概述

● 一个int成员变量 state 表示同步状态
● 通过内置的FIFO队列来完成资源获取线程的排队工作

属性

AbstractQueuedSynchronizer属性

   /**
     * 同步队列的头节点     
     */
    private transient volatile Node head;

    /**
     * 同步队列尾节点，enq 加入
     */
    private transient volatile Node tail;

    /**
     * 同步状态
     */
    private volatile int state;

    /**
     * 获取状态
     */
    protected final int getState() {
        return state;
    }

    /**
     * 设置状态
     */
    protected final void setState(int newState) {
        state = newState;
    }

    /**
     * CAS 设置状态
     */
    protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

    /**
     * The number of nanoseconds for which it is faster to spin
     * rather than to use timed park. A rough estimate suffices
     * to improve responsiveness with very short timeouts.
     */
    static final long spinForTimeoutThreshold = 1000L;

Node 节点属性

 static final class Node {
        /** 共享节点 */
        static final Node SHARED = new Node();
     
        /** 独占节点 */
        static final Node EXCLUSIVE = null;

        // 在同步队列中等待的线程等待超时或被中断, 需要从同步队列中取消等待, 状态不会变化 |
        static final int CANCELLED = 1;
          
        // 后继节点处于等待状态, 当前节点释放了同步状态或者被取消, 通知后续节点, 使后续节点得以运行
        static final int SIGNAL = -1;
        
        // 值为-2, 节点在等待队列, 当其他线程 signal(),从等待队列中移除到同步队列中 |
        static final int CONDITION = -2;
        
        // 值为-3, 下一次共享获取同步状态将会无条件传播下去
        static final int PROPAGATE = -3;

        /**
         * 节点初始状态，初始化为0
         */
        volatile int waitStatus;

        /**
         * 前一个节点
         */
        volatile Node prev;

        /**
         * 后一个节点
         */
        volatile Node next;

        /*
          * 节点的线程
          */
        volatile Thread thread;

        /**
         * 下一个等待者
         */
        Node nextWaiter;

        /**
         * 是否是共享节点
         */
        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        /**
         *  前一个节点
         */
        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;
        }
    }

常用方法

同步状态的三个方法：
● getState() 获取同步状态
● setState(int newState) 设置当前同步状态
● compareAndSetState(int expect, int update) CAS设置同步状态,原子操作

AbstractQueuedSynchronizer可重写的方法：

方法名称	方法描述
boolean tryAcquire(int arg)	独占式获取同步状态，查询当前状态是否符合预期，并且CAS设置
boolean tryRelease(int arg)	独占式释放同步状态，释放后，等待获取同步状态的线程有机会获取同步状态
int tryAcquireShared(int arg)	共享式获取同步状态，如果大于等于0，表示获取成功
boolean tryReleaseShared(int arg)	共享式释放同步状态
boolean isHeldExclusively()	在独占模式下被线程占用，表示是否被当前线程独占

AbstractQueuedSynchronizer提供的模版方法

方法名称	方法描述
boolean acquire(int arg)	独占式获取同步状态, 成功返回, 失败队列等待, 调用tryAcquire()
boolean acquireInterruptibly(int arg)	acquire 相同, 但是可以中断
int tryAcquireNanos(int arg, long nanos)	acquireInterruptibly 基础上增加了超时限制, 超时返回false, 返回true
acquireShared(int arg)	共享式获取同步状态, 和acquire差不多, 区别是同一时刻可以有多个线程获取同步状态
acquireSharedInterruptibly(int arg)	acquireShared 相同, 但是可以中断
int tryAcquireSharedInterruptibly(int arg, long nanos)	acquireSharedInterrup

流程图

流程图主要方法源码阅读

acquire

独占式获取同步状态, 成功返回, 失败队列等待

public final void acquire(int arg) {
    // tryAcquire获取信号量
    // 如果失败 tryAcquire(arg)=false addWaiter入队列、acquireQueued 排队获取锁
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}

final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
            // 前一个节点是头节点 尝试获取锁 获取锁成功 设置自己为头节点 
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }

           // 前面节点设置为 singal，自己就可以睡眠了
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                // 被中断 尝试获取信号量
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

addWaiter

节点进入同步队列

    private Node addWaiter(Node mode) {
        // 创建节点
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        // 尾节点不为空
        if (pred != null) {
            // 设置当前节点的前一个节点为尾节点
            node.prev = pred;
            // cas 设置自己为尾节点
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        // 尾节点为空 或 cas 设置自己为尾节点失败了
        enq(node);
        return node;
    }
    
    /**
     * 入队
     */
    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
           // 尾节点为空，设置新的头节点
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                // 设置当前节点的前一个节点为尾节点
                node.prev = t;
                // cas 设置自己为尾节点
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

shouldParkAfterFailedAcquire

前面节点设置为 singal，设置成功返回true，失败false

 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        // 前面的节点SIGNAL自己就可以park了
        if (ws == Node.SIGNAL)
            return true;
        if (ws > 0) {
            // 找到第一个不是取消状态的节点
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
            /*
             * 设置 WaitStatus SIGNAL
             */
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

parkAndCheckInterrupt

   private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

acquireInterruptibly

acquire 相同, 但是可以中断

 public final void acquireInterruptibly(int arg)
            throws InterruptedException {
        // 被中断抛出InterruptedException
        if (Thread.interrupted())
            throw new InterruptedException();
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }
    
private void doAcquireInterruptibly(int arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.EXCLUSIVE);
    boolean failed = true;
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                // 被中断抛出InterruptedException
                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }

tryAcquireNanos

acquireInterruptibly 基础上增加了超时限制, 超时返回false, 返回true

   public final boolean tryAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquire(arg) ||
            doAcquireNanos(arg, nanosTimeout);
    }
    
    private boolean doAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (nanosTimeout <= 0L)
            return false;
        final long deadline = System.nanoTime() + nanosTimeout;
        final Node node = addWaiter(Node.EXCLUSIVE);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return true;
                }
                nanosTimeout = deadline - System.nanoTime();
                // 超时返回false
                if (nanosTimeout <= 0L)
                    return false;
                if (shouldParkAfterFailedAcquire(p, node) &&
                    nanosTimeout > spinForTimeoutThreshold)
                    // park指定时间
                    LockSupport.parkNanos(this, nanosTimeout);
                // 中断抛出异常
                if (Thread.interrupted())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

release

释放信号量， 如果头节点不为空 状态为SINGAL, 唤醒头节点的下一个节点

public final boolean release(int arg) {
    if (tryRelease(arg)) {
        // 释放arg信号量成功
        Node h = head;
        // 如果头节点不为空 状态为SINGAL, 唤醒头节点的下一个节点
        if (h != null && h.waitStatus != 0)
            unparkSuccessor(h);
        return true;
    }
    return false;
}

private void unparkSuccessor(Node node) {
    int ws = node.waitStatus;
    // 唤醒先修改waitStatus从SINGAL->0初始化
    if (ws < 0)
        compareAndSetWaitStatus(node, ws, 0);

    // 找到node之后第一个不被取消的节点， LockSupport.unpark唤醒该节点
    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);
}

参考文献

• Java并发编程的艺术第二版 方腾飞、魏鹏、程晓明