java并发编程学习之AQS

摘要：原理全称，当线程去获取资源的时候，会根据状态值来判断是否有锁，如果有锁，则加入到链表，链表里的线程，通过自旋，判断资源是否已经释放，如果释放，则获取资源。

全称AbstractQueuedSynchronizer，当线程去获取资源的时候，会根据状态值state来判断是否有锁，如果有锁，则加入到链表，链表里的线程，通过自旋，判断资源是否已经释放，如果释放，则获取资源。

volatile Node head：阻塞的头节点

volatile Node tail：阻塞的尾节点，新的阻塞节点加到最后

volatile int state：锁的状态，0说明未占用，大于等于1说明已占用

Thread exclusiveOwnerThread：占用锁的当前线程

Node EXCLUSIVE：独占模式

volatile Thread thread：当前线程

volatile Node prev：前置节点

volatile Node next：后置节点

volatile int waitStatus：状态字段，-1：等待被唤醒，大于0，被取消

这里以非公平锁为例

获取锁

final void lock() {
    if (compareAndSetState(0, 1))//如果状态是0，则设置为1
        setExclusiveOwnerThread(Thread.currentThread());//设置占用锁为当前的线程
    else
        acquire(1);//资源被占用
}

锁被占用后，再尝试获取，获取不到，进入阻塞队列

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

调用的是Sync的nonfairTryAcquire方法。

final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();//获取当前线程
    int c = getState();//获取当前状态
    if (c == 0) {
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {//为当前线程，重入
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

把线程封装成node，加入队列

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;//获取尾节点
    if (pred != null) {
        node.prev = pred;//当前节点的前置节点为获取到的尾节点
        if (compareAndSetTail(pred, node)) {//设置当前节点为尾节点
            pred.next = node;//如果cas操作成功，设置双向链表
            return node;
        }
    }
    enq(node);
    return node;
}

通过自旋的方式，加入到尾节点

private Node enq(final Node node) {
    for (;;) {
        Node t = tail;//获取尾节点
        if (t == null) { // 如果尾节点为空，初始化头节点和尾节点，地址为同一个
            if (compareAndSetHead(new Node()))
                tail = head;
        } else {
            node.prev = t;
            if (compareAndSetTail(t, node)) {//加入尾节点
                t.next = node;
                return t;
            }
        }
    }
}

final boolean acquireQueued(final Node node, int arg) {
    boolean failed = true;
    try {
        boolean interrupted = false;
        for (;;) {
            final Node p = node.predecessor();//获取前置节点
            if (p == head && tryAcquire(arg)) {//如果前置节点是head，并且获取到了锁
                setHead(node);//把当前节点设置到head上面
                p.next = null; // help GC
                failed = false;
                return interrupted;
            }
            //如果既不是队头，或者没有抢过其他线程
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())//如果队头是唤醒的状态，就用parkAndCheckInterrupt挂起
                interrupted = true;
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
    int ws = pred.waitStatus;
    if (ws == Node.SIGNAL)
        /*
         * This node has already set status asking a release
         * to signal it, so it can safely park.
         */
        return true;//唤醒，返回true
    if (ws > 0) {
        /*
         * Predecessor was cancelled. Skip over predecessors and
         * indicate retry.
         */
        do {
            node.prev = pred = pred.prev;//如果被取消了，被取消的前置节点替换当前节点的前置节点
        } while (pred.waitStatus > 0);
        pred.next = node;//双向链表
    } 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);//前置节点状态设置为唤醒
    }
    return false;
}

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);//唤醒头节点
        return true;
    }
    return false;
}

protected final boolean tryRelease(int releases) {
    int c = getState() - releases;//可能重入的情况
    if (Thread.currentThread() != getExclusiveOwnerThread())//非当前线程抛异常
        throw new IllegalMonitorStateException();
    boolean free = false;
    if (c == 0) {//不用cas是因为仅有当前显示有锁
        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);//如果头节点当前waitStatus<0, 修改为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;
    //唤醒下一个节点，如果第一个为空，从尾部遍历上去，获取最前面的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;
    }
    if (s != null)
        LockSupport.unpark(s.thread);//唤醒
}