读写锁（ReentrantReadWriteLock）源码解读

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• 读写锁允许同一时刻被多个读线程访问，但是在写线程访问时，所有的读线程和其他的写线程都会被阻塞。

• ReentrantReadWriteLock特性：

  1. 公平性选择：支持非公平性（默认）和公平的锁获取方式，吞吐量还是非公平优于公平；
  2. 重入性：支持重入，读锁获取后能再次获取，写锁获取之后能够再次获取写锁，同时也能够获取读锁；
  3. 锁降级：遵循获取写锁，获取读锁再释放写锁的次序，写锁能够降级成为读锁。

• 阅读之前的一些思考：

  1. 读写锁是怎样记录读写状态？
  2. 写锁怎样获取锁、释放锁？
  3. 读锁怎样获取锁、释放锁？

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• 类关系图如下所示：

ReentrantReadWriteLock类

ReadWriteLock接口

readLock方法

writeLock方法

AQS抽象类

AOS抽象类

ReadLock类

WriteLock类

Sync类

Lock接口

tryLock方法

tryLock超时方法

unlock方法

lock方法

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• 获取读锁

  • ReentrantReadWriteLock->ReadLock.lock()->Sync.acquireShared(1)->AQS->tryAcquireShared(arg)

• 场景：多线程安全操作下，读的情况多于写的情况，满足多线程操作的安全性，也要确保性能不能太差。

        // AQS中的获取共享锁的方法
        public final void acquireShared(int arg) {
            // 小于0表示获取失败进入进入队列
            if (tryAcquireShared(arg) < 0)
                doAcquireShared(arg);
        }

        // ReentrantReadWriteLock->Sync中获取共享锁的实现
        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();
            // 获取锁当前状态
            int c = getState();
            // 如果当前有线程获取了写锁，且不是当前线程，返回失败
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;
                
            // 获取当前读锁的数量，读状态存储在高位
            // c >>> 16取高16位的值
            int r = sharedCount(c);
            
            // 是否阻塞判断
            // 锁数量没有达到最大值
            // 尝试CAS将状态修改为c+65536， 修改成功则获取锁
            if (!readerShouldBlock() &&
                r < MAX_COUNT &&
                compareAndSetState(c, c + SHARED_UNIT)) {
                // 如果谁也没有获取过读锁
                if (r == 0) {
                    // 第一个reader指向当前线程，读锁持有数量为1
                    firstReader = current;
                    firstReaderHoldCount = 1;
                } else if (firstReader == current) {
                   // 如果当前线程再次获取到读锁，持有数量+1
                    firstReaderHoldCount++;
                } else {
                   // 获取读锁的不是第一个线程，记录每个线程获取的次数，存到线程的ThreadLocalMap中
                    HoldCounter rh = cachedHoldCounter;
                     // 缓存为空 || 缓存的不是当前线程的读锁计数
                    if (rh == null || rh.tid != getThreadId(current))
                        cachedHoldCounter = rh = readHolds.get();                
                    // 缓存的是当前线程的读锁计数，而当前线程的读锁计数在上次release时被删除
                    else if (rh.count == 0)
                        readHolds.set(rh);
                    rh.count++;
                }
                return 1;
            }
            
            // 循环尝试获取锁
            return fullTryAcquireShared(current);
        }


   // 默认进入非公平锁代码段
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = -8159625535654395037L;
        final boolean writerShouldBlock() {
            return false; // writers can always barge
        }
        final boolean readerShouldBlock() {
            /* As a heuristic to avoid indefinite writer starvation,
             * block if the thread that momentarily appears to be head
             * of queue, if one exists, is a waiting writer.  This is
             * only a probabilistic effect since a new reader will not
             * block if there is a waiting writer behind other enabled
             * readers that have not yet drained from the queue.
             */
            return apparentlyFirstQueuedIsExclusive();
        }
    }
    
    
    // 这里返回false，则不进行阻塞
    // 如果头节点为空
    // 如果头结点不为空，但头结点后面节点为空
    // 如果头结点不为空，后继节点不为空，后继节点是共享的
    // 如果头结点不为空，后继节点不为空，后继节点不是共享的，后继节点中线程为空
    final boolean apparentlyFirstQueuedIsExclusive() {
        Node h, s;
        return (h = head) != null &&
            (s = h.next)  != null &&
            !s.isShared()         &&
            s.thread != null;
    }



    /**
     * 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 = null;
        for (;;) {
            int c = getState();
            // 有线程获取写锁，且不是当前线程返回失败
            if (exclusiveCount(c) != 0) {
                if (getExclusiveOwnerThread() != current)
                    return -1;
                // else we hold the exclusive lock; blocking here
                // would cause deadlock.
                
            // 读线程需要阻塞
            } else if (readerShouldBlock()) {
                // Make sure we're not acquiring read lock reentrantly
                // 当前线程占有了锁
                if (firstReader == current) {
                    // assert firstReaderHoldCount > 0;
                } else {
                    // 其它线程占有了锁
                    if (rh == null) {
                        rh = cachedHoldCounter;
                        if (rh == null || rh.tid != getThreadId(current)) {
                            rh = readHolds.get();
                            
                            // 当前线程的读锁计数为0
                            if (rh.count == 0)
                                // 在线程局部变量中删除当前线程的读锁计数
                                readHolds.remove();
                        }
                    }
                    
                    // 当前线程的读锁计数为0
                    if (rh.count == 0)
                        // 应排队取锁
                        return -1;
                }
            }
            
            // 读锁达到最大值，异常
            if (sharedCount(c) == MAX_COUNT)
                throw new Error("Maximum lock count exceeded");
                
            // 尝试CAS更新状态，成功则返回
            if (compareAndSetState(c, c + SHARED_UNIT)) {
                if (sharedCount(c) == 0) {
                    firstReader = current;
                    firstReaderHoldCount = 1;
                } else if (firstReader == current) {
                    firstReaderHoldCount++;
                } else {
                    if (rh == null)
                        rh = cachedHoldCounter;
                    if (rh == null || rh.tid != getThreadId(current))
                        rh = readHolds.get();
                    else if (rh.count == 0)
                        readHolds.set(rh);
                    rh.count++;
                    cachedHoldCounter = rh; // cache for release
                }
                return 1;
            }
        }
    }

• 释放读锁

    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
    
    
    // 尝试释放读锁
    protected final boolean tryReleaseShared(int unused) {
        Thread current = Thread.currentThread();
        // 当前线程为首个读线程
        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) {
                // 在线程局部变量中删除当前线程的读锁计数
                readHolds.remove();
                if (count <= 0)
                    throw unmatchedUnlockException();
            }
            // 当前线程的读锁计数（缓存的线程读锁计数）--
            --rh.count;
        }
        for (;;) {
            // CAS失败将回到此处
            int c = getState();
            // 读锁计数--
            int nextc = c - SHARED_UNIT;
            // CAS重置读锁status
            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.
                // CAS(state)成功
                return nextc == 0;
        }
    }

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• 获取写锁

  • 主要逻辑为：当读锁已经被读线程获取或者写锁已经被其他写线程获取，则写锁获取失败；否则，获取成功并支持重入，增加写状态。

    // WriteLock
    public void lock() {
        sync.acquire(1);
    }


    // AQS
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
    
    // Sync
    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();
        // 获取当前锁的状态
        int c = getState();
        // 获取写锁的次数
        int w = exclusiveCount(c);
        if (c != 0) {
            // (Note: if c != 0 and w == 0 then shared count != 0)
            // 当前线程不是获取写锁的线程 或者有线程获取到读锁
            if (w == 0 || current != getExclusiveOwnerThread())
                return false;
            
            // 写锁数量大于最大值，抛异常
            if (w + exclusiveCount(acquires) > MAX_COUNT)
                throw new Error("Maximum lock count exceeded");
            
            // Reentrant acquire
            // 当前线程获取写锁
            // ??? 当前线程已经获取写锁，且没有其它线程获取到读锁
            setState(c + acquires);
            return true;
        }
        
        // 写锁未被任何线程获取，CAS成功
        if (writerShouldBlock() ||
            !compareAndSetState(c, c + acquires))
            return false;
        
        setExclusiveOwnerThread(current);
        return true;
    }
    
    static final int SHARED_SHIFT   = 16;
    static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;
    
    // 同步状态的低16位用来表示写锁的获取次数
    static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
    // 1 & ((1<<16)-1)
    
    // 同步状态的高16位用来表示读锁被获取的次数
    static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }
    // 2>>> 16

• 释放写锁

protected final boolean tryRelease(int releases) {
    if (!isHeldExclusively())
        throw new IllegalMonitorStateException();
    // 同步状态减去写状态
    int nextc = getState() - releases;
    // 当前写状态是否为0，为0则释放写锁
    boolean free = exclusiveCount(nextc) == 0;
    if (free)
        setExclusiveOwnerThread(null);
    // 不为0则更新同步状态
    setState(nextc);
    return free;
}

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