【源码】ReentrantLock

ReentrantLock

  ReentrantLock内部有sync接口，sync有NonfairSyn和FairSync两个实现，这两个实现了lock和unlock方法
竞争锁用的是cas

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

  竞争不到就重新尝试，依然得不到就加入队列，设置中断状态

    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

  加入队列时判断需要阻塞就会阻塞当前线程，使用了LockSupport工具的pack方法来实现阻塞

    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

  LockSupport工具的大多方法利用了UNSAFE类里的native方法

        public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        UNSAFE.park(false, 0L);
        setBlocker(t, null);
    }

    public native void park(boolean isAbsolute, long time);