AQS

简述

AQS全称AbstractQueuedSynchronizer，提供实现阻塞锁和相关的框架
JDK中使用AQS来实现的同步工具类有ReentrantLock、ReentrantReadWriteLock、Semaphore、CountDownLatch。
AQS中有一个状态变量，子类通过改变这个状态变量来改变锁的状态。

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AQS原理解析

通过Lock来讲解,讲解ReentrantLock中的非公平锁

final void lock() {
            //通过cas将状态设置成1
            if (compareAndSetState(0, 1))
                //将线程设置成当前线程
                setExclusiveOwnerThread(Thread.currentThread());
            else
            //获取线程
                acquire(1);
}
 public final void acquire(int arg) {
        //尝试获取（子类实现），获取失败加入CLH队列、中断当前线程
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
}

protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
}
final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {//锁未被占用
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {//当前线程是本线程
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
}
//添加等待者节点、采用尾插法
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;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
}
/**
* 通过死循环遍历，只有遍历到p是头节点并且能够获得资源才可以退出。
* shouldParkAfterFailedAcquire判断当前节点的状态，
* 通过parkAndCheckInterrupt将线程设成waitting状态
* 需要中断或unpack唤醒
*/
 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);
        }
}

release

public void unlock() {
        sync.release(1);
}
public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
}

protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
}

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);

        /*找到队列中可以运行的，最靠近头部的节点
         */
        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;
        }
        //唤醒该线程
        if (s != null)
            LockSupport.unpark(s.thread);
}

讲解CLH队列

     *      +------+  prev +-----+       +-----+
     * head |      | <---- |     | <---- |     |  tail
     *      +------+       +-----+       +-----+
     * 

CLH队列通过双向链表实现队列的功能，将等待的线程放入到队列中。

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AQS优点

• 用户可以通过AQS快速的实现自己的同步工具类
• AQS相对于内置锁有更高的性能。AQS通过CAS实现，实现了无锁化。