Java并发之ReentrantReadWriteLock

ReentrantReadWriteLock介绍

可重入的读写锁，将读写分离，读共享，写互斥，适用于读多写少的场景，其特点如下：

State复用

　　读写锁内部维护者两把锁，读锁和写锁。写锁需要一个变量表示写锁被重入的次数，读锁需要一个变量表示读锁被占用的次数。读写锁内部实现将state属性复用，将其切分为两部分，高16位表示读，低16位表示写，通过位运算得到读锁和写锁的状态。

        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; }

写锁

写锁的获取

protected final boolean tryAcquire(int acquires) {
            /*
             * Walkthrough:
             * 1. If read count nonzero or write count nonzero
             *    and owner is a different thread, fail.
             * 2. If count would saturate, fail. (This can only
             *    happen if count is already nonzero.)
             * 3. Otherwise, this thread is eligible for lock if
             *    it is either a reentrant acquire or
             *    queue policy allows it. If so, update state
             *    and set owner.
             */
            Thread current = Thread.currentThread();
            int c = getState();
            //获取写锁的状态
            int w = exclusiveCount(c);
            if (c != 0) {
                // 当存在读锁或者写锁被占用时，线程阻塞
                if (w == 0 || current != getExclusiveOwnerThread())
                    return false;
                //超出最大范围
                if (w + exclusiveCount(acquires) > MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                // Reentrant acquire
                setState(c + acquires);
                return true;
            }
            // 按照公平策略判断是否应该阻塞
            if (writerShouldBlock() ||
                !compareAndSetState(c, c + acquires))
                return false;
            setExclusiveOwnerThread(current);
            return true;
        }

写锁的释放

 protected final boolean tryRelease(int releases) {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int nextc = getState() - releases;
            //若写锁的新线程数为0，则将锁的持有者设置为null
            boolean free = exclusiveCount(nextc) == 0;
            if (free)
                setExclusiveOwnerThread(null);
            setState(nextc);
            return free;
        }

读锁

存在两类计数器：

1. state的高16位表示所有线程占用的读锁次数；
2. HoldCounter线程内部计数器，表示当前线程重入次数；

读锁的获取

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();
            //如果存在写锁且锁的持有者不是当前线程则阻塞
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;

            //读锁计数器加1，当前线程内部计数器加1
            int r = sharedCount(c);
            if (!readerShouldBlock() &&
                r < MAX_COUNT &&
                compareAndSetState(c, c + SHARED_UNIT)) {
                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);
        }

读锁的释放

protected final boolean tryReleaseShared(int unused) {
             //当前线程的内部计数器减1
            Thread current = Thread.currentThread();
            if (firstReader == current) {
                // assert firstReaderHoldCount > 0;
                if (firstReaderHoldCount == 1)
                    firstReader = null;
                else
                    firstReaderHoldCount--;
            } else {
                HoldCounter rh = cachedHoldCounter;
                if (rh == null || rh.tid != getThreadId(current))
                    rh = readHolds.get();
                int count = rh.count;
                if (count <= 1) {
                    readHolds.remove();
                    if (count <= 0)
                        throw unmatchedUnlockException();
                }
                --rh.count;
            }

            //读锁计数器减1
            for (;;) {
                int c = getState();
                int nextc = c - SHARED_UNIT;
                if (compareAndSetState(c, nextc))
                    // Releasing the read lock has no effect on readers,
                    // but it may allow waiting writers to proceed if
                    // both read and write locks are now free.
                    return nextc == 0;
            }
        }

优化方案

  JDK1.8 StampedLock