ReentrantReadWriteLock 读写锁解析

java中锁是个很重要的概念，当然这里的前提是你会涉及并发编程。

除了语言提供的锁关键字 synchronized和volatile之外，jdk还有其他多种实用的锁。

不过这些锁大多都是基于AQS队列同步器。ReadWriteLock 读写锁就是其中一个。

读写锁的含义是，将读锁与写锁分开对待，读锁可以任意个一起读，因为读并不涉及数据变更，而遇到写锁后，所有后续的读写都将被阻塞。这特性有什么用呢？比如我们有一个缓存，我们可以用它来提高访问速度，但是当数据变更时，怎样能保证能读到准确的数据？

在没有读写锁之前，我们可以使用wait/notify机制，我们可以以写锁作为一个同步介质，当写锁被占用时，读只能等待，写操作完成后，通知所有读继续。这看起来不那么好实现！

当有了读写锁后，我们就不需要这么麻烦了，只需要读操作使用读锁，写操作获取写锁操作。大家可能会想，既然都要获取锁，那和其他锁有什么差别呢，一般看到锁咱们都会想到串行，阻塞。但其实读写锁不是这样的。看起来你是每次都获取读锁，但其实单纯的读锁并不会阻塞线程，所以同样是并行无阻，读锁只有在一种情况下会阻塞，那就是写锁被某线程占用时。因为写锁被占用则意味着，数据可能马上发生变化，如果此再允许读操作任意进行的话，多半可能读到写了一半或者是老数据，而这简直太糟了。而写锁则只每次都会真正进行后续操作的阻塞动作，使写操作保证强一致性。

好了，以上就是咱们从概念上来理解读写锁。

而实际上呢？ReadWriteLock只是一个接口，而其实现则可能是n多的。我们就以jdk实现的 ReentrantReadWriteLock 为契机，看一下读写锁的实现吧。

在介绍 ReetrantReadWriteLock 之前，我们要先简单说下 ReentrantLock 重入锁，从字面意思理解，就是可重新进入的锁。那么，到底是什么意思呢？我们想一下，如果我们有2个资源锁可用，那么，如果我在本线程上上锁两次，是不是资源就没有了呢，那第三次进行锁获取的时候，是不是就把自己给锁死了呢？想想应该是这样的，但是为啥平时咱们都遇不到这种情况呢？原因就在于可重入性。可重入的意思是说，如果当前线程进行多次加锁操作，那么无论如何它自己都是可以进入的。简单从实现来说就是，锁会排除当前线程，从而避免自身阻塞。这些需求看起来很理所当然，但是咱们自己实现的时候可能会因为场景不一样，从而不一定需要这种特性呢。syncronized也是一种重入锁。好了，说了这么多，还是没有看到 ReetrantLock是怎么实现的！

用个不恰当的图描绘下：

我们来看下源码就一目了然了。

        /**
         * Fair version of tryAcquire
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    // 第一次进入获取到锁后，标记获得锁的线程，后续判定重入
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            // 重入锁判定，否则失败
            else if (current == getExclusiveOwnerThread()) {
                // 最多可重入 int 次
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

重入锁介绍完后，咱们可以安心的来说说 ReentrantReadWriteLock了。该读写锁也是一种可重入锁。它要实现的特性就是，读读锁无阻塞，写锁必阻塞（包括写读锁/写写锁），读写锁阻塞（需等待读锁释放后才能获取写锁从而保证无脏读）。

从上面可以看出，读和写是两个锁，但是他们的状态却是互相关联的，那怎样设计其数据结构呢？用两个变量去推导往往不太可行，因为其本身就是锁，如果再用两个变量去判定锁状态，那么又如何保证变量自身的可靠性呢？ReentrantReadWriteLock 是通过一个状态变量来控制的，具体为 高16位保存读锁状态，低16位保存写锁状态，而在改变状态时，使用cas保证写入的可靠性。（其实这里可以看出，锁个数不应该超过16位即65536个，这种锁数量已经完全被忽略掉了）。有了数据结构，咱们再看下怎么控制读写互联。读锁的获取，写锁没被占用时，即低位为0时，高位大于0即可代表获取了读锁，所以，读锁是n个可用的。而写锁的获取，则要依赖高低位判定了，高位大于0，即代表还有读锁存在，不能进入，如果高位为0，也不一定可进入，低位不为0则代表有写锁在占用，所以只有高低位都为0时，写锁才可用。

下面，来看下读写锁的具体实现！

来个例子先:

public class ReadWriteLockTest {

    private ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock();
    /**
     * 读锁
     */
    private Lock r = reentrantReadWriteLock.readLock();

    /**
     * 写锁
     */
    private Lock w = reentrantReadWriteLock.writeLock();

    /**
     * 执行线程池
     */
    private ExecutorService executorService = Executors.newCachedThreadPool();

    @Test
    public void testReadLock() {
        for (int i = 0; i < 10; i++) {
            Thread readWorker = new ReadWorker();
            executorService.submit(readWorker);
        }
        waitForExecutorFinish();
    }

    @Test
    public void testWriteLock() {
        for (int i = 0; i < 10; i++) {
            Thread writeWorker = new WriteWorker();
            executorService.submit(writeWorker);
        }
        waitForExecutorFinish();
    }

    @Test
    public void testReadWriteLock() {
        for (int i = 0; i < 10; i++) {
            Thread readWorker = new ReadWorker();
            Thread writeWorker = new WriteWorker();
            executorService.submit(readWorker);
            executorService.submit(writeWorker);
        }
        waitForExecutorFinish();
    }

    /**
     * 线程模拟完成后，关闭线程池
     */
    private void waitForExecutorFinish() {
        executorService.shutdown();
        try {
            executorService.awaitTermination(100, TimeUnit.SECONDS);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private final class ReadWorker extends Thread {
        @Override
        public void run() {
            r.lock();
            try {
                SleepUtils.second(1);
                System.out.println(System.currentTimeMillis() + ": " + Thread.currentThread().getName() + " reading...");
                SleepUtils.second(1);
            }
            finally {
                r.unlock();
            }
        }
    }

    private final class WriteWorker extends Thread {
        @Override
        public void run() {
            w.lock();
            try {
                SleepUtils.second(1);
                System.out.println(System.currentTimeMillis() + ": " + Thread.currentThread().getName() + " writing...");
                SleepUtils.second(1);
            }
            finally {
                w.unlock();
            }
        }
    }

}

可以看到 testReadLock(), 无阻塞，立即完成10个读任务！

而 testWriteLock()，则是全部阻塞执行，20秒完成串行10个任务！

而 testReadWriteLock(), 则是 读锁与写锁交替执行，在执行写锁时，所有锁等待，在执行读锁时，可能存在多个锁同时运行！执行结果样例如下:

1543816105277: pool-1-thread-1 reading...
1543816107278: pool-1-thread-2 writing...
1543816109278: pool-1-thread-20 writing...
1543816111278: pool-1-thread-16 writing...
1543816113279: pool-1-thread-12 writing...
1543816115279: pool-1-thread-8 writing...
1543816117280: pool-1-thread-19 reading...
1543816117280: pool-1-thread-15 reading...
1543816119280: pool-1-thread-4 writing...
1543816121280: pool-1-thread-18 writing...
1543816123281: pool-1-thread-3 reading...
1543816123281: pool-1-thread-7 reading...
1543816125287: pool-1-thread-14 writing...
1543816127290: pool-1-thread-6 writing...
1543816129290: pool-1-thread-10 writing...
1543816131290: pool-1-thread-11 reading...
1543816131290: pool-1-thread-13 reading...
1543816131290: pool-1-thread-9 reading...
1543816131290: pool-1-thread-5 reading...
1543816131290: pool-1-thread-17 reading...

ok, 现象已经展示了，是时候透过现象看本质了！

1. 读锁的获取过程 r.lock(), 其实现为 ReadLock！

        public void lock() {
            // 调用 AQS 的 acquireShared() 方法，进行统一调度
            sync.acquireShared(1);
        }
    // AQS 获取共享读锁    
    public final void acquireShared(int arg) {
        // 调用 ReentrantReadWriteLock.Sync.tryAcquireShared(), 定义锁获取方式
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }
    
    
    // 获取读锁，unused 传参未使用，直接使用内置的高位加1方式处理
        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();
            // 写锁使用中，则直接获取失败
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;
            int r = sharedCount(c);
            // 读锁任意获取，除了超过最大限制
            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;
            }
            // 对读锁阻塞情况，进行处理
            return fullTryAcquireShared(current);
        }
        
        // 获取低位数，即写锁状态值
        static int exclusiveCount(int c) {
            return c & EXCLUSIVE_MASK; 
        }
        // 获取高位数，即读锁状态值
        static int sharedCount(int c) { 
            return c >>> SHARED_SHIFT; 
        }
        
        /**
         * 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();
                if (exclusiveCount(c) != 0) {
                    if (getExclusiveOwnerThread() != current)
                        return -1;
                    // else we hold the exclusive lock; blocking here
                    // would cause deadlock.
                } else if (readerShouldBlock()) {
                    // Make sure we're not acquiring read lock reentrantly
                    if (firstReader == current) {
                        // assert firstReaderHoldCount > 0;
                    } else {
                        if (rh == null) {
                            rh = cachedHoldCounter;
                            if (rh == null || rh.tid != getThreadId(current)) {
                                rh = readHolds.get();
                                if (rh.count == 0)
                                    readHolds.remove();
                            }
                        }
                        if (rh.count == 0)
                            return -1;
                    }
                }
                if (sharedCount(c) == MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                // 验证通过，cas更新锁状态，使用 SHARED_UNIT 进行高位加1
                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;
                }
            }
        }

以上是获取读锁的过程，其实际控制很简单，只是多了很多的状态统计，所以看起来复杂！

2. 下面，来看写锁的获取过程，WriteLock.lock()

        public void lock() {
            // AQS获取独占锁
            sync.acquire(1);
        }
        
    // AQS 锁调度
    public final void acquire(int arg) {
        // 如果获取锁失败，则加入到等待队列中
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    // ReentrantReadWriteLock.Sync.tryAcquire(), 写锁获取过程
        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();
            int w = exclusiveCount(c);
            // 如果是0，则说明不存在读写锁，直接成功
            // 否则分有读锁和有写锁两种情况判断
            if (c != 0) {
                // (Note: if c != 0 and w == 0 then shared count != 0)
                // 存在读锁，或者不是当前线程（重入），则直接失败
                if (w == 0 || current != getExclusiveOwnerThread())
                    return false;
                if (w + exclusiveCount(acquires) > MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                // Reentrant acquire
                setState(c + acquires);
                return true;
            }
            // cas 更新 state 
            if (writerShouldBlock() ||
                !compareAndSetState(c, c + acquires))
                return false;
            setExclusiveOwnerThread(current);
            return true;
        }
        
    /**
     * Creates and enqueues node for current thread and given mode.
     *
     * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
     * @return the new node
     */
    private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }

    // AQS 的锁入队列操，从队列中进行锁获取，如果获取失败，则产线一个中断标志
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                // 这里是公平锁的实现方式，只会从队列头获取锁
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                // 阻塞判定，响应中断
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

ok, 读写锁的获取已经完成，再来看一下释放的过程！

3. 读锁的释放 ReadLock.unlock()

        public void unlock() {
            // AQS 的释放控制
            sync.releaseShared(1);
        }
        
    // AQS 释放锁
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
    // ReentrantReadWriteLock.Sync.tryReleaseShared() 自定义释放
        protected final boolean tryReleaseShared(int unused) {
            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;
            }
            for (;;) {
                int c = getState();
                int nextc = c - SHARED_UNIT;
                // cas更新状态，每次减1，直到为0，锁才算真正释放
                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;
            }
        }
        
    /**
     * Release action for shared mode -- signals successor and ensures
     * propagation. (Note: For exclusive mode, release just amounts
     * to calling unparkSuccessor of head if it needs signal.)
     */
    private void doReleaseShared() {
        /*
         * Ensure that a release propagates, even if there are other
         * in-progress acquires/releases.  This proceeds in the usual
         * way of trying to unparkSuccessor of head if it needs
         * signal. But if it does not, status is set to PROPAGATE to
         * ensure that upon release, propagation continues.
         * Additionally, we must loop in case a new node is added
         * while we are doing this. Also, unlike other uses of
         * unparkSuccessor, we need to know if CAS to reset status
         * fails, if so rechecking.
         */
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }

4. 读锁的释放, WriteLock.unlock()

        public void unlock() {
            // AQS 释放控制
            sync.release(1);
        }
    // AQS
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            // 释放锁
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
        // Sync.tryRelease()
        protected final boolean tryRelease(int releases) {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int nextc = getState() - releases;
            // 如果写锁状态为0，则意味着当前线程完全释放锁，将 owner 线各设置为null
            boolean free = exclusiveCount(nextc) == 0;
            if (free)
                setExclusiveOwnerThread(null);
            setState(nextc);
            return free;
        }
    
    /**
     * Wakes up node's successor, if one exists.
     *
     * @param node the node
     */
    private void unparkSuccessor(Node node) {
        /*
         * If status is negative (i.e., possibly needing signal) try
         * to clear in anticipation of signalling.  It is OK if this
         * fails or if status is changed by waiting thread.
         */
        int ws = node.waitStatus;
        if (ws < 0)
            compareAndSetWaitStatus(node, ws, 0);

        /*
         * Thread to unpark is held in successor, which is normally
         * just the next node.  But if cancelled or apparently null,
         * traverse backwards from tail to find the actual
         * non-cancelled successor.
         */
        Node s = node.next;
        if (s == null || s.waitStatus > 0) {
            s = null;
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        // 调用 LockSupport 释放锁
        if (s != null)
            LockSupport.unpark(s.thread);
    }

综上，读写锁的简要解析就算完成了。 其主要使用 AQS 的基础组件，进行锁调度！ 使用CAS进行状态的安全设置！ 而锁的阻塞，则是使用 LockSupport 工具组件进行实际阻塞！