Lock介绍(二)

Lock接口的unLock介绍

分析下ReentrantLock的unLock源码

方法一
public void unlock() {
    sync.release(1);
}
方法二
public final boolean release(int arg) {
    if (tryRelease(arg)) {
        Node h = head;
        //如果头节点不为null并且头(node)节点的等待状态(waitStatus)不为0,说明之前的node列表存放在等待获取锁的线程对象
        if (h != null && h.waitStatus != 0)
            //执行unpark方法
            unparkSuccessor(h);
        return true;
    }
    return false;
}
方法三
protected final boolean tryRelease(int releases) {
    //当前锁对象state-1
    int c = getState() - releases;
    //判断当前线程对象是否等于当前锁对象的线程是否是同一对象
    if (Thread.currentThread() != getExclusiveOwnerThread())
        //不是说明程序处于不正常的状态,所以抛出非法监控状态异常
        throw new IllegalMonitorStateException();
    boolean free = false;
    if (c == 0) {
        free = true;
        //将当前锁对象的线程属性值赋值为null
        setExclusiveOwnerThread(null);
    }
    //state属性值为原始值-1
    setState(c);
    //如果state减为0说明锁释放成功,返回true;如果state不为0说明锁还未被释放成功,返回fasle;
    return free;
}
方法四
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.
     */
    //获取头节点的等待状态属性值
    int ws = node.waitStatus;
    //如果为负数,说明当前节点的内的线程处于park阻塞状态
    if (ws < 0)
        采用cas将当前的waitStatus设置为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.
     */
    //s为头节点的下一个节点
    Node s = node.next;
    //如果s为null或者s的等待状态大于0,找尾节点最接近头节点的节点赋值为s.
    if (s == null || s.waitStatus > 0) {
        //将s赋值为空
        s = null;
        //遍历尾节点,尾节点不为空并且尾节点不为头节点
        //尾节点取上一个节点
        for (Node t = tail; t != null && t != node; t = t.prev)
            //如果t的状态为小于等于0
            if (t.waitStatus <= 0)
                //将节点赋值为s
                s = t;
    }
    if (s != null)
        //解锁头节点下一个节点的线程或者尾节点向前遍历最接近头节点的等待节点小于等于0的线程,唤醒对应线程会继续执行acquireQueued的(for(;;)方法体内容去竞争获取锁)
        LockSupport.unpark(s.thread);
}

总结

ReentrantLock的unLock源码逻辑相对加锁逻辑会显得不够特别复杂,但是里面关于node的waitStatus和state的逻辑控制处理的判断值得再研究学习下.