JUC(八)-AbstractQueuedSynchronizer之AQS

AQS即是抽象的队列式的同步器，熟知的ReentrantLock、ReentrantReadWriteLock、CountDownLatch、Semaphore等都是基于AQS来实现的。

基本原理

是用来构建锁或者其它同步器组件的重量级基础框架及整个JUC体系的基石，通过内置的FIFO队列来完成资源获取线程的排队工作，并通过一个int类变量表示持有锁的状态

图片来自网络

CLH:Craig、Landin and Hagersten 队列，是一个单向链表，AQS中的队列是CLH变体的虚拟双向队列FIFO

AQS中 维护了一个volatile int state（代表共享资源）和一个FIFO线程等待队列（多线程争用资源被阻塞时会进入此队列）

这里volatile能够保证多线程下的可见性，当state=1则代表当前对象锁已经被占有，其他线程来加锁时则会失败，加锁失败的线程会被放入一个FIFO的等待队列中，比列会被UNSAFE.park()操作挂起，等待其他获取锁的线程释放锁才能够被唤醒

另外state的操作都是通过CAS来保证其并发修改的安全性

AQS的基本API

• getState()：获取锁的标志state值
• setState()：设置锁的标志state值
• tryAcquire(int)：独占方式获取锁。尝试获取资源，成功则返回true，失败则返回false
• tryRelease(int)：独占方式释放锁。尝试释放资源，成功则返回true，失败则返回false

AQS源码

以ReentrantLock为例，加锁过程

1、尝试加锁

2、加锁失败，线程放入队列

3、线程入队列后，进入阻塞状态

非公平锁NonfairSync为例，案例都是理想正常流程执行 A-B-C 线程执行，实际上有差异

Lock方法（挂起线程）

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);
    }
}

compareAndSetState 比较state，这里A先进入比较并交换 设置state0到1，AQS实现，

protected final boolean compareAndSetState(int expect, int update) {
    return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}

protected final void setExclusiveOwnerThread(Thread thread) {
    exclusiveOwnerThread = thread;
}

此时B线程执行lock开始抢占锁，此时非常抱歉，此时state=1,那么B只能进入acquire方法

模板方法进入AbstractQueuedSynchronizer的acquire方法，注意：tryAcquire 方法AbstractQueuedSynchronizer本身不实现，会抛出UnsupportedOperationException异常

public final void acquire(int arg) {
    if (!tryAcquire(arg) &&
        acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
        selfInterrupt();
}

而非公平NonfairSync里面的实现

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) {
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

由于此时C=1,并且getExclusiveOwnerThread=A ,所以此时返回false

接下来执行父类模板定义的方法

acquireQueued(addWaiter(Node.EXCLUSIVE), arg)

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;
}

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;
            }
        }
    }
}

此时tail=null,enq 自旋先创建哨兵节点,用于占位，再创建b节点入队，并将Node 头尾节点设置

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);
    }
}

​ for (;;)这里又是一个自旋，

第一次自旋，tryAcquire去抢锁，这里默认A 休眠五分钟，所以这里肯定抢占失败返回false，执行shouldParkAfterFailedAcquire 将p也就是当前的哨兵节点，此时哨兵节点pred的waitStatus=0，条件进入比较并交换为-1。

第二次自旋，shouldParkAfterFailedAcquire 哨兵节点pred的waitStatus=-1，此方法返回true,便执行parkAndCheckInterrupt中的方法，此时就把线程B挂起

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

unLock 方法（唤醒线程）

unlock—>sync.release(1)—>tryRelease 子类实现

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; // 1-1=0
    if (Thread.currentThread() != getExclusiveOwnerThread())
        throw new IllegalMonitorStateException();
    boolean free = false;
    if (c == 0) {
        free = true;
        setExclusiveOwnerThread(null);
    }
    setState(c);
    return free;
}

tryRelease=0,返回true,执行unparkSuccessor，此时node是头节点是哨兵节点 waitStatus=-1,下一节点B将执行

LockSupport.unpark(s.thread)。也就是B节点线程唤醒

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);
}