java中鎖是個很重要的概念，當然這裡的前提是你會涉及併發程式設計。

　　除了語言提供的鎖關鍵字 synchronized和volatile之外，jdk還有其他多種實用的鎖。

　　不過這些鎖大多都是基於AQS佇列同步器。ReadWriteLock 讀寫鎖就是其中一個。

　　讀寫鎖的含義是，將讀鎖與寫鎖分開對待，讀鎖可以任意個一起讀，因為讀並不涉及資料變更，而遇到寫鎖後，所有後續的讀寫都將被阻塞。這特性有什麼用呢？比如我們有一個快取，我們可以用它來提高訪問速度，但是當資料變更時，怎樣能保證能讀到準確的資料？

　　在沒有讀寫鎖之前，我們可以使用wait/notify機制，我們可以以寫鎖作為一個同步介質，當寫鎖被佔用時，讀只能等待，寫操作完成後，通知所有讀繼續。這看起來不那麼好實現！

　　當有了讀寫鎖後，我們就不需要這麼麻煩了，只需要讀操作使用讀鎖，寫操作獲取寫鎖操作。大家可能會想，既然都要獲取鎖，那和其他鎖有什麼差別呢，一般看到鎖咱們都會想到序列，阻塞。但其實讀寫鎖不是這樣的。看起來你是每次都獲取讀鎖，但其實單純的讀鎖並不會阻塞執行緒，所以同樣是並行無阻，讀鎖只有在一種情況下會阻塞，那就是寫鎖被某執行緒佔用時。因為寫鎖被佔用則意味著，資料可能馬上發生變化，如果此再允許讀操作任意進行的話，多半可能讀到寫了一半或者是老資料，而這簡直太糟了。而寫鎖則只每次都會真正進行後續操作的阻塞動作，使寫操作保證強一致性。

　　好了，以上就是咱們從概念上來理解讀寫鎖。

　　而實際上呢？ReadWriteLock只是一個介面，而其實現則可能是n多的。我們就以jdk實現的 ReentrantReadWriteLock 為契機，看一下讀寫鎖的實現吧。

　　在介紹 ReetrantReadWriteLock 之前，我們要先簡單說下 ReentrantLock 重入鎖，從字面意思理解，就是可重新進入的鎖。那麼，到底是什麼意思呢？我們想一下，如果我們有2個資源鎖可用，那麼，如果我在本執行緒上上鎖兩次，是不是資源就沒有了呢，那第三次進行鎖獲取的時候，是不是就把自己給鎖死了呢？想想應該是這樣的，但是為啥平時咱們都遇不到這種情況呢？原因就在於可重入性。可重入的意思是說，如果當前執行緒進行多次加鎖操作，那麼無論如何它自己都是可以進入的。簡單從實現來說就是，鎖會排除當前執行緒，從而避免自身阻塞。這些需求看起來很理所當然，但是咱們自己實現的時候可能會因為場景不一樣，從而不一定需要這種特性呢。syncronized也是一種重入鎖。好了，說了這麼多，還是沒有看到 ReetrantLock是怎麼實現的！

我們來看下原始碼就一目瞭然了。

        /**
         * 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)) {
                    // 第一次進入獲取到鎖後，標記獲得鎖的執行緒，後續判定重入
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            // 重入鎖判定，否則失敗
            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;
        }
    }

　　重入鎖介紹完後，咱們可以安心的來說說 ReentrantReadWriteLock了。該讀寫鎖也是一種可重入鎖。它要實現的特性就是，讀讀鎖無阻塞，寫鎖必阻塞（包括寫讀鎖/寫寫鎖），讀寫鎖阻塞（需等待讀鎖釋放後才能獲取寫鎖從而保證無髒讀）。

　　從上面可以看出，讀和寫是兩個鎖，但是他們的狀態卻是互相關聯的，那怎樣設計其資料結構呢？用兩個變數去推導往往不太可行，因為其本身就是鎖，如果再用兩個變數去判定鎖狀態，那麼又如何保證變數自身的可靠性呢？ReentrantReadWriteLock 是通過一個狀態變數來控制的，具體為 高16位儲存讀鎖狀態，低16位儲存寫鎖狀態，而在改變狀態時，使用cas保證寫入的可靠性。（其實這裡可以看出，鎖個數不應該超過16位即65536個，這種鎖數量已經完全被忽略掉了）。有了資料結構，咱們再看下怎麼控制讀寫互聯。讀鎖的獲取，寫鎖沒被佔用時，即低位為0時，高位大於0即可代表獲取了讀鎖，所以，讀鎖是n個可用的。而寫鎖的獲取，則要依賴高低位判定了，高位大於0，即代表還有讀鎖存在，不能進入，如果高位為0，也不一定可進入，低位不為0則代表有寫鎖在佔用，所以只有高低位都為0時，寫鎖才可用。

　　下面，來看下讀寫鎖的具體實現！

來個例子先:

public class ReadWriteLockTest {

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

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

    /**
     * 執行執行緒池
     */
    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();
    }

    /**
     * 執行緒模擬完成後，關閉執行緒池
     */
    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(), 無阻塞，立即完成10個讀任務！

　　而 testWriteLock()，則是全部阻塞執行，20秒完成序列10個任務！

　　而 testReadWriteLock(), 則是 讀鎖與寫鎖交替執行，在執行寫鎖時，所有鎖等待，在執行讀鎖時，可能存在多個鎖同時執行！執行結果樣例如下:

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, 現象已經展示了，是時候透過現象看本質了！

1. 讀鎖的獲取過程 r.lock(), 其實現為 ReadLock！

        public void lock() {
            // 呼叫 AQS 的 acquireShared() 方法，進行統一排程
            sync.acquireShared(1);
        }
    // AQS 獲取共享讀鎖    
    public final void acquireShared(int arg) {
        // 呼叫 ReentrantReadWriteLock.Sync.tryAcquireShared(), 定義鎖獲取方式
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }
    
    
    // 獲取讀鎖，unused 傳參未使用，直接使用內建的高位加1方式處理
        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();
            // 寫鎖使用中，則直接獲取失敗
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;
            int r = sharedCount(c);
            // 讀鎖任意獲取，除了超過最大限制
            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;
            }
            // 對讀鎖阻塞情況，進行處理
            return fullTryAcquireShared(current);
        }
        
        // 獲取低位數，即寫鎖狀態值
        static int exclusiveCount(int c) {
            return c & EXCLUSIVE_MASK; 
        }
        // 獲取高位數，即讀鎖狀態值
        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");
                // 驗證通過，cas更新鎖狀態，使用 SHARED_UNIT 進行高位加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;
                }
            }
        }

　　以上是獲取讀鎖的過程，其實際控制很簡單，只是多了很多的狀態統計，所以看起來複雜！

2. 下面，來看寫鎖的獲取過程，WriteLock.lock()

        public void lock() {
            // AQS獲取獨佔鎖
            sync.acquire(1);
        }
        
    // AQS 鎖排程
    public final void acquire(int arg) {
        // 如果獲取鎖失敗，則加入到等待佇列中
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    // ReentrantReadWriteLock.Sync.tryAcquire(), 寫鎖獲取過程
        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，則說明不存在讀寫鎖，直接成功
            // 否則分有讀鎖和有寫鎖兩種情況判斷
            if (c != 0) {
                // (Note: if c != 0 and w == 0 then shared count != 0)
                // 存在讀鎖，或者不是當前執行緒（重入），則直接失敗
                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 的鎖入佇列操，從佇列中進行鎖獲取，如果獲取失敗，則產線一箇中斷標誌
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                // 這裡是公平鎖的實現方式，只會從佇列頭獲取鎖
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                // 阻塞判定，響應中斷
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

　　ok, 讀寫鎖的獲取已經完成，再來看一下釋放的過程！

3. 讀鎖的釋放 ReadLock.unlock()

        public void unlock() {
            // AQS 的釋放控制
            sync.releaseShared(1);
        }
        
    // AQS 釋放鎖
    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
    // ReentrantReadWriteLock.Sync.tryReleaseShared() 自定義釋放
        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更新狀態，每次減1，直到為0，鎖才算真正釋放
                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. 讀鎖的釋放, WriteLock.unlock()

        public void unlock() {
            // AQS 釋放控制
            sync.release(1);
        }
    // AQS
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            // 釋放鎖
            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;
            // 如果寫鎖狀態為0，則意味著當前執行緒完全釋放鎖，將 owner 線各設定為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;
        }
        // 呼叫 LockSupport 釋放鎖
        if (s != null)
            LockSupport.unpark(s.thread);
    }

　　綜上，讀寫鎖的簡要解析就算完成了。 其主要使用 AQS 的基礎元件，進行鎖排程！ 使用CAS進行狀態的安全設定！ 而鎖的阻塞，則是使用 LockSupport 工具元件進行實際阻塞！