并发中有一块很重要的东西就是AQS。接下来一周的目标就是它。
看复杂源码时，一眼望过去，这是什么？不要慌，像剥洋葱一样，一层层剥开（哥，喜欢"扒开"这个词）。
参考资源：
https://www.cnblogs.com/waterystone/p/4920797.html
https://javadoop.com/post/AbstractQueuedSynchronizer#toc4
一概述
大师Doug Lea依赖FIFO（First-in-first-out）等待队列，创建了AQS框架来实现锁和同步器的操作。AQS是LimitLatch，countDownLacth，ReentrantLock，etc 这些类的基础。它们都是继承了AQS类，继承的时候，protected的方法根据业务需要必须重写，也就是tryAcquire，tryRelease，tryAcquireShared，tryReleaseShared，isHeldExclusively中的一部分或是全部。
二AQS结构
1.我们看看AQS的属性：
静态内部类 Node （为什么要写静态内部类，个人觉得，Node是链表结构，在这里作为阻塞队列来使用，只有单独一个地方（AQS中）使用，所以写成静态内部类，也提高了封装性）
// 头结点
private transient volatile Node head;
// 尾结点
private transient volatile Node tail;
// 资源状态，等于0：没有被任何thread占有；大于0：资源已经被其他线程占有
private volatile int state;
2.Node的内部构造
Node类至关重要，我懒了，贴一下代码，发现里面的注释真的很详细，就不再翻译一遍了。
1 /** Marker to indicate a node is waiting in shared mode */
// 共享模式
2 static final Node SHARED = new Node(); 3 /** Marker to indicate a node is waiting in exclusive mode */
// 独占模式
4 static final Node EXCLUSIVE = null; 5 6 /** waitStatus value to indicate thread has cancelled */ 7 static final int CANCELLED = 1;
// 在这里，大家注意一下。waitStatus 值有1，-1，-2，中间越过了0.
// 其实，waitStatus 的值，有0 这种情况，初始值为0 8 /** waitStatus value to indicate successor's thread needs unparking */ 9 static final int SIGNAL = -1; 10 /** waitStatus value to indicate thread is waiting on condition */ 11 static final int CONDITION = -2; 12 /** 13 * waitStatus value to indicate the next acquireShared should 14 * unconditionally propagate 15 */ 16 static final int PROPAGATE = -3; 17 18 /** 19 * Status field, taking on only the values: 20 * SIGNAL: The successor of this node is (or will soon be) 21 * blocked (via park), so the current node must 22 * unpark its successor when it releases or 23 * cancels. To avoid races, acquire methods must 24 * first indicate they need a signal, 25 * then retry the atomic acquire, and then, 26 * on failure, block. 27 * CANCELLED: This node is cancelled due to timeout or interrupt. 28 * Nodes never leave this state. In particular, 29 * a thread with cancelled node never again blocks. 30 * CONDITION: This node is currently on a condition queue. 31 * It will not be used as a sync queue node 32 * until transferred, at which time the status 33 * will be set to 0. (Use of this value here has 34 * nothing to do with the other uses of the 35 * field, but simplifies mechanics.) 36 * PROPAGATE: A releaseShared should be propagated to other 37 * nodes. This is set (for head node only) in 38 * doReleaseShared to ensure propagation 39 * continues, even if other operations have 40 * since intervened. 41 * 0: None of the above 42 * 43 * The values are arranged numerically to simplify use. 44 * Non-negative values mean that a node doesn't need to 45 * signal. So, most code doesn't need to check for particular 46 * values, just for sign. 47 * 48 * The field is initialized to 0 for normal sync nodes, and 49 * CONDITION for condition nodes. It is modified using CAS 50 * (or when possible, unconditional volatile writes). 51 */ 52 volatile int waitStatus; 53 65 volatile Node prev; 79 */ 80 volatile Node next; 81 82 /** 83 * The thread that enqueued this node. Initialized on 84 * construction and nulled out after use. 85 */ 86 volatile Thread thread; 87 98 Node nextWaiter;
接下来，进入到AQS源码分析环节。
概述中提到过下面这几个方法tryAcquire，tryRelease，tryAcquireShared，tryReleaseShared
它们分别是独占锁的获取和释放，共享锁的获取和释放。
这里，我们从独占锁的获取开始讲起。
3.独占锁方法解析
3.1 acquire 方法
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 */
// 尝试获取锁，tryAcquire放回true，表示成功获取锁，就不会再往下走了。 13 public final void acquire(int arg) {
// 尝试获取锁失败，并且，acquireQueued成功，那么就会进入方法中，执行自我中断
// 接下来，开始剥洋葱，方法逐个分析解惑 14 if (!tryAcquire(arg) && 15 acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) 16 selfInterrupt(); 17 }
3.2 tryAcquire 方法
1 /** 2 * Attempts to acquire in exclusive mode. This method should query 3 * if the state of the object permits it to be acquired in the 4 * exclusive mode, and if so to acquire it. 5 * 6 * <p>This method is always invoked by the thread performing 7 * acquire. If this method reports failure, the acquire method 8 * may queue the thread, if it is not already queued, until it is 9 * signalled by a release from some other thread. This can be used 10 * to implement method {@link Lock#tryLock()}. 11 * 12 * <p>The default 13 * implementation throws {@link UnsupportedOperationException}. 14 * 15 * @param arg the acquire argument. This value is always the one 16 * passed to an acquire method, or is the value saved on entry 17 * to a condition wait. The value is otherwise uninterpreted 18 * and can represent anything you like. 19 * @return {@code true} if successful. Upon success, this object has 20 * been acquired. 21 * @throws IllegalMonitorStateException if acquiring would place this 22 * synchronizer in an illegal state. This exception must be 23 * thrown in a consistent fashion for synchronization to work 24 * correctly. 25 * @throws UnsupportedOperationException if exclusive mode is not supported 26 */
// 哇咔咔，这么长注释，不过别着急。慢慢看。
// 这个方法用来查询对象的state是否允许以独占方式来获取锁，如果可以，尝试获取。
// 接下里大家会疑问，为什么这个方法里面只有throw这一行代码。因为，这个方法需要在子类继承的时候需要被重写，这个就是设计模式中的模板方法。
// 同时，这个方法没有被做成abstract方法，因为，子类继承的时候为了自己的需求，只需要实现独占模式的方法或是共享模式的方法即可，不用都去实现。 27 protected boolean tryAcquire(int arg) { 28 throw new UnsupportedOperationException(); 29 }
3.3 acquireQueued方法
1 /** 2 * Acquires in exclusive uninterruptible mode for thread already in 3 * queue. Used by condition wait methods as well as acquire. 4 * 5 * @param node the node 6 * @param arg the acquire argument 7 * @return {@code true} if interrupted while waiting 8 */
// 线程挂起后，被解锁，就是在这个方法里实现的
// 方法返回结果：等待时，是否被中断
9 final boolean acquireQueued(final Node node, int arg) {
// failed true：表示没有拿到资源 false：表示成功拿到资源 10 boolean failed = true; 11 try {
// interrupted 是否被中断 12 boolean interrupted = false;
// 又一个自旋的使用哦 13 for (;;) {
// 获取前一个节点Node 14 final Node p = node.predecessor(); 15 if (p == head && tryAcquire(arg)) {
// 设置头结点，将原先的头结点与node节点的连接，断开 16 setHead(node); 17 p.next = null; // help GC 18 failed = false; 19 return interrupted; 20 }
// 获取锁失败，是否应该挂起当前线程
// p 是前驱节点，node是当前线程节点 21 if (shouldParkAfterFailedAcquire(p, node) &&
// 获取锁失败，挂起线程的操作在下面方法里实施 22 parkAndCheckInterrupt()) 23 interrupted = true; 24 } 25 } finally { 26 if (failed) 27 cancelAcquire(node); 28 } 29 }
3.3.1 shouldParkAfterFailedAcquire方法
1 /** 2 * Checks and updates status for a node that failed to acquire. 3 * Returns true if thread should block. This is the main signal 4 * control in all acquire loops. Requires that pred == node.prev. 5 * 6 * @param pred node's predecessor holding status 7 * @param node the node 8 * @return {@code true} if thread should block 9 */
// 这个方法用途：获取锁失败，判断是否需要挂起线程 10 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { 11 int ws = pred.waitStatus;
// 等待状态 SIGNAL表示前驱节点状态正常，当前线程需要挂起，直接返回true 12 if (ws == Node.SIGNAL) 13 /* 14 * This node has already set status asking a release 15 * to signal it, so it can safely park. 16 */
// 这个节点已经让请求release的状态来标识，所以它可以安全的park了 17 return true; 18 if (ws > 0) { 19 /* 20 * Predecessor was cancelled. Skip over predecessors and 21 * indicate retry. 22 */
// ws大于0的状态1，表示取消了排队。
// 如果取消了排队，接着再去找前一个，前一个也被取消了，就找前一个的前一个，总会有一个没被取消的。 23 do { 24 node.prev = pred = pred.prev; 25 } while (pred.waitStatus > 0); 26 pred.next = node; 27 } else { 28 /* 29 * waitStatus must be 0 or PROPAGATE. Indicate that we 30 * need a signal, but don't park yet. Caller will need to 31 * retry to make sure it cannot acquire before parking. 32 */
// else的情况，就是waitStatus 为0，-2，-3
// 用CAS将前驱节点状态设为-1（Node.SIGNAL） 33 compareAndSetWaitStatus(pred, ws, Node.SIGNAL); 34 } 35 return false; 36 }
3.3.2 parkAndCheckInterrupt方法
1 /** 2 * Convenience method to park and then check if interrupted 3 * 4 * @return {@code true} if interrupted 5 */
// 在shouldParkAfterFailedAcquire返回true的时候，才会执行这个方法，这个方法的作用就是挂起线程 6 private final boolean parkAndCheckInterrupt() {
// 调用park方法，使线程挂起 （使用unpark方法来唤醒线程） 7 LockSupport.park(this);
// 查看线程是否被中断 8 return Thread.interrupted(); 9 }
3.4 addWaiter 方法
1 /** 2 * Creates and enqueues node for current thread and given mode. 3 * 4 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared 5 * @return the new node 6 */
// 将Node放进阻塞队列的末尾
7 private Node addWaiter(Node mode) {
// 生成一个node节点，保存当前的线程和占用模式（独占模式或是共享模式） 8 Node node = new Node(Thread.currentThread(), mode); 9 // Try the fast path of enq; backup to full enq on failure 10 Node pred = tail;
// 查询tail节点是否有值，有值代表的是此链表不为空 11 if (pred != null) {
// node相连接，很简单，就是改变指针指向，可以参考数据结构 链表 12 node.prev = pred;
// compareAndSetTail这个方法的操作就是比较赋值，经典的CAS方法，不明白的可以参照下面资源
// http://www.cnblogs.com/lihao007/p/8654787.html 13 if (compareAndSetTail(pred, node)) { 14 pred.next = node; 15 return node; 16 } 17 } 18 enq(node); 19 return node; 20 }
3.4.1 eng（）方法
1 /** 2 * Inserts node into queue, initializing if necessary. See picture above. 3 * @param node the node to insert 4 * @return node's predecessor 5 */
// 来到这个方法有两种可能。一是等待线程队列为空，二是其他线程更新了队列 6 private Node enq(final Node node) {
// 又是自旋方法（乐观锁），AQS源码中出现多次，因为需要线程的不安全 7 for (;;) { 8 Node t = tail; 9 if (t == null) { // Must initialize 10 if (compareAndSetHead(new Node())) 11 tail = head; 12 } else {
// 将node放在队列最后，有线程竞争的话，排不上重排 13 node.prev = t; 14 if (compareAndSetTail(t, node)) { 15 t.next = node; 16 return t; 17 } 18 } 19 } 20 }
独占模式获取锁，就分析到这里了。
4.release独占模式 释放锁
1 /** 2 * Releases in exclusive mode. Implemented by unblocking one or 3 * more threads if {@link #tryRelease} returns true. 4 * This method can be used to implement method {@link Lock#unlock}. 5 * 6 * @param arg the release argument. This value is conveyed to 7 * {@link #tryRelease} but is otherwise uninterpreted and 8 * can represent anything you like. 9 * @return the value returned from {@link #tryRelease} 10 */
// 独占模式下，释放锁 11 public final boolean release(int arg) {
// tryRelease方法是模板方法，留给子类定义。 12 if (tryRelease(arg)) { 13 Node h = head;
// 首先判断阻塞队列是否为空。接下来，判断waitStatus的状态，0的状态是初始化是被赋予的值。只有是非0的状态，才说明head节点后面是有其它节点的。 14 if (h != null && h.waitStatus != 0) 15 unparkSuccessor(h); 16 return true; 17 } 18 return false; 19 }
4.1 unparkSuccessor
1 /** 2 * Wakes up node's successor, if one exists. 3 * 4 * @param node the node 5 */ 6 private void unparkSuccessor(Node node) { 7 /* 8 * If status is negative (i.e., possibly needing signal) try 9 * to clear in anticipation of signalling. It is OK if this 10 * fails or if status is changed by waiting thread. 11 */ 12 int ws = node.waitStatus; 13 if (ws < 0) 14 compareAndSetWaitStatus(node, ws, 0); 15 16 /* 17 * Thread to unpark is held in successor, which is normally 18 * just the next node. But if cancelled or apparently null, 19 * traverse backwards from tail to find the actual 20 * non-cancelled successor. 21 */ 22 Node s = node.next; 23 if (s == null || s.waitStatus > 0) { 24 s = null;
// 这里，是从队列末尾向前查找没有被取消的节点
// 这里，我当时对为什么从后向前查找有疑问，后来看了文章明白了。地址：https://javadoop.com/post/AbstractQueuedSynchronizer#toc4 25 for (Node t = tail; t != null && t != node; t = t.prev) 26 if (t.waitStatus <= 0) 27 s = t; 28 } 29 if (s != null) 30 LockSupport.unpark(s.thread); 31 }
就写到这里了，仍需努力，以后有想法了，慢慢补充，写的不对的地方，欢迎指正，共同探讨。