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 should14          * unconditionally propagate15          */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 must22          *               unpark its successor when it releases or23          *               cancels. To avoid races, acquire methods must24          *               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 node32          *               until transferred, at which time the status33          *               will be set to 0. (Use of this value here has34          *               nothing to do with the other uses of the35          *               field, but simplifies mechanics.)36          *   PROPAGATE:  A releaseShared should be propagated to other37          *               nodes. This is set (for head node only) in38          *               doReleaseShared to ensure propagation39          *               continues, even if other operations have40          *               since intervened.41          *   0:          None of the above42          *43          * The values are arranged numerically to simplify use.44          * Non-negative values mean that a node doesn't need to45          * signal. So, most code doesn't need to check for particular46          * values, just for sign.47          *48          * The field is initialized to 0 for normal sync nodes, and49          * CONDITION for condition nodes.  It is modified using CAS50          * (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 on84          * 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 to10      *        {
    @link #tryAcquire} but is otherwise uninterpreted and11      *        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      * 
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 used10      * to implement method {
    @link Lock#tryLock()}.11      *12      * 
The default13      * implementation throws {
    @link UnsupportedOperationException}.14      *15      * @param arg the acquire argument. This value is always the one16      *        passed to an acquire method, or is the value saved on entry17      *        to a condition wait.  The value is otherwise uninterpreted18      *        and can represent anything you like.19      * @return {
    @code true} if successful. Upon success, this object has20      *         been acquired.21      * @throws IllegalMonitorStateException if acquiring would place this22      *         synchronizer in an illegal state. This exception must be23      *         thrown in a consistent fashion for synchronization to work24      *         correctly.25      * @throws UnsupportedOperationException if exclusive mode is not supported26      */        // 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Node14                 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 GC18                     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true12         if (ws == Node.SIGNAL)13             /*14              * This node has already set status asking a release15              * 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 and21              * 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 we30              * need a signal, but don't park yet.  Caller will need to31              * 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 interrupted3      *4      * @return {
    @code true} if interrupted5      */ // 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 failure10         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 initialize10                 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 this10          * 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 normally18          * just the next node.  But if cancelled or apparently null,19          * traverse backwards from tail to find the actual20          * 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#toc425             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。