ReentrantLock源码解析(二)

可重入原理

仍然以非公平锁为例

加锁

/**
 * Performs non-fair tryLock.  tryAcquire is implemented in
 * subclasses, but both need nonfair try for trylock method.
 */
// Sync的方法
final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
      // 如果已经获得了锁, 线程还是当前线程, 表示发生了锁重入
    else if (current == getExclusiveOwnerThread()) {
        //state++
        int nextc = c + acquires;
        if (nextc 0或者cas失败 唤醒Thread-0
        LockSupport.unpark(node.thread);
    return true;
}

private Node enq(final Node node) {
    for (;;) {
        Node t = tail;//此时同步队列尾指针指向的是Thread-3
        if (t == null) { // Must initialize
            if (compareAndSetHead(new Node()))
                tail = head;
        } else {
            node.prev = t;//Thread-0的前指针指向Thread-3
            if (compareAndSetTail(t, node)) {//同步队列尾指针指向Thread-0
                t.next = node;//Thread-3的后指针指向Thread-0
                return t;//返回Thead-3
            }
        }
    }
}

unlock 流程

接着Thread-1 会释放锁，进入 unlock 流程，然后唤醒Thread-2去尝试获取锁

unlock释放锁的流程已经在从ReentrantLock源码解读AQS中讲解过了，这里就不再继续赘述了。