parkAndCheckInterrupt()
尝试获取一次，若state=0则获得锁，若state不为0且当前独占线程=current，则state+1，表示重入
acquireShared(1)
acquire(1);
state改变是volatile的
AQS=cas+volatile
ReentrantLock的非公平锁的lock方法调用实现
nonfairTryAcquire(acquires)
cas失败
unlock就比较简单：1.将state-1，2.如果state==0则完全释放锁3.设置独占线程为null4.state=0则调用LockSupport.unpark
readLock-->lock-->AQS
selfInterrupt();
cas成功设置独占线程
线程阻塞的核心
调用的LockSupport.park
writeLock
/** Returns the number of shared holds represented in count */ static int sharedCount(int c) { return c >>> SHARED_SHIFT; } /** Returns the number of exclusive holds represented in count */ static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
tryAcquire(arg)
setExclusiveOwnerThread（currentThread（））
tryAcquireShared
非公平锁每个线程进来都先cas，unsafe中cas AQS中的state能否成功
addWaiter 将当前线程入队，修改head和tail都是cas操作
ReentrantLock-->AQS-->NonFairAQSlock（）
