11.深入理解读写锁ReentrantReadWriteLock

最新推荐文章于 2024-10-14 23:27:49 发布

weixin_33928137

最新推荐文章于 2024-10-14 23:27:49 发布

阅读量57

点赞数

CC 4.0 BY-SA版权

原文链接：http://www.cnblogs.com/itxiaok/p/10356582.html

本文深入分析了ReentrantReadWriteLock中读锁的获取过程，详细解释了tryAcquireShared方法的实现原理，包括状态检查、重入性处理及CAS更新状态等关键步骤。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

protected final int tryAcquireShared(int unused) {
/*
* Walkthrough:
* 1. If write lock held by another thread, fail.
* 2. Otherwise, this thread is eligible for
* lock wrt state, so ask if it should block
* because of queue policy. If not, try
* to grant by CASing state and updating count.
* Note that step does not check for reentrant
* acquires, which is postponed to full version
* to avoid having to check hold count in
* the more typical non-reentrant case.
* 3. If step 2 fails either because thread
* apparently not eligible or CAS fails or count
* saturated, chain to version with full retry loop.
*/
Thread current = Thread.currentThread();
int c = getState();
//1. 如果写锁已经被获取并且获取写锁的线程不是当前线程的话
// 线程获取读锁失败返回-1
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return -1;
int r = sharedCount(c);
if (!readerShouldBlock() &&
r < MAX_COUNT &&
//2. 当前线程获取读锁
compareAndSetState(c, c + SHARED_UNIT)) {
//3. 下面的代码主要是新增的一些功能，比如getReadHoldCount()方法
//返回当前获取读锁的次数
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;
}
//4. 处理在第二步中CAS操作失败的自旋已经实现重入性
return fullTryAcquireShared(current);
}