JUC回顾之-AQS同步器的实现原理

1.什么是AQS?

     AQS的核心思想是基于volatile int state这样的volatile变量,配合Unsafe工具对其原子性的操作来实现对当前锁状态进行修改。同步器内部依赖一个FIFO的双向队列来完成资源获取线程的排队工作。

2.同步器的应用

 同步器主要使用方式是继承,子类通过继承同步器并实现它的抽象方法来管理同步状态,对同步状态的修改或者访问主要通过同步器提供的3个方法:

  • getState() 获取当前的同步状态
  • setState(int newState) 设置当前同步状态
  • compareAndSetState(int expect,int update) 使用CAS设置当前状态,该方法能够保证状态设置的原子性。

     同步器可以支持独占式的获取同步状态,也可以支持共享式的获取同步状态,这样可以方便实现不同类型的同步组件。

     同步器也是实现锁的关键,在锁的实现中聚合同步器,利用同步器实现锁的语义。

3.AQS同步队列

   同步器AQS内部的实现是依赖同步队列(一个FIFO的双向队列,其实就是数据结构双向链表)来完成同步状态的管理。

   当前线程获取同步状态失败时,同步器AQS会将当前线程和等待状态等信息构造成为一个节点(node)加入到同步队列,同时会阻塞当前线程;

   当同步状态释放的时候,会把首节点中的线程唤醒,使首节点的线程再次尝试获取同步状态。AQS是独占锁和共享锁的实现的父类。   

 

4.AQS锁的类别:分为独占锁和共享锁两种。

  • 独占锁:锁在一个时间点只能被一个线程占有。根据锁的获取机制,又分为“公平锁”和“非公平锁”。等待队列中按照FIFO的原则获取锁,等待时间越长的线程越先获取到锁,这就是公平的获取锁,即公平锁。而非公平锁,线程获取的锁的时候,无视等待队列直接获取锁。ReentrantLock和ReentrantReadWriteLock.Writelock是独占锁。
  • 共享锁:同一个时候能够被多个线程获取的锁,能被共享的锁。JUC包中ReentrantReadWriteLock.ReadLock,CyclicBarrier,CountDownLatch和Semaphore都是共享锁。

  JUC包中的锁的包括:Lock接口,ReadWriteLock接口;Condition条件,LockSupport阻塞原语。      

  AbstractOwnableSynchronizer/AbstractQueuedSynchronizer/AbstractQueuedLongSynchronizer三个抽象类,

  ReentrantLock独占锁,ReentrantReadWriteLock读写锁。CountDownLatch,CyclicBarrier和Semaphore也是通过AQS来实现的。

  下面是AQS和使用AQS实现的一些锁,以及通过AQS实现的一些工具类的架构图:

 

                         图 1.依赖AQS实现的锁和工具类

                                                                                

 

5.AQS同步器的结构:同步器拥有首节点(head)和尾节点(tail)。同步队列的基本结构如下:

 

                                                       图 1.同步队列的基本结构 compareAndSetTail(Node expect,Node update)

  • 同步队列设置尾节点(未获取到锁的线程加入同步队列): 同步器AQS中包含两个节点类型的引用:一个指向头结点的引用(head),一个指向尾节点的引用(tail),当一个线程成功的获取到锁(同步状态),其他线程无法获取到锁,而是被构造成节点(包含当前线程,等待状态)加入到同步队列中等待获取到锁的线程释放锁。这个加入队列的过程,必须要保证线程安全。否则如果多个线程的环境下,可能造成添加到队列等待的节点顺序错误,或者数量不对。因此同步器提供了CAS原子的设置尾节点的方法(保证一个未获取到同步状态的线程加入到同步队列后,下一个未获取的线程才能够加入)。  如下图,设置尾节点:

 图 2.尾节点的设置  节点加入到同步队列

  •  同步队列设置首节点(原头节点释放锁,唤醒后继节点):同步队列遵循FIFO,头节点是获取锁(同步状态)成功的节点,头节点在释放同步状态的时候,会唤醒后继节点,而后继节点将会在获取锁(同步状态)成功时候将自己设置为头节点。设置头节点是由获取锁(同步状态)成功的线程来完成的,由于只有一个线程能够获取同步状态,则设置头节点的方法不需要CAS保证,只需要将头节点设置成为原首节点的后继节点 ,并断开原头结点的next引用。如下图,设置首节点:

图 3.首节点的设置

 6.独占式的锁的获取:调用同步器的acquire(int arg)方法可以获取同步状态,该方法对中断不敏感,即线程获取同步状态失败后进入同步队列,后续对线程进行中断操作时,线程不会从同步队列中移除。

    (1) 当前线程实现通过tryAcquire()方法尝试获取锁,获取成功的话直接返回,如果尝试失败的话,进入等待队列排队等待,可以保证线程安全(CAS)的获取同步状态。

    (2) 如果尝试获取锁失败的话,构造同步节点(独占式的Node.EXCLUSIVE),通过addWaiter(Node node,int args)方法,将节点加入到同步队列的队列尾部。

    (3) 最后调用acquireQueued(final Node node, int args)方法,使该节点以死循环的方式获取同步状态,如果获取不到,则阻塞节点中的线程。acquireQueued方法当前线程在死循环中获取同步状态,而只有前驱节点是头节点的时候才能尝试获取锁(同步状态)( p == head && tryAcquire(arg))。

    原因是:1.头结点是成功获取同步状态的节点,而头结点的线程释放锁以后,将唤醒后继节点,后继节点线程被唤醒后要检查自己的前驱节点是否为头结点。

                2.维护同步队列的FIFO原则,节点进入同步队列以后,就进入了一个自旋的过程,每个节点(后者说每个线程)都在自省的观察。

 下图为节点自旋检查自己的前驱节点是否为头结点:

                              图 4 节点自旋获取同步状态

 

独占式的锁的获取源码:

acquire方法源码如下
/**
     * Acquires in exclusive(互斥) mode, ignoring(忽视) interrupts.  Implemented
     * by invoking at least once {@link #tryAcquire},
     * returning on success.  Otherwise the thread is queued(排队), possibly
     * repeatedly(反复的) blocking and unblocking, invoking {@link
     * #tryAcquire} until success.  This method can be used
     * to implement method {@link Lock#lock}.
     *
     * @param arg the acquire argument.  This value is conveyed(传达) to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     *        
     *  独占式的获取同步状态      
     *        
     */
    public final void acquire(int arg) {
        if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }


 

  尝试获取锁:tryAcquire方法:如果获取到了锁,tryAcquire返回true,反之,返回false。

//方法2:
    protected final boolean tryAcquire(int acquires) {
        // 获取当前线程
        final Thread current = Thread.currentThread();
        // 获取“独占锁”的状态,获取父类AQS的标志位
        int c = getState();
        //c == 0 意思是锁(同步状态)没有被任何线程所获取
        //1.当前线程是否是同步队列中头结点Node,如果是的话,则获取该锁,设置锁的状态,并设置锁的拥有者为当前线程
        if (c == 0) {
            if (!hasQueuedPredecessors() &&

// 修改下状态为,这里的acquires的值是1,是写死的调用子类的lock的方法的时候传进来的,如果c == 0,compareAndSetState操作会更新成功为1. compareAndSetState(0, acquires)) {
// 上面CAS操作更新成功为1,表示当前线程获取到了锁,因为将当前线程设置为AQS的一个变量中,代表这个线程拿走了锁。 setExclusiveOwnerThread(current);
return true; } } //2.如果c不为0,即状态不为0,表示锁已经被拿走。
//因为ReetrantLock是可重入锁,是可以重复lock和unlock的,所以这里还要判断一次,获取锁的线程是否为当前请求锁的线程。 else if (current == getExclusiveOwnerThread()) {
//如果是,state继续加1,这里nextc的结果就会 > 1,这个判断表示获取到的锁的线程,还可以再获取锁,这里就是说的可重入的意思
int nextc = c + acquires; if (nextc < 0) throw new Error("Maximum lock count exceeded"); setState(nextc); return true; } return false; }

addWaiter方法的源码:回到aquire方法,如果尝试获取同步状态(锁)失败的话,则构造同步节点(独占式的Node.EXCLUSIVE),
通过addWaiter(Node node,int args)方法
将该节点加入到同步队列的队尾。

/**
    * Creates and enqueues node for current thread and given mode.
    *
    * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
    * @return the new node
    * 
    * 
    * 如果尝试获取同步状态失败的话,则构造同步节点(独占式的Node.EXCLUSIVE),通过addWaiter(Node node,int args)方法将该节点加入到同步队列的队尾。
    * 
    */
    private Node addWaiter(Node mode) {
// 用当前线程够着一个Node对象,mode是一个表示Node类型的字段,或者说是这个节点是独占的还是共享的,或者说AQS的这个队列中,哪些节点是独占的,哪些节点是共享的。 Node node
= new Node(Thread.currentThread(), mode); // Try the fast path of enq; backup to full enq on failure Node pred = tail;
//队列不为空的时候
if (pred != null) { node.prev = pred; // 确保节点能够被线程安全的添加,使用CAS方法
// 尝试修改为节点为最新的节点,如果修改失败,意味着有并发,这个时候进入enq中的死循环,进行“自旋”的方式修改 if (compareAndSetTail(pred, node)) { pred.next = node; return node; } }
//进入自旋 enq(node);
return node; }




enq方法的源码:同步器通过死循环的方式来保证节点的正确添加,在“死循环” 中通过CAS将节点设置成为尾节点之后,当前线程才能从该方法中返回,否则
当前线程不断的尝试设置。

enq方法将并发添加节点的请求通过CAS变得“串行化”了。
/**
     * Inserts node into queue, initializing if necessary. See picture above.
     * @param node the node to insert
     * @return node's predecessor
     * 
     * 同步器通过死循环的方式来保证节点的正确添加,在“死循环” 中通过CAS将节点设置成为尾节点之后,当前线程才能从该方法中返回,否则当前线程不断的尝试设置。
     * enq方法将并发添加节点的请求通过CAS变得“串行化”了。
     * 
     */
    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

 

acquireQueued方法:在队列中的线程获取锁的过程:
/**
    * Acquires in exclusive uninterruptible mode for thread already in
    * queue. Used by condition wait methods as well as acquire.
    *
    * @param node the node
    * @param arg the acquire argument
    * @return {@code true} if interrupted while waiting
    * 
    * acquireQueued方法当前线程在死循环中获取同步状态,而只有前驱节点是头节点才能尝试获取同步状态(锁)( p == head && tryAcquire(arg))
    *     原因是:1.头结点是成功获取同步状态(锁)的节点,而头节点的线程释放了同步状态以后,将会唤醒其后继节点,后继节点的线程被唤醒后要检查自己的前驱节点是否为头结点。
    *           2.维护同步队列的FIFO原则,节点进入同步队列之后,就进入了一个自旋的过程,每个节点(或者说是每个线程)都在自省的观察。
    * 
    */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
//死循环检查(自旋检查)当前节点的前驱节点是否为头结点,才能获取锁
for (;;) {
// 获取节点的前驱节点
final Node p = node.predecessor(); if (p == head && tryAcquire(arg)) {//节点中的线程循环的检查,自己的前驱节点是否为头节点
//将当前节点设置为头结点,移除之前的头节点 setHead(node); p.next
= null; // help GC failed = false; return interrupted; }
// 否则检查前一个节点的状态,看当前获取锁失败的线程是否要挂起
if (shouldParkAfterFailedAcquire(p, node) &&
//如果需要挂起,借助JUC包下面的LockSupport类的静态方法park挂起当前线程,直到被唤醒
parkAndCheckInterrupt()) interrupted = true; } } finally {
//如果有异常
if (failed)
//取消请求,将当前节点从队列中移除 cancelAcquire(node); } }


 

独占式的获取同步状态的流程如下: 

图5 独占式的获取同步状态的流程

 7.独占锁的释放:下面直接看源码:

 

 /* 
1. unlock():unlock()是解锁函数,它是通过AQS的release()函数来实现的。 * 在这里,“1”的含义和“获取锁的函数acquire(1)的含义”一样,它是设置“释放锁的状态”的参数。 * 由于“公平锁”是可重入的,所以对于同一个线程,每释放锁一次,锁的状态-1。 unlock()在ReentrantLock.java中实现的,源码如下:
*/ public void unlock() { sync.release(1); }

 

release()会调用tryRelease方法尝试释放当前线程持有的锁(同步状态),成功的话唤醒后继线程,并返回true,否则直接返回false

    /**
    * Releases in exclusive mode.  Implemented by unblocking one or
    * more threads if {@link #tryRelease} returns true.
    * This method can be used to implement method {@link Lock#unlock}.
    *
    * @param arg the release argument.  This value is conveyed to
    *        {@link #tryRelease} but is otherwise uninterpreted and
    *        can represent anything you like.
    * @return the value returned from {@link #tryRelease}
    * 
    * 
    * 
    */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

 

 // tryRelease() 尝试释放当前线程的同步状态(锁)
  protected final boolean tryRelease(int releases) {
            //c为释放后的同步状态
          int c = getState() - releases;
          //判断当前释放锁的线程是否为获取到锁(同步状态)的线程,不是抛出异常(非法监视器状态异常)
          if (Thread.currentThread() != getExclusiveOwnerThread())
              throw new IllegalMonitorStateException();
          boolean free = false;
          //如果锁(同步状态)已经被当前线程彻底释放,则设置锁的持有者为null,同步状态(锁)变的可获取
          if (c == 0) {
              free = true;
              setExclusiveOwnerThread(null);
          }
          setState(c);
          return free;
      }
      

 释放锁成功后,找到AQS的头结点,并唤醒它即可:

// 4. 唤醒头结点的后继节点
     
     private void unparkSuccessor(Node node) {
         //获取头结点(线程)的状态
        int ws = node.waitStatus;
        //如果状态<0,设置当前线程对应的锁的状态为0
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);
            
        Node s = node.next;
        
         //解释:Thread to unpark is held in successor, which is normally just the next node. 
         //But if cancelled or apparently(显然) null, traverse backwards(向后遍历) from tail to find the actual(实际的) non-cancelled successor(前继节点).
         //从队列尾部开始往前去找最前面的一个waitStatus小于0的节点。
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        //唤醒后继节点对应的线程
        if (s != null)
            LockSupport.unpark(s.thread);
    }
    

 上面说的是ReentrantLock的公平锁获取和释放的AQS的源码,唯独还剩下一个非公平锁NonfairSync没说,其实,它和公平锁的唯一区别就是获取锁的方式不同,公平锁是按前后顺序一次获取锁,非公平锁是抢占式的获取锁,那ReentrantLock中的非公平锁NonfairSync是怎么实现的呢?

 /**
     * Sync object for non-fair locks
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

非公平锁的lock的时候多了上面加粗的代码:在lock的时候先直接用cas判断state变量是否为0(尝试获取锁),成功的话更新成1,表示当前线程获取到了锁,不需要在排队,从而直接抢占的目的。而对于公平锁的lock方法是一开始就走AQS的双向队列排队获取锁。更详细的关于ReentrantLock的实现请看后面写的一篇文章:http://www.cnblogs.com/200911/p/6035765.html

 

 总结:在获取同步状态的时候,同步器维护一个同步队列,获取失败的线程会被加入到队列中并在队列中自旋;移除队列(或停止自旋)的条件是前驱节点为头结点并且获取到了同步状态。在释放同步状态时,同步器调用tryRelease(int args)方法释放同步状态,然后唤醒头结点的后继节点。AQS的实现思路其实并不复杂,用一句话准确的描述的话,其实就是使用标志状态位status(volatile int state)和 一个双向队列的入队和出队来实现。AQS维护一个线程何时访问的状态,它只是对状态负责,而这个状态的含义,子类可以自己去定义。

 

 

 

自己注释的AQS的源码:如下:

 

 

public class AbstractQueuedSynchronizerTest {

    /**
     * 
     * (AQS节点的定义,同步队列的节点定义)
     *
     * <p>
     * 修改历史:                                            <br>  
     * 修改日期            修改人员       版本             修改内容<br>  
     * -------------------------------------------------<br>  
     * 2016年7月4日 上午10:26:38   user     1.0        初始化创建<br>
     * </p> 
     *
     * @author        Peng.Li 
     * @version        1.0  
     * @since        JDK1.7
     */
    static final class Node {

        /** Marker to indicate a node is waiting in shared mode */
        static final Node SHARED = new Node();
        /** Marker to indicate a node is waiting in exclusive mode 
         * 
         * */
        static final Node EXCLUSIVE = null;

        /** waitStatus value to indicate thread has cancelled 
         *     在同步队列中等待的线程等待超时或者被中断,需要从同步队列中取消等待
         * */
        static final int CANCELLED = 1;

        /** waitStatus value to indicate successor's thread needs unparking(唤醒)
         *     后继节点的线程处于等待状态,而当前的节点如果释放了同步状态或者被取消,将会通知后继节点,使后继节点的线程得以运行。
         **/
        static final int SIGNAL = -1;

        /** waitStatus value to indicate thread is waiting on condition 
         *  节点在等待队列中,节点的线程等待在Condition上,当其他线程对Condition调用了signal()方法后,该节点会从等待队列中转移到同步队列中,加入到同步状态的获取中
         **/
        static final int CONDITION = -2;
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally(无条件的) propagate(传播)
         * 
         * 表示下一次共享式同步状态获取将会被无条件地传播下去
         */
        static final int PROPAGATE = -3;

        /**
         * Status field, taking on only the values:
         *   SIGNAL:     The successor of this node is (or will soon be)
         *               blocked (via park), so the current node must
         *               unpark its successor when it releases or
         *               cancels. To avoid races, acquire methods must
         *               first indicate they need a signal,
         *               then retry the atomic acquire, and then,
         *               on failure, block.
         *   CANCELLED:  This node is cancelled due to timeout or interrupt.
         *               Nodes never leave this state. In particular,
         *               a thread with cancelled node never again blocks.
         *   CONDITION:  This node is currently on a condition queue.
         *               It will not be used as a sync queue node
         *               until transferred, at which time the status
         *               will be set to 0. (Use of this value here has
         *               nothing to do with the other uses of the
         *               field, but simplifies mechanics.)
         *   PROPAGATE:  A releaseShared should be propagated(传播) to other
         *               nodes. This is set (for head node only) in
         *               doReleaseShared to ensure propagation
         *               continues, even if other operations have
         *               since intervened(干涉).
         *   0:          None of the above
         *
         * The values are arranged numerically to simplify use.
         * Non-negative values mean that a node doesn't need to
         * signal. So, most code doesn't need to check for particular
         * values, just for sign.
         *
         * The field is initialized to 0 for normal sync nodes, and
         * CONDITION for condition nodes.  It is modified using CAS
         * (or when possible, unconditional volatile writes).
         * 
         * 使用CAS更改状态,volatile保证线程可见性,即被一个线程修改后,状态会立马让其他线程可见。
         * 
         */
        volatile int waitStatus;

        /**
         * Link to predecessor node that current node/thread relies on
         * for checking waitStatus. Assigned during enqueing(入队), and nulled
         * out (for sake of GC) only upon dequeuing.  Also, upon
         * cancellation of a predecessor, we short-circuit while
         * finding a non-cancelled one, which will always exist
         * because the head node is never cancelled: A node becomes
         * head only as a result of successful acquire. A
         * cancelled thread never succeeds in acquiring, and a thread only
         * cancels itself, not any other node.
         * 
         * 前驱节点,当前节点加入到同步队列中被设置
         */
        volatile Node prev;

        /**
         * Link to the successor node that the current node/thread
         * unparks upon release. Assigned during enqueuing, adjusted
         * when bypassing cancelled predecessors, and nulled out (for
         * sake of GC) when dequeued.  The enq operation does not
         * assign next field of a predecessor until after attachment,
         * so seeing a null next field does not necessarily mean that
         * node is at end of queue. However, if a next field appears
         * to be null, we can scan prev's from the tail to
         * double-check.  The next field of cancelled nodes is set to
         * point to the node itself instead of null, to make life
         * easier for isOnSyncQueue.
         * 
         * 后继节点
         */
        volatile Node next;

        /**
         * The thread that enqueued this node.  Initialized on
         * construction and nulled out after use.
         * 
         * 获取同步状态的线程
         */
        volatile Thread thread;

        /**
         * Link to next node waiting on condition, or the special
         * value SHARED.  Because condition queues are accessed only
         * when holding in exclusive(独有的) mode, we just need a simple
         * linked queue to hold nodes while they are waiting on
         * conditions. They are then transferred(移动到) to the queue(同步队列) to
         * re-acquire. And because conditions can only be exclusive,
         * we save a field by using special value to indicate shared
         * mode.
         * 
         * 等待队列中的后继节点,如果当前节点是共享的,那么这个字段是一个SHARED常量,
         * 也就是说节点类型(独占和共享)和等待队列中的后继节点共用同一个字段。
         */
        Node nextWaiter;

        /**
         * Returns true if node is waiting in shared mode
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        /**
         * Returns previous node, or throws NullPointerException if null.
         * Use when predecessor cannot be null.  The null check could
         * be elided, but is present to help the VM.
         *
         * @return the predecessor of this node
         */
        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }

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

        Node(Thread thread, Node mode) { // Used by addWaiter
            this.nextWaiter = mode;
            this.thread = thread;
        }

        Node(Thread thread, int waitStatus) { // Used by Condition
            this.waitStatus = waitStatus;
            this.thread = thread;
        }
    }

    /**
     * Head of the wait queue, lazily initialized.  Except for (除...以外)
     * initialization(初始化), it is modified only via method setHead.  Note:
     * If head exists, its waitStatus is guaranteed not to be
     * CANCELLED.(如果head引用已经存在,等待状态保证不会被取消)
     */
    private transient volatile Node head;

    /**
     * Tail of the wait queue(等待队列), lazily initialized.  Modified only via
     * method enq to add new wait node.
     */
    private transient volatile Node tail;

    /**
     * The synchronization state.
     * 同步状态,线程可见的,共享内存里面保存
     * 
     */
    private volatile int state;

    /**
     * Returns the current value of synchronization state.
     * This operation has memory semantics of a <tt>volatile</tt> read.
     * @return current state value
     * 
     * 得到同步状态的值
     * 
     */
    protected final int getState() {
        return state;
    }

    /**
     * Sets the value of synchronization state.
     * This operation has memory semantics of a <tt>volatile</tt> write.
     * @param newState the new state value
     */
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * Acquires in exclusive(互斥) mode, ignoring(忽视) interrupts.  Implemented
     * by invoking at least once {@link #tryAcquire},
     * returning on success.  Otherwise the thread is queued(排队), possibly
     * repeatedly(反复的) blocking and unblocking, invoking {@link
     * #tryAcquire} until success.  This method can be used
     * to implement method {@link Lock#lock}.
     *
     * @param arg the acquire argument.  This value is conveyed(传达) to
     *        {@link #tryAcquire} but is otherwise uninterpreted and
     *        can represent anything you like.
     *        
     *  独占式的获取同步状态      
     *        
     */
    public final void acquire(int arg) {
        if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
    * Creates and enqueues node for current thread and given mode.
    *
    * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
    * @return the new node
    * 
    * 
    * 如果尝试获取同步状态失败的话,则构造同步节点(独占式的Node.EXCLUSIVE),通过    addWaiter(Node node,int args)方法将该节点加入到同步队列的队尾。

    * 
    */
    private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            // 确保节点能够被安全的添加
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

    /**
     * Convenience method to interrupt current thread.
     * 分析:如果在acquireQueued()中,当前线程被中断过,则执行selfInterrupt();否则不会执行。
     * 线程在阻塞状态被“中断唤醒”而获取CPU的执行权;但是该线程前面还有其他等待锁的线程,根据公平性原则,该线程仍然无法获取到锁,他会再次阻塞。
     * 直到该线程被他前面等待锁的线程唤醒;线程才会获取锁。该线程“成功获取锁并真正执行起来之前”,他的中断会被忽略并且中断标记会被清除,因为在parkAndCheckInterrupt()中,
     * 我们线程的中断状态时调用了Thread.interrupted(),这个函数在返回中断状态之后,还会清除中断状态,正因为清除了中断状态,所以在selfInterrupt重新产生一个中断。
     * 
     * 
     * 当前线程自己产生一个中断
     */
    private static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

    /**
    * Acquires in exclusive uninterruptible mode for thread already in
    * queue. Used by condition wait methods as well as acquire.
    *
    * @param node the node
    * @param arg the acquire argument
    * @return {@code true} if interrupted while waiting
    * 
    * acquireQueued方法当前线程在死循环中获取同步状态,而只有前驱节点是头节点才能尝试获取同步状态( p == head && tryAcquire(arg))
    *     原因是:1.头结点是成功获取同步状态的节点,而头节点的线程释放了同步状态以后,将会唤醒其后继节点,后继节点的线程被唤醒后要检查自己的前驱节点是否为头结点。
    *           2.维护同步队列的FIFO原则,节点进入同步队列之后,就进入了一个自旋的过程,每个节点(或者说是每个线程)都在自省的观察。
    * 
    */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     * @param node the node to insert
     * @return node's predecessor
     * 
     * 同步器通过死循环的方式来保证节点的正确添加,在“死循环” 中通过CAS将节点设置成为尾节点之后,当前线程才能从该方法中返回,否则当前线程不断的尝试设置。
     * enq方法将并发添加节点的请求通过CAS变得“串行化”了。
     * 
     */
    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

    /**
    * Convenience method to park and then check if interrupted
    *
    * @return {@code true} if interrupted
    * 
    * 阻塞当前线程
    * 
    */
    private final boolean parkAndCheckInterrupt() {
        // 阻塞当前线程
        LockSupport.park(this);
        // 线程被唤醒之后的中断状态
        return Thread.interrupted();
    }

    /**
    * Releases in exclusive mode.  Implemented by unblocking one or
    * more threads if {@link #tryRelease} returns true.
    * This method can be used to implement method {@link Lock#unlock}.
    *
    * @param arg the release argument.  This value is conveyed to
    *        {@link #tryRelease} but is otherwise uninterpreted and
    *        can represent anything you like.
    * @return the value returned from {@link #tryRelease}
    * 
    * 释放公平锁
    * 
    */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

    /**
     * Attempts to set the state to reflect a release in exclusive
     * mode.
     *
     * <p>This method is always invoked by the thread performing release.
     *
     * <p>The default implementation throws
     * {@link UnsupportedOperationException}.
     *
     * @param arg the release argument. This value is always the one
     *        passed to a release method, or the current state value upon
     *        entry to a condition wait.  The value is otherwise
     *        uninterpreted and can represent anything you like.
     * @return {@code true} if this object is now in a fully released
     *         state, so that any waiting threads may attempt to acquire;
     *         and {@code false} otherwise.
     * @throws IllegalMonitorStateException if releasing would place this
     *         synchronizer in an illegal state. This exception must be
     *         thrown in a consistent fashion for synchronization to work
     *         correctly.
     * @throws UnsupportedOperationException if exclusive mode is not supported
     */
    protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }

    /**
    * Checks and updates status for a node that failed to acquire.
    * Returns true if thread should block. This is the main signal
    * control in all acquire loops.  Requires that pred == node.prev
    *
    * @param pred node's predecessor holding status
    * @param node the node
    * @return {@code true} if thread should block
    * 返回当前线程是否应该阻塞
    * 
    * 说明:
    (01) 关于waitStatus请参考下表(中扩号内为waitStatus的值),更多关于waitStatus的内容,可以参考前面的Node类的介绍。

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

    规则1:如果前继节点状态为SIGNAL,表明当前节点需要被unpark(唤醒),此时则返回true。
    规则2:如果前继节点状态为CANCELLED(ws>0),说明前继节点已经被取消,则通过先前回溯找到一个有效(非CANCELLED状态)的节点,并返回false。
    规则3:如果前继节点状态为非SIGNAL、非CANCELLED,则设置前继的状态为SIGNAL,并返回false。
    * 
    */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        // 前驱节点的状态
        int ws = pred.waitStatus;
        // 如果前驱节点是SIGNAL状态,则意味着当前线程需要unpark唤醒,此时返回true
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release to signal it, so it can safely park.
             */
            return true;
        // 如果前继节点是取消的状态,则设置当前节点的“当前前继节点为”原节点的前继节点
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and indicate retry.
             */
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * waitStatus must be 0 or PROPAGATE. Indicate that we need a signal, but don't park yet. Caller will need to retry to make sure
             * it cannot acquire before parking.
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

    /**
     * Cancels an ongoing attempt to acquire.
     *
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node == null)
            return;

        node.thread = null;

        // Skip cancelled predecessors
        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        // predNext is the apparent node to unsplice. CASes below will
        // fail if not, in which case, we lost race vs another cancel
        // or signal, so no further action is necessary.
        Node predNext = pred.next;

        // Can use unconditional write instead of CAS here.
        // After this atomic step, other Nodes can skip past us.
        // Before, we are free of interference from other threads.
        node.waitStatus = Node.CANCELLED;

        // If we are the tail, remove ourselves.
        if (node == tail && compareAndSetTail(node, pred)) {
            compareAndSetNext(pred, predNext, null);
        } else {
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
            int ws;
            if (pred != head && ((ws = pred.waitStatus) == Node.SIGNAL || (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL)))
                    && pred.thread != null) {
                Node next = node.next;
                if (next != null && next.waitStatus <= 0)
                    compareAndSetNext(pred, predNext, next);
            } else {
                unparkSuccessor(node);
            }

            node.next = node; // help GC
        }
    }

    /**
    * Wakes up node's successor, if one exists.
    *
    * @param node the node
    */
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try to clear in anticipation of signalling. It is OK if this fails or if
         * status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally just the next node. But if cancelled or apparently null, traverse
         * backwards from tail to find the actual non-cancelled successor.
         */
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
            LockSupport.unpark(s.thread);
    }

    /**
     * Sets head of queue to be node, thus dequeuing. Called only by
     * acquire methods.  Also nulls out unused fields for sake of GC
     * and to suppress unnecessary signals and traversals.
     *
     * @param node the node
     */
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }

    /**
     * Atomically sets synchronization state to the given updated
     * value if the current state value equals the expected value.
     * This operation has memory semantics of a <tt>volatile</tt> read
     * and write.
     *
     * @param expect the expected value
     * @param update the new value
     * @return true if successful. False return indicates that the actual
     *         value was not equal to the expected value.
     */
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

    /**
     * CAS waitStatus field of a node.
     */
    private static final boolean compareAndSetWaitStatus(Node node, int expect, int update) {
        return unsafe.compareAndSwapInt(node, waitStatusOffset, expect, update);
    }

    /**
     * CAS next field of a node.
     */
    private static final boolean compareAndSetNext(Node node, Node expect, Node update) {
        return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
    }

    /**
     * CAS tail field. Used only by enq.
     */
    private final boolean compareAndSetTail(Node expect, Node update) {
        return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
    }

    /**
     * CAS head field. Used only by enq.
     */
    private final boolean compareAndSetHead(Node update) {
        return unsafe.compareAndSwapObject(this, headOffset, null, update);
    }

    /**
     * Setup to support compareAndSet. We need to natively implement
     * this here: For the sake of permitting future enhancements, we
     * cannot explicitly subclass AtomicInteger, which would be
     * efficient and useful otherwise. So, as the lesser of evils, we
     * natively implement using hotspot intrinsics(编译器内部函数) API. And while we
     * are at it, we do the same for other CASable fields (which could
     * otherwise be done with atomic field updaters).
     */
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long stateOffset;
    private static final long headOffset;
    private static final long tailOffset;
    private static final long waitStatusOffset;
    private static final long nextOffset;

    static {
        try {
            stateOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("state"));
            headOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("head"));
            tailOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
            waitStatusOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("waitStatus"));
            nextOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("next"));

        } catch (Exception ex) {
            throw new Error(ex);
        }
    }

}
View Code

 

AbstractOwnableSynchronizer的源码如下:

 

package concurrentMy.aqs;

/**
 * 
 * (设置和获取锁的持有者线程)
 *
 * <p>
 * 修改历史:                                            <br>  
 * 修改日期            修改人员       版本             修改内容<br>  
 * -------------------------------------------------<br>  
 * 2016年7月5日 下午3:42:37   user     1.0        初始化创建<br>
 * </p> 
 *
 * @author        Peng.Li 
 * @version        1.0  
 * @since        JDK1.7
 */
public abstract class AbstractOwnableSynchronizerTest implements java.io.Serializable {

    /** Use serial ID even though all fields transient. */
    private static final long serialVersionUID = 3737899427754241961L;

    /**
     * Empty constructor for use by subclasses.
     */
    protected AbstractOwnableSynchronizerTest() {
    }

    /**
     * The current owner of exclusive mode synchronization.
     * 
     * 加 transient 表示exclusiveOwnerThread不能被串行化,不会被作为序列化的一部分
     * 
     * 锁的持有线程
     */
    private transient Thread exclusiveOwnerThread;

    /**
     * Sets the thread that currently owns exclusive access. A
     * <tt>null</tt> argument indicates that no thread owns access.
     * This method does not otherwise impose any synchronization or
     * <tt>volatile</tt> field accesses.
     * 
     * protected final来修饰,表示子类可以使用这个方法,但是不能重载这个方法,也就是不能修改这个方法
     */
    protected final void setExclusiveOwnerThread(Thread t) {
        exclusiveOwnerThread = t;
    }

    /**
     * Returns the thread last set by
     * <tt>setExclusiveOwnerThread</tt>, or <tt>null</tt> if never
     * set.  This method does not otherwise impose any synchronization
     * or <tt>volatile</tt> field accesses.
     * @return the owner thread
     */
    protected final Thread getExclusiveOwnerThread() {
        return exclusiveOwnerThread;
    }
}
View Code

 

 

参考文章:

1.Doug Lea的论文: http://gee.cs.oswego.edu/dl/papers/aqs.pdf 

2. 深度解析Java 8:JDK1.8 AbstractQueuedSynchronizer的实现分析(上): http://www.infoq.com/cn/articles/jdk1.8-abstractqueuedsynchronizer

3. 深度解析Java 8:AbstractQueuedSynchronizer的实现分析(下): http://www.infoq.com/cn/articles/java8-abstractqueuedsynchronizer

4. 深入浅出 Java Concurrency (7): 锁机制 part 2 AQS: http://www.blogjava.net/xylz/archive/2010/07/06/325390.html

5 AQS:http://www.cnblogs.com/leesf456/p/5350186.html

6.参考:https://tech.meituan.com/Java_Lock.html

 

  

posted @ 2016-11-04 18:13  积淀  阅读(...)  评论(...编辑  收藏