Skip to main content

ReentrantLock

重入锁

  • 可重入
  • 有公平锁的实现，按照申请事件获取锁
  • 可以绑定多个条件，这就意味着可以精准的唤醒某个线程，而不是像Synchronized那样只能用notifyall
  • 提供了tryLock()方法，可以在不阻塞线程的情况下尝试获取锁资源，避免死锁问题

源码实现​

AbstractQueuedSynchronizer​

基本流程​

  • 尝试获取锁（待子类实现）
  • 尝试加入队尾
  • 尝试检查是否排到队头， 尝试获取锁（待子类实现）
  • 检查前置节点状态，自旋或者阻塞

阻塞和唤醒线程​

LockSupport工具类：对Unsafe类中原语进行了封装

阻塞
    public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        UNSAFE.park(false, 0L);
        setBlocker(t, null);
    }
唤醒
    public static void unpark(Thread thread) {
        if (thread != null)
            UNSAFE.unpark(thread);
    }

多线程的管理​

利用双向链表和CAS实现了一个阻塞队列

    public final void acquire(int arg) {
    //尝试一次获取锁
        if (!tryAcquire(arg) &&
//        acquireQueued：等待并尝试获取锁
//        addWaiter：向队尾添加当前线程的node节点
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            // 自旋
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    // 获取到锁之后，删掉当前线程的节点
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //阻塞等待
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    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;
        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 {
            /*
             * 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);
        }
        return false;
    }

公平和非公平的实现​

非公平只是在排队之前去尝试获取一次锁，公平就老老实实排队，先进先出

NonfairSync
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

tryLock的实现​

直接调用tryAcquire，尝试一次

    public boolean tryLock() {
        return sync.nonfairTryAcquire(1);
    }

Q&A​

  1. 为什么AQS需要双向队列 为了获取前置节点的状态，在尝试获取锁的过程中，需要先判断前置节点是否是head节点，因为只有获取锁之后，该线程节点才会被设置成head
  2. AQS在java18做了哪些改进 tryLock加上了时间参数
  3. AQS在哪些地方用了cas和自旋
  • cas
    • 设置节点状态
    • 设置头尾节点
  • 自旋
    • 添加队尾
    • 尝试获取锁
    • 阻塞线程