Java - ReentrantReadWriteLock的读写锁竞争原理

夹胡碰

发布于 2020-12-29 11:21:26

5120

发布于 2020-12-29 11:21:26

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

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);
}

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