AQS源码分析07-（独占式释放同步状态--release(int arg)）

独占式释放同步状态–release(int arg)

该方法为AQS提供的模板方法

//该方法为AQS提供的模板方法
public final boolean release(int arg) {
    if (tryRelease(arg)) {//自行实现释放同步状态的方法
        Node h = head;
        if (h != null && h.waitStatus != 0)
            //唤醒后继节点
            unparkSuccessor(h);
        return true;
    }
    return false;
}

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)
        //如果当前节点的状态小于0，就通过CAS操作尝试修改状态为0(初始状态)
        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.
     */
    //取到节点的下一个节点
    Node s = node.next;
    if (s == null || s.waitStatus > 0) {
        //如果为空或者已经取消/被中断
        s = null;
        //从尾部倒着遍历，找到离当前节点最近的节点状态小于等于0的节点（回溯）
        for (Node t = tail; t != null && t != node; t = t.prev)
            if (t.waitStatus <= 0)
                s = t;
    }
    if (s != null)
        //唤醒后边的节点
        LockSupport.unpark(s.thread);
}