并发编程之ReentrantLock

一、ReentrantLock的核心组成

1、CAS

2、abstract static class Sync extends AbstractQueuedSynchronizer  ---AQS  （双向链表）

3、锁的类型

 4、 UNSAFE.park(false, 0L); 、 UNSAFE.unpark(thread);

特点：

1、单个线程，交替执行时与AQS队列无关

二、加锁源码解读

protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    //获取线程状态
    int c = getState();
    if (c == 0) {
    //hasQueuedPredecessors 判断队列是否有排队的线程
    //compareAndSetState  底层CAS 
        if (!hasQueuedPredecessors() &&
            compareAndSetState(0, acquires)) {
            //加锁成功将exclusiveOwnerThread 设置为当前线程
            setExclusiveOwnerThread(current);
            return true;
        }
    }
　　//当前来获取锁的线程 与 持有锁的线程比较，如果是同一个线程，则state累加 是一种重入的思想  重入锁
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0)
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

1、由此可以看出当队列中没有线程的时候，整个加锁过程核心就是CAS ；队列中没有线程的场景单线程工作或者多线程交替执行

 2、竞争执行

//入队操作
    private Node addWaiter(Node mode) {
        //创建当前线程Node节点对象
        Node node = new Node(Thread.currentThread(), mode);
        Node pred = tail;
        if (pred != null) {
            //node 入队
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }
    //AQS 队头元素 Thread=null
    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            //初始化 时 AQS 队头 元素 Thread=null
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                //将传入的node入队
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }


    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                //获取当前节点的上一个节点
                final Node p = node.predecessor();
                //判断 p是不是头结点，自有  tryAcquire()并且自己再尝试拿一次锁(自旋)
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //自旋后判断是不是需要park()
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }



    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        //线程状态
        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;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            do {
                node.prev = pred = pred.prev;
            } while (pred.waitStatus > 0);
            pred.next = node;
        } else {
             // 将上一个节点pred的WS状态设置为 -1 （当前线程只能把上一个线程的ws状态设置为-1，因为他自己park()后，无法设置自己的WS=-1，所以他的WS=-1只能由下一个线程来设置）
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }
    //让当前线程阻塞 park()
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }