ReentrantReadWriteLock是可重入读写锁，底层依赖AQS实现，读写锁的竞争通过state的高位和低位进行区分。
1. 原理展示
代码语言：javascript复制static final int SHARED_SHIFT = 16;
static final int SHARED_UNIT = (1 << SHARED_SHIFT);
static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1;
static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;
/** 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; }
2. 源码解析
- 读锁的获取锁
protected final int tryAcquireShared(int unused) {
Thread current = Thread.currentThread();
int c = getState();
if (exclusiveCount(c) != 0 && // 低位
getExclusiveOwnerThread() != current)
return -1;
int r = sharedCount(c);
if (!readerShouldBlock() &&
r < MAX_COUNT &&
compareAndSetState(c, c SHARED_UNIT)) { // 高位
if (r == 0) {
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
firstReaderHoldCount ;
} else {
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count ;
}
return 1;
}
return fullTryAcquireShared(current);
}
