公平锁和非公平锁

Lock锁分为公平锁和非公平锁两种

公平锁：线程获取锁的顺序是按照线程加锁的顺序来分配的，即先来先得的FIFO先进先出顺序

非公平锁：一种获取锁的抢占机制，是随机获取锁的，和公平锁的区别就是先来的不一定先得到锁，导致某些线程可能一直拿不到锁，所以是不公平的

　　公平锁，就是很公平，在并发环境中，每个线程在获取锁时会先查看此锁维护的等待队列，如果为空，或者当前线程是等待队列的第一个，就占有锁，否则就会加入到等待队列中，以后会按照FIFO的规则从队列中取到自己；非公平锁比较粗鲁，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

--------------------------------------公平锁--------------------------------------

    protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {//状态是0，说明当前线程没有占有锁
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);//当前线程占有锁
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {//如果当前线程本身持有锁，叠加状态值，继续持有
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
------------------------------------------------------------------
    public final boolean hasQueuedPredecessors() {
        // The correctness of this depends on head being initialized
        // before tail and on head.next being accurate if the current
        // thread is first in queue.
        Node t = tail; // Read fields in reverse initialization order
        Node h = head;
        Node s;
        return h != t &&
            ((s = h.next) == null || s.thread != Thread.currentThread());
    }

--------------------------------------非公平锁--------------------------------------

　　　　final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                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;
        }