成员：sync
方法：acquire acquire(int) acquireUnInterruptibly acquireUnInterruptily(int) tryAcquire tryAcquire(int) tryAcquire(long, TimeUnit) tryAcquire(int, long, TimeUnit)
release release(int)
availablePermits drainPermits isFair hasQueueThreads getQueueLength
内部类：Sync NonFairSync FairSync
两个子Sync都覆盖了tryAcquireShared
非公平的是拿当前状态和acquires相减然后compareAndSetState
公平的比非公平的多一个步骤，就是在之前通过getFirstQueuedThread获取线程和当前线程相比，FIFO公平
tryAcquireShared
tryAcquire