理解ReentrantReadWriteLock
本文主要内容是对并发包中的读写锁的认识，主要解释读写锁的请求过程，锁降级的实现以及锁升级的不可能性。
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首先来了解一些常量和简单方法，贴下代码
/** * Full version of acquire for reads, that handles CAS misses * and reentrant reads not dealt with in tryAcquireShared. */final int fullTryAcquireShared(Thread current) { /* * This code is in part redundant with that in * tryAcquireShared but is simpler overall by not * complicating tryAcquireShared with interactions between * retries and lazily reading hold counts. */ HoldCounter rh = cachedHoldCounter; if (rh == null || rh.tid != current.getId()) rh = readHolds.get(); for (;;) { int c = getState(); int w = exclusiveCount(c); if ((w != 0 && getExclusiveOwnerThread() != current) || ((rh.count | w) == 0 && readerShouldBlock(current))) return -1; //标记2 if (sharedCount(c) == MAX_COUNT) throw new Error("Maximum lock count exceeded"); if (compareAndSetState(c, c + SHARED_UNIT)) { cachedHoldCounter = rh; // cache for release rh.count++; return 1; } }}它在标记2处返回-1，表示读线程获取锁失败，将进入同步队列。所以从读取锁升级写入锁是不可能的。
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最后就是锁的释放，在理解了本文，相信看读写锁的释放操作还是比较简单的。
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