a predecessor until after attachment,
74 * so seeing a null next field does not necessarily mean that
75 * node is at end of queue. However, if a next field appears
76 * to be null, we can scan prev's from the tail to
77 * double-check. The next field of cancelled nodes is set to
78 * point to the node itself instead of null, to make life
79 * easier for isOnSyncQueue.
80 */
81 volatile Node next;
82
83 /**
84 * The thread that enqueued this node. Initialized on
85 * construction and nulled out after use.
86 */
87 volatile Thread thread;
88
89 /**
90 * Link to next node waiting on condition, or the special
91 * value SHARED. Because condition queues are accessed only
92 * when holding in exclusive mode, we just need a simple
93 * linked queue to hold nodes while they are waiting on
94 * conditions. They are then transferred to the queue to
95 * re-acquire. And because conditions can only be exclusive,
96 * we save a field by using special value to indicate shared
97 * mode.
98 */
99 Node nextWaiter;
100
101 /**
102 * Returns true if node is waiting in shared mode.
103 */
104 final boolean isShared() {
105 return nextWaiter == SHARED;
106 }
107
108 /**
109 * Returns previous node, or throws NullPointerException if null.
110 * Use when predecessor cannot be null. The null check could
111 * be elided, but is present to help the VM.
112 *
113 * @return the predecessor of this node
114 */
115 final Node predecessor() throws NullPointerException {
116 Node p = prev;
117 if (p == null)
118 throw new NullPointerException();
119 else
120 return p;
121 }
122
123 Node() { // Used to establish initial head or SHARED marker
124 }
125 // 线程加入等待结点
126 Node(Thread thread, Node mode) { // Used by addWaiter
127 this.nextWaiter = mode;
128 this.thread = thread;
129 }
130 // 线程加入条件对列，会带上线程的状态值waitStatus
131 Node(Thread thread, int waitStatus) { // Used by Condition
132 this.waitStatus = waitStatus;
133 this.thread = thread;
134 }
135 } ②ConditionObject：条件队列，这个类的作用从AQS的注释上可知。
该类主要是为了让子类实现独占模式。AQS框架下独占模式的获取资源、释放等操作到最后都是基于这个类实现的。只有在独占模式下才会去使用该类。
ConditionObject源码如下（对主要代码进行了注释）：
1 public class ConditionObject implements Condition, java.io.Serializable {
2 private static final long serialVersionUID = 1173984872572414699L;
3 /** First node of condition queue. */
4 private transient Node firstWaiter; // 存储条件对列中第一个节点
5 /** Last node of condition queue. */
6 private transient Node lastWaiter; // 存储条件对列中最后一个节点
7
8 /**
9 * Creates a new {@code ConditionObject} instance.
10 */
11 public ConditionObject() { }
12
13 // Internal methods
14
15 /**
16 * Adds a new waiter to wait queue. // 增加一个新的节点到等待队列中
17 * @return its new wait node
18 */
19 private Node addConditionWaiter() {
20 Node t = lastWaiter;
21 // 如果最后一个节点的状态已经结束，则直接清理掉
22 // If lastWaiter is cancelled, clean out.
23 if (t != null && t.waitStatus != Node.CONDITION) {
24 // 拆分已经处于结束状态的节点 也就是清除掉这类节点
25 unlinkCancelledWaiters();
26 t = lastWaiter;
27 }
28 // 创建一个新的节点，带上结点状态，表明结点处于条件对列上
29 Node node = new Node(Thread.currentThread(), Node.CONDITION);
30 /**
31 条件队列中加入节点都是从队尾加入，并且从下面代码可知，每次都会存储最后一个节点的值。
32 当最后一个节点为空时，说明队列中不存在节点，所以将node赋值给第一个节点,否则将节点加入对列尾
33 */
34 if (t == null)
35 firstWaiter = node;
36 else
37 t.nextWaiter = node;
38 lastWaiter = node; // 存储最后一个节点的值
39 return node;
40 }
41
42 /**
43 * 唤醒节点
44 * 移除和转换节点直到节点状态处于未结束或者为空 (节点移除相当于唤醒)
45 * Removes and transfers nodes until hit non-cancelled one or
46 * null. Split out from signal in part to encourage compilers
47 * to inline the case of no waiters.
48 * @param first (non-null) the first node on condition queue
49 */
50 private void doSignal(Node first) {
51 do {
52 // 当next节点为null时，则将lastWaiter赋值为null
53 if ( (firstWaiter = first.nextWaiter) == null)
54 lastWaiter = null;
55 first.nextWaiter = null; // 切断当前节点
56 } while (!transferForSignal(first) &&
57 (first = firstWaiter) != null);
58 }
59
60 /**
61 * 唤醒所有节点
62 * Removes and transfers all nodes.
63 * @param first (non-null) the first node on condition queue
64 */
65 private void doSignalAll(Node first) {
66 lastWaiter = firstWaiter = null;
67 do {
68 // 循环唤醒所有节点，代码还是比较容易理解
69 // 将每个节点直接截断即可
70 Node next = first.nextWaiter;
71 first.nextWaiter = null;
72 transferForSignal(first);
73 first = next;
74 } while (first != null);
75 }
76
77 /**
78 * Unlinks cancelled waiter nodes from condition queue.
79 * Called only while holding lock. This is called when
80 * cancellation occurred during condition wait, and upon
81 * insertion of a new waiter when lastWaiter is seen to have
82 * been cancelled. This method is needed to avoid garbage
83 * retention in the absence of signals. So even though it may
84 * require a full traversal, it comes into play only when
85 * timeouts or cancellations occur in the absence of
86 * signals. It traverses all nodes rather than stopping at a
87 * particular target to unlink all pointers to garbage nodes
88 * without requiring many re-traversals during cancellation
89 * storms.
90 */
91 private void unlinkCancelledWaiters() { // 删除处于结束状态的节点
92 Node t = firstWaiter;
93 Node trail = null;
94 // 第一个节点为空，直接返回
95 // 这里会遍历所有节点
96 while (t != null) {
97 Node next = t.nextWaiter; // 记录下一个节点的值
98 // 当节点状态不为CONDITION
99 if (t.waitStatus != Node.CONDITION) {
100 // 首先将当前节点的下一个节点赋值为空，切断当前节点链路
101 t.nextWaiter = null;
102 // 如果追踪节点为空的时候，则存储第一个节点的值为next，因为当前节点状态不为CONDITION需要清理
103 if (trail == null)
104 firstWaiter = next;
105 else // 在追踪节点串联下一个节点，主要是为了存储最后一个节点的值
106 trail.nextWaiter = next;
107 if (next == null) // 当next为空时，则存储trail为最后一个节点，将最后一个节点值存储下来
108 lastWaiter = trail;
109 }
110 else // 当节点状态为CONDITION时，将该节点赋值给trail
111 trail = t;
112 t = next; // 将next赋值给t，继续遍历
113 }
114 }
115
116 // public methods
117
118 /**
119 * 唤醒等待时间最长的节点，使其拥有锁
120 * Moves the longest-waiting thread, if one exists, from the
121 * wait queue for this condition to the wait queue for the
122 * owning lock.
123 *
124 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
125 * returns {@code false}
126 */
127 public final void signal() {
128 // 如果线程不是独占资源，则抛出异常，从这里也说明ConditionObject只能用在独占模式中
129 if (!isHeldExclusively())
130 throw new IllegalMonitorStateException();
131 Node first = firstWaiter;
132 if (first != null)
133 doSignal(first);
134 }
135
136 /**
137 * 唤醒所有等待节点
138 * Moves all threads from the wait queue for this condition to
139 * the wait queue for the owning lock.
140 *
141 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
142 * returns {@code false}
143 */
144 public final void signalAll() {
145 if (!isHeldExclusively())
146 throw new IllegalMonitorStateException();
147 Node first = firstWaiter;
148 if (first != null)
149 doSignalAll(first);
150 }
151
152 /**
153 * 节点不间断等待
154 * Implements uninterruptible condition wait.
155 * <ol>
156 * <li> Save lock state returned by {@link #getState}.
157 * <li> Invoke {@link #release} with saved state as argument,
158 * throwing IllegalMonitorStateException if it fails.
159 * <li> Block until signalled.
160 * <li> Reacquire by invoking specialized version of
161 * {@link #acquire} with saved state as argument.
162 * </ol>
163 */
164 public final void awaitUninterruptibly() {
165 Node node = addConditionWaiter();
166 int savedState = fullyRelease(node);
167 boolean interrupted = false;
168 while (!isOnSyncQueue(node)) {
169 LockSupport.park(this);
170 if (Thread.interrupted())
171 interrupted = true;
172 }
173 if (acquireQueued(node, savedState) || interrupted)
174 selfInterrupt();
175 }
176
177 /*
178 * For interruptible waits, we need to track whether to throw
179 * InterruptedException, if interrupted while blocked on
180 * condition, versus reinterrupt current thread, if
181 * interrupted while blocked waiting to re-acquire.
182 */
183
184 /** Mode meaning to reinterrupt on exit from wait */
185 private static final int REINTERRUPT = 1;
186 /** Mode meaning to throw InterruptedException on exit from wait */
187 private static final int THROW_IE = -1;
188
189 /**
190 * Checks for interrupt, returning THROW_IE if interrupted
191 * before signalled, REINTERRUPT if after signalled, or
192 * 0 if not interrupted.
193 */
194 private int checkInterruptWhileWaiting(Node node) {
195 return Thread.interrupted() ?
196 (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
197 0;
198 }
199
200 /**
201 * Throws InterruptedException, reinterrupts current thread, or
202 * does nothing, depending on mode.
203 */
204 private void reportInterruptAfterWait(int interruptMode)
205 throws InterruptedException {
206 if (interruptMode == THROW_IE)
207 throw new InterruptedException();
208 else if (interruptMode == REINTERRUPT)
209 selfInterrupt();
210 }
211
212 /**
213 * Implements interruptible condition wait.
214 * <ol>
215 * <li> If current thread is interrupted, throw InterruptedException.
216 * <li> Save lock state returned by {@link #getState}.
217 * <li> Invoke {@link #release} with saved state as argument,
218 * throwing IllegalMonitorStateException if it fails.
219 * <li> Block until signalled or interrupted.
220 * <li> Reacquire by invoking specialized version of
221 * {@link #acquire} with saved state as argument.
222 * <li> If interrupted while blocked in step 4, throw InterruptedException.
223 * </ol>
224 */
225 public final void await() throws InterruptedException {
226 if (Thread.interrupted())
227 throw new InterruptedException();
228 Node node = addConditionWaiter();
229 int savedState = fullyRelease(node);
230 int interruptMode = 0;
231 while (!isOnSyncQueue(node)) {
232 LockSupport.park(this);
233 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
234 break;
235 }
236 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
237 interruptMode = REINTERRUPT;
238 if (node.nextWaiter != null) // clean up if cancelled
239 unlinkCancelledWaiters();
240 if (interruptMode != 0)
241 reportInterruptAfterWait(interruptMode);
242 }
243
244 /**
245 * Implements timed condition wait.
246 * <ol>
247 * <li> If current thread is interrupted, throw InterruptedException.
248 * <li> Save lock state returned by {@link #getState}.
249 * <li> Invoke {@link #release} with saved state as argument,
250 * throwing IllegalMonitorStateException if it fails.
251 * <li> Block until signalled, interrupted, or timed out.
252 * <li> Reacquire by invoking specialized version of
253 * {@link #acquire} with saved state as argument.
254 * <li> If interrupted while blocked in step 4, throw InterruptedException.
255 * </ol>
256 */
257 public final long awaitNanos(long nanosTimeout)
258 throws InterruptedException {
259 if (Thread.interrupted())
260 throw new InterruptedException();
261 Node node = addConditionWaiter();
262 int savedState = fullyRelease(node);
263 final long deadline = System.nanoTime() + nanosTimeout;
264 int interruptMode = 0;
265 while (!isOnSyncQueue(node)) {
266 if (nanosTimeout <= 0L) {
267 transferAfterCancelledWait(node);
268 break;
269 }
270 if (nanosTimeout >= spinForTimeoutThreshold)
271 LockSupport.parkNanos(this, nanosTimeout);
272 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
273 break;
274 nanosTimeout = deadline - System.nanoTime();
275 }
276 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
277 interruptMode = REINTERRUPT;
278 if (node.nextWaiter != null)
279 unlinkCancelledWaiters();
280 if (interruptMode != 0)
281 reportInterruptAfterWait(interruptMode);
282 return deadline - System.nanoTime();
283 }
284
285 /**
286 * Implements absolute timed condition wait.
287 * <ol>
288 * <li> If current thread is interrupted, throw InterruptedException.
289 * <li> Save lock state returned by {@link #getState}.
290 * <li> Invoke {@link #release} with saved state as argument,
291 * throwing IllegalMonitorStateException if it fails.
292 * <li> Block until signalled, interrupted, or timed out.
293 * <li> Reacquire by invoking specialized version of
294 * {@link #acquire} with saved state as argument.
295 * <li> If interrupted while blocked in step 4, throw InterruptedException.
296 * <li> If timed out while blocked in step 4, return false, else true.
297 * </ol>
298 */
299 public final boolean awaitUntil(Date deadline)
300 throws InterruptedException {
301 long abstime = deadline.getTime();
302 if (Thread.interrupted())
303 throw new InterruptedException();
304 Node node = addConditionWaiter();
305 int savedState = fullyRelease(node);
306 boolean timedout = false;
307 int interruptMode = 0;
308 while (!isOnSyncQueue(node)) {
309 if (System.currentTimeMillis() > abstime) {
310 timedout = transferAfterCancelledWait(node);
311 break;
312 }
313 LockSupport.parkUntil(this, abstime);
314 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
315 break;
316 }
317 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
318 interruptMode = REINTERRUPT;
319 if (node.nextWaiter != null)
320 unlinkCancelledWaiters();
321 if (interruptMode != 0)
322 reportInterruptAfterWait(interruptMode);
323 return !timedout;
324 }
325
326 /**
327 * Implements timed condition wait.
328 * <ol>
329 * <li> If current thread is interrupted, throw InterruptedException.
330 * <li> Save lock state returned by {@link #getState}.
331 * <li> Invoke {@link #release} with saved state as argument,
332 * throwing IllegalMonitorStateException if it fails.
333 * <li> Block until signalled, interrupted, or timed out.
334 * <li> Reacquire by invoking specialized version of
335 * {@link #acquire} with saved state as argument.
336 * <li> If interrupted while blocked in step 4, throw InterruptedException.
337 * <li> If timed out while blocked in step 4, return false, else true.
338 * </ol>
339 */
340 public final boolean await(long time, TimeUnit unit)
341 throws InterruptedException {
342 long nanosTimeout = unit.toNanos(time);
343 if (Thread.interrupted())
344 throw new InterruptedException();
345 Node node = addConditionWaiter();
346 int savedState = fullyRelease(node);
347 final long deadline = System.nanoTime() + nanosTimeout;
348 boolean timedout = false;
349 int interruptMode = 0;
350 while (!isOnSyncQueue(node)) {
351 if (nanosTimeout <= 0L) {
352 timedout = transferAfterCancelledWait(node);
353 break;
354 }
355 if (nanosTimeout >= spinForTimeoutThreshold)
356 LockSupport.parkNanos(this, nanosTimeout);
357 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
358 break;
359 nanosTimeout = deadline - System.nanoTime();
360 }
361 if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
362 interruptMode = REINTERRUPT;
363 if (node.nextWaiter != null)
364 unlinkCancelledWaiters();
365 if (interruptMode != 0)
366 reportInterruptAfterWait(interruptMode);
367 return !timedout;
368 }
369
370 // support for instrumentation
371
372 /**
373 * Returns true if this condition was created by the given
374 * synchronization object.
375 *
376 * @return {@code true} if owned
377 */
378 final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
379 return sync == AbstractQueuedSynchronizer.this;
380 }
381
382 /**
383 * Queries whether any threads are waiting on this condition.
384 * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}.
385 *
386 * @return {@code true} if there are any waiting threads
387 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
388 * returns {@code false}
389 */
390 protected final boolean hasWaiters() {
391 if (!isHeldExclusively())
392 throw new IllegalMonitorStateException();
393 for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
394 if (w.waitStatus == Node.CONDITION)
395 return true;
396 }
397 return false;
398 }
399
400 /**
401 * Returns an estimate of the number of threads waiting on
402 * this condition.
403 * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}.
404 *
405 * @return the estimated number of waiting threads
406 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
407 * returns {@code false}
408 */
409 protected final int getWaitQueueLength() {
410 if (!isHeldExclusively())
411 throw new IllegalMonitorStateException();
412 int n = 0;
413 for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
414 if (w.waitStatus == Node.CONDITION)
415 ++n;
416 }
417 return n;
418 }
419
420 /**
421 * Returns a collection containing those threads that may be
422 * waiting on this Condition.
423 * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}.
424 *
425 * @return the collection of threads
426 * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
427 * returns {@code false}
428 */
429 protected final Collection<Thread> getWaitingThreads() {
430 if (!isHeldExclusively())
431 throw new IllegalMonitorStateException();
432 ArrayList<Thread> list = new ArrayList<Thread>();
433 for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
434 if (w.waitStatus == Node.CONDITION) {
435 Thread t = w.thread;
436 if (t != null)
437 list.add(t);
438 }
439 }
440 return list;
441 }
442 } View Code
3.AQS成员函数
由于AQS分独占模式和共享模式，因此这里按独占、共享模式的顺序对AQS的成员函数进行分析。
①acquire(int arg)
独占模式下获取资源，如果获取到资源，线程直接返回，否则进入等待队列，直到获取到资源为止，整个过程忽略中断。源码如下：
1 /**
2 * Acquires in exclusive mode, ignoring interrupts. Implemented
3 * by invoking at least once {@link #tryAcquire},
4 * returning on success. Otherwise the thread is queued, possibly
5 * repeatedly blocking and unblocking, invoking {@link
6 * #tryAcquire} until success. This method can be used
7 * to implement method {@link Lock#lock}.
8 *
9 * @param arg the acquire argument. This value is conveyed to
10 * {@link #tryAcquire} but is otherwise uninterpreted and
11 * can represent anything you like.
12 */
13 public final void acquire(int arg) {
14 if (!tryAcquire(arg) &&
15 acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
16 selfInterrupt();
17 } 该函数执行流程：
A.如果tryAcquire()成功获取资源，则直接返回。
B.直接获取资源失败，则通过addWaiter()将线程加入队列尾，并标记为独占模式。
C.通过acquireQueued()让线程在等待队列中获取资源，通过自旋方式，一直获取到后才返回。如果在等待过程中被中断过，则返回true，否则返回false。
D.如果线程在等待获取资源的过程中被中断，只有在获取到资源后才会去响应，执行selfInterrupt进行自我中断。
#1.tryAcquire(int)
该方法是在独占模式下获取资源，成功-ture，失败-false。
1 protected boolean tryAcquire(int arg) {
2 throw new UnsupportedOperationException();
3 } 直接调用该方法会抛出异常，因为AQS只是一个框架，只是定义该接口，具体实现需在子类中实现。
#2.addWaiter(Node mode)
将当前线程加入等待队列的队尾，并返回当前线程所在的节点。
1 private Node addWaiter(Node mode) {
2 // 创建节点，以独占模式
3 Node node = new Node(Thread.currentThread(), mode);
4 // Try the fast path of enq; backup to full enq on failure
5 // 尝试将节点快速放入队尾
6 Node pred = tail;
7 if (pred != null) {
8 node.prev = pred;
9 // 主要通过CAS入队尾
10 if (compareAndSetTail(pred, node)) {
11 pred.next = node;
12 return node;
13 }
14 }
15 // 如果快速入队尾失败，则通过enq方式入对尾
16 enq(node);
17 return node;
18 } CAS操作后面讨论，这里先看enq(final Node node)入队尾操作。
1 private Node enq(final Node node) {
2 // 这里是CAS的“自旋”操作，直到将节点成功加入队尾
3 for (;;) {
4 Node t = tail;
5 // 因为每次入队都是从队尾加入，当队尾为null，则表明队列为null,则需初始化头结点
6 // 并将尾节点也指向头节点
7 if (t == null) { // Must initialize
8 if (compareAndSetHead(new Node()))
9 tail = head;
10 } else { // 通过CAS入队尾，自旋操作
11 node.prev = t;
12 if (compareAndSetTail(t, node)) {
13 t.next = node;
14 return t;
15 }
16 }
17 }
18 } 在线程入队尾后，就需要acquireQueued函数了，该函数的作用是让线程拿到资源，当然还是通过自旋的方式来拿资源，也是就是一个排队的过程。
1 final boolean acquireQueued(final Node node, int arg) {
2 boolean failed = true; // 标记是否成功拿到资源
3 try {
4 boolean interrupted = false; // 标记在等待过程中是否被中断过
5 // 自旋操作
6 for (;;) {
7 final Node p = node.predecessor(); // 拿到当前节点的前向节点
8 // 如果前向节点为head，则表明当前节点排在第二位了，已经得到获取资源的资格
9 if (p == head && tryAcquire(arg)) {
10 // 成功拿到资源后，将head节点指向当前节点
11 // 从这里可以看出，head节点就是当前获取到锁的节点
12 setHead(node);
13 // 将原来head节点的next设置为null，方便GC回收以前的head节点，也就意味着之前拿到锁的节点出队列了
14 p.next = null; // help GC
15 failed = false;
16 return interrupted; // 返回在排队过程中线程是否被中断过
17 }
18 // 到这里，表明线程处于等待状态，自旋直到被unpark
19 if (shouldParkAfterFailedAcquire(p, node) &&
20 parkAndCheckInterrupt())
21 interrupted = true;
22 }
23 } finally {
24 if (failed) // 获取资源失败，则将节点标记为结束状态
25 cancelAcquire(node);
26 }
27 } 在线程排队等待的过程中，有两个关键函数shouldParkAfterFailedAcquire(Node pred, Node node)和parkAndCheckInterrupt()。
1 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
2 int ws = pred.waitStatus; // 前驱节点的状态
3 if (ws == Node.SIGNAL)
4 // 如果前驱节点正处于被唤醒的状态，则正常排队等待即可
5 /*
6 * This node has already set status asking a release
7 * to signal it, so it can safely park.
8 */
9 return true;
10 if (ws > 0) { // 前驱节点处于结束状态
11 /*
12 * Predecessor was cancelled. Skip over predecessors and
13 * indicate retry.
14 */
15 /*
16 *继续向下找，一直找到处于正常等待状态的节点，将当前节点插入其后，其他
17 *无用节点形成一个链，会被GC
18 */
19 do {
20 node.prev = pred = pred.prev;
21 } while (pred.waitStatus > 0);
22 pred.next = node;
23 } else {
24 /*
25 * waitStatus must be 0 or PROPAGATE. Indicate that we
26 * need a signal, but don't park yet. Caller will need to
27 * retry to make sure it cannot acquire before parking.
28 */
29 // 前驱节点状态正常，则把前驱节点的状态设置为SIGNAL，这样前驱节点拿到资源后，可通知下当前节点
30 compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
31 }
32 return false;
33 } 分析以上源码可知：只有当前驱节点的状态为SIGNAL时，当前节点才能正常排队等待，否则需找到一个合适的节点next位置来进行排队等待。
1 private final boolean parkAndCheckInterrupt() {
2 // 使线程进入waitting状态
3 LockSupport.park(this);
4 return Thread.interrupted(); // 返回线程是否被中断过
5 } 该函数作用：当节点正常进入排队后，让线程进入等待状态。
至此acquireQueued()函数总结完成，该函数的具体执行流程:
#1.首先检查节点是否可以立即获取资源。
#2.如果不能立即获取资源，则进行排队，这里需要找到正确的排队点，直到unpark或interrupt唤醒自己。
#3.唤醒后，判断自己是否有资格获取资源，如果拿到资源，则将head指向当前节点，并返回在等待过程是否被中断过，如果没拿到资源，则继续流程2。
acquire小结
到这里acquire(int)函数分析结束，这个函数非常重要，这里再贴上源码：
1 public final void acquire(int arg) {
2 if (!tryAcquire(arg) &&
3 acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
4 selfInterrupt();
5 } #1.调用子类的tryAcquire直接获取资源，如果成功则返回。
#2.如果流程1失败，则将线程加入等待队列的队尾（独占模式）。
#3.在acquireQueued中排队，通过自旋获取资源，直到获取资源才返回。如果在排队过程中线程被中断过返回true，否则返回false。
#4.在排队过程中被中断是不响应的，只有获取到资源后，才进行自我中断，补上中断标记。
整个过程的流程图如下：
②release(int)独占模式释放资源。
1 public final boolean release(int arg) {
2 // 尝试释放资源
3 if (tryRelease(arg)) {
4 Node h = head;
5 if (h != null && h.waitStatus != 0)
6 unparkSuccessor(h); // 唤醒队列中下一个线程
7 return true;
8 }
9 return false;
10 } 释放锁的函数很简单，通过tryRelease尝试释放资源，然后唤醒队列中的其他线程。
tryRelease(int):
1 protected boolean tryRelease(int arg) {
2 throw new UnsupportedOperationException();
3 } 与tryAcquire函数一样，该方法需要子类去实现，如果直接调用会抛异常。
unparkSuccessor(Node node)：
唤醒等待队列中的下一个线程，这里唤醒的是等待队列中最前边那个未放弃的线程，注意看代码注释。
1 private void unparkSuccessor(Node node) {
2 /*
3 * If status is negative (i.e., possibly needing signal) try
4 * to clear in anticipation of signalling. It is OK if this
5 * fails or if status is changed by waiting thread.
6 */
7 int ws = node.waitStatus; // 获取当前线程的状态
8 if (ws < 0) // 如果当前线程状态处于可用状态，则直接将状态值置0
9 compareAndSetWaitStatus(node, ws, 0);
10
11 /*
12 * Thread to unpark is held in successor, which is normally
13 * just the next node. But if cancelled or apparently null,
14 * traverse backwards from tail to find the actual
15 * non-cancelled successor.
16 */
17 Node s = node.next; // 下一个节点
18 if (s == null || s.waitStatus > 0) { // 如果节点为null或节点已处于结束状态
19 s = null;
20 // 从队列尾向前遍历，找到next可用的节点，状态小于0就可用，这里的节点是队列中最前边的可用节点
21 for (Node t = tail; t != null && t != node; t = t.prev)
22 if (t.waitStatus <= 0)
23 s = t;
24 }
25 if (s != null)
26 LockSupport.unpark(s.thread);// 唤醒next线程
27 } 独占模式的主要函数分析完毕，接下来看共享模式。
②acquireShared(int)
共享模式下获取资源，如果成功则直接返回，否则进入等待队列，通过自旋直到获取资源为止。
1 public final void acquireShared(int arg) {
2 // 共享模式下获取资源，如果获取失败，则进入等待队列
3 // 同样该函数需要子类去实现
4 if (tryAcquireShared(arg) < 0)
5 doAcquireShared(arg); // 进入等待队列直到锁获取到为止
6 } tryAcquireShared(int)函数返回值，需要注意下：
负数：表示获取失败；
0：获取成功，但没有剩余资源；
正数：获取成功，且有剩余资源；
#1.doAcquireShared(int)
将线程加入队列尾，然后通过自旋获取资源，直到得到资源才返回。
1 private void doAcquireShared(int arg) {
2 final Node node = addWaiter(Node.SHARED); // 将线程加入队尾，通过共享模式
3 boolean failed = true;// 是否成功
4 try {
5 boolean interrupted = false; // 在自旋过程中是否被中断过
6 for (;;) {
7 final Node p = node.predecessor(); // 前驱节点
8 if (p == head) { // 这里表明当前节点处于head的next位，此时node被唤醒，很可能是head用完来唤醒
9 int r = tryAcquireShared(arg); // 获取资源
10 if (r >= 0) { // 成功
11 setHeadAndPropagate(node, r);// 将head指向自己，还有剩余资源可用的话再唤醒之后的线程
12 p.next = null; // help GC 无用链，帮助GC
13 if (interrupted) // 如果等待过程中被中断过，将中断补上
14 selfInterrupt();
15 failed = false;
16 return;
17 }
18 }
19 // 线程未排在head之后，继续排队，进入waiting状态，等着unpark
20 if (shouldParkAfterFailedAcquire(p, node) &&
21 parkAndCheckInterrupt())
22 interrupted = true; // 中断标记
23 }
24 } finally {
25 if (failed)
26 cancelAcquire(node);
27 }
28 } 整个流程与独占模式的acquireQueued很相似，只是共享模式下，在唤醒自己后，如果还有剩余资源，需要唤醒后续节点。
setHeadAndPropagate(node, int)
将head节点设置为当前节点，如果还有剩余资源，则唤醒下一个线程。
1 private void setHeadAndPropagate(Node node, int propagate) {
2 Node h = head; // Record old head for check below
3 setHead(node); // 将队列中的head执行当前节点
4 /*
5 * Try to signal next queued node if:
6 * Propagation was indicated by caller,
7 * or was recorded (as h.waitStatus either before
8 * or after setHead) by a previous operation
9 * (note: this uses sign-check of waitStatus because
10 * PROPAGATE status may transition to SIGNAL.)
11 * and
12 * The next node is waiting in shared mode,
13 * or we don't know, because it appears null
14 *
15 * The conservatism in both of these checks may cause
16 * unnecessary wake-ups, but only when there are multiple
17 * racing acquires/releases, so most need signals now or soon
18 * anyway.
19 */
20 // 如果还有剩余资源，则唤醒后续线程
21 if (propagate > 0 || h == null || h.waitStatus < 0 ||
22 (h = head) == null || h.waitStatus < 0) {
23 Node s = node.next;
24 if (s == null || s.isShared())
25 doReleaseShared();
26 }
27 } 这里除了将head设置成当前线程，如果有剩余资源，需要唤醒后续节点。
doReleaseShared()
1 private void doReleaseShared() {
2 /*
3 * Ensure that a release propagates, even if there are other
4 * in-progress acquires/releases. This proceeds in the usual
5 * way of trying to unparkSuccessor of head if it needs
6 * signal. But if it does not, status is set to PROPAGATE to
7 * ensure that upon release, propagation continues.
8 * Additionally, we must loop in case a new node is added
9 * while we are doing this. Also, unlike other uses of
10 * unparkSuccessor, we need to know if CAS to reset status
11 * fails, if so rechecking.
12 */
13 // 自旋操作
14 for (;;) {
15 Node h = head;
16 if (h != null && h != tail) {
17 int ws = h.waitStatus;
18 if (ws == Node.SIGNAL) { // 如果head状态为SIGNAL，则需唤醒后续节点
19 // CAS一下当前节点的状态，判断是否为SIGNAL，如果是则置为0，否则继续循环
20 if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
21 continue; // loop to recheck cases
22 unparkSuccessor(h); // 唤醒后继节点
23 }
24 // 如果head节点状态为0,且CAS置为传播状态失败，则继续循环，因为if操作中会改变节点的状态
25 else if (ws == 0 &&
26 !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
27 continue; // loop on failed CAS
28 }
29 if (h == head) // 如果head节点发生了改变，则继续自旋操作，防止上述操作过程中添加了节点的情况 // loop if head changed
30 break;
31 }
32 } 该方法的作用主要是用于唤醒后续节点。
共享模式获取锁操作与独占模式基本相同：先直接获取资源，如果成功，直接返回；如果失败，则将线程加入等待队列尾，直到获取到资源才返回，整个过程忽略中断。不同点在于共享模式下自己拿到资源后，还需要唤醒后续节点。
#2.releaseShared(int）
同享模式下释放资源
1 public final boolean releaseShared(int arg) {
2 if (tryReleaseShared(arg)) { // 尝试释放资源
3 doReleaseShared(); // 唤醒后续节点，前面已经分析
4 return true;
5 }
6 return false;
7 } 共享模式释放资源与独占模式类似，但是独占模式下需要完全释放资源后，才会返回true，而共享模式没有这种要求。
总结
这里只是对AQS的顶层框架进行了简要的分析，具体需要深入其子类中去，AQS的子类按模式分类可聚合成以下几类：
#1.独占模式：
ReentrantLock：可重入锁。state=0独占锁，或者同一线程可多次获取锁（获取+1，释放-1）。
Worker(java.util.concurrent.ThreadPoolExecutor类中的内部类)线程池类。shutdown关闭空闲工作线程，中断worker工作线程是独占的，互斥的。
#2.共享模式：
Semaphore：信号量。 控制同时有多少个线程可以进入代码段。（互斥锁的拓展）
CountDownLatch：倒计时器。 初始化一个值，多线程减少这个值，直到为0，倒计时完毕，执行后续代码。
#3.独占+共享模式：
ReentrantReadWriteLock：可重入读写锁。独占写+共享读，即并发读，互斥写。
后续对这些类进行详细分析。
by Shawn Chen，2019.1.29日，下午。