公平锁和非公平锁

公平锁： Threads acquire a fair lock in the order in which they requested it
非公平锁： a nonfair lock permits barging: threads requesting a lock can jump ahead of the queue
of waiting threads if the lock happens to be available when it is requested.

所谓公平锁指的是哪个线程先运行，那就可以先得到锁。
非公平锁是不管线程是否是先运行，上来就直接尝试占有锁，如果尝试失败，就再采用类似公平锁那种方式。

我们看下ReentrantLock 类的源码：

公平锁代码：

 protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            //状态为0，说明当前没有线程占有锁
            if (c == 0) {
               //判断当前队列前面是有等待线程 ，没有的话尝试做cas操作，有点话就等待
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
           //判断当前获得锁的线程是否为自己，这里体现出了 可重入锁的机制
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

非公平锁代码：

/**
         * Performs non-fair tryLock.  tryAcquire is implemented in
         * subclasses, but both need nonfair try for trylock method.
         */
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            ////状态为0，说明当前没有线程占有锁
            if (c == 0) {
                //上来就直接尝试占有锁
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }