摘要：在線程處理任務期間，其它線程要么循環訪問，要么一直阻塞等著線程喚醒，再不濟就真的如我所說，放棄鎖的競爭，去處理別的任務。寫鎖的話，獨占寫計數，排除一切其他線程。

在上一篇 Java并發核心淺談 我們大概了解到了Lock和synchronized的共同點，再簡單總結下：

Lock主要是自定義一個 counter，從而利用CAS對其實現原子操作，而synchronized是c++ hotspot實現的 monitor（具體的咱也沒看，咱就不說）

二者都可重入（遞歸，互調，循環），其本質都是維護一個可計數的 counter，在其它線程訪問加鎖對象時會判斷 counter 是否為 0

理論上講二者都是阻塞式的，因為線程在拿鎖時，如果拿不到，最終的結果只能等待（前提是線程的最終目的就是要獲取鎖）讀寫鎖分離成兩把鎖了，所以不一樣

舉個例子：線程 A 持有了某個對象的 monitor，其它線程在訪問該對象時，發現 monitor 不為 0，所以只能阻塞掛起或者加入等待隊列，等著線程 A 處理完退出后將 monitor 置為 0。在線程 A 處理任務期間，其它線程要么循環訪問 monitor，要么一直阻塞等著線程 A 喚醒，再不濟就真的如我所說，放棄鎖的競爭，去處理別的任務。但是應該做不到去處理別的任務后，任務處理到一半，被線程 A 通知后再回去搶鎖

不共享 counter

        // 非公平鎖在第一次拿鎖失敗也會調用該方法
        public final void acquire(int arg) {
        // 沒拿到鎖就加入隊列
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
        }
        
        // 非公平鎖方法
        final void lock() {
            // 走來就嘗試獲取鎖
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1); // 上面那個方法
        }
        
        // 公平鎖 Acquire 計數
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            // 拿到計數
            int c = getState();
            if (c == 0) {
                // 公平鎖會先嘗試排隊 非公平鎖少個 !hasQueuedPredecessors() 其它代碼一樣
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)  // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
        
        /**
         * @return {@code true} if there is a queued thread preceding the // 當前線程前有線程等待，則排隊
         *         current thread, and {@code false} if the current thread
         *         is at the head of the queue or the queue is empty // 隊列為空不用排隊
         * @since 1.7
         */
        public final boolean hasQueuedPredecessors() {
            // The correctness of this depends on head being initialized
            // before tail and on head.next being accurate if the current
            // thread is first in queue.
            Node t = tail; // Read fields in reverse initialization order
            Node h = head;
            Node s;
            // 當前線程處于頭節點的下一個且不為空則不用排隊
            // 或該線程就是當前持有鎖的線程，即重入鎖，也不用排隊
            return h != t &&
                ((s = h.next) == null || s.thread != Thread.currentThread());
        }
        
        // 加入等待隊列
        final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                // 獲取失敗會檢查節點狀態
                // 然后 park 線程
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
        /** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED =  1; // 線程取消加鎖
        /** waitStatus value to indicate successor"s thread needs unparking */
        static final int SIGNAL    = -1;  // 解除線程 park
        /** waitStatus value to indicate thread is waiting on condition */ // 
        static final int CONDITION = -2; // 線程被阻塞
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
        static final int PROPAGATE = -3; // 廣播
        
        // 官方注釋
        /**
         * Status field, taking on only the values:
         *   SIGNAL:     The successor of this node is (or will soon be)
         *               blocked (via park), so the current node must
         *               unpark its successor when it releases or
         *               cancels. To avoid races, acquire methods must
         *               first indicate they need a signal,
         *               then retry the atomic acquire, and then,
         *               on failure, block.
         *   CANCELLED:  This node is cancelled due to timeout or interrupt.
         *               Nodes never leave this state. In particular,
         *               a thread with cancelled node never again blocks.
         *   CONDITION:  This node is currently on a condition queue.
         *               It will not be used as a sync queue node
         *               until transferred, at which time the status
         *               will be set to 0. (Use of this value here has
         *               nothing to do with the other uses of the
         *               field, but simplifies mechanics.)
         *   PROPAGATE:  A releaseShared should be propagated to other
         *               nodes. This is set (for head node only) in
         *               doReleaseShared to ensure propagation
         *               continues, even if other operations have
         *               since intervened.
         *   0:          None of the above
         *
         * The values are arranged numerically to simplify use.
         * Non-negative values mean that a node doesn"t need to
         * signal. So, most code doesn"t need to check for particular
         * values, just for sign.
         *
         * The field is initialized to 0 for normal sync nodes, and
         * CONDITION for condition nodes.  It is modified using CAS
         * (or when possible, unconditional volatile writes).
         */
        volatile int waitStatus;

讀鎖：共享 counter

寫鎖：不共享 counter

        // 讀寫鎖和線程池的類似之處
        // 高 16 位為讀計數，低 16 位為寫計數
        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. */ // 獲取讀計數
        static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }
        /** Returns the number of exclusive holds represented in count. */ // 獲取寫計數
        static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
        
        /**
         * A counter for per-thread read hold counts. 每個線程自己的讀計數
         * Maintained as a ThreadLocal; cached in cachedHoldCounter.
         */
        static final class HoldCounter {
            int count;          // initially 0
            // Use id, not reference, to avoid garbage retention
            final long tid = LockSupport.getThreadId(Thread.currentThread()); // 線程 id
        }
        
    // 嘗試獲取讀鎖
    protected final int tryAcquireShared(int unused) {
            // ReentrantReadWriteLock ReadLock 讀鎖
            /*
             * 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();
            // 如果寫鎖計數不為零，且當前線程不是寫鎖持有線程，則可以獲得讀鎖
            // 言外之意，獲得寫鎖的線程不可以再獲得讀鎖
            // 個人認為不用判斷寫計數也行
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;
            // 獲得讀計數
            int r = sharedCount(c);
            // 檢查等待隊列 讀計數上限
            if (!readerShouldBlock() &&
                r < MAX_COUNT &&
                // 在高 16 位更新
                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 != LockSupport.getThreadId(current))
                        // cachedHoldCounter 每個線程自己的讀計數，非共享。但是鎖計數與其它讀操作共享，不與寫操作共享
                        // readHolds 為ThreadLocalHoldCounter，繼承于 ThreadLocal，存 cachedHoldCounter
                        cachedHoldCounter = rh = readHolds.get();
                    else if (rh.count == 0)
                        readHolds.set(rh);
                    rh.count++;
                }
                return 1;
            }
            // 說明在排隊中，就一直遍歷，直到隊首，實際起作用的代碼和上面代碼差不多
            // 大師本人也說了代碼有冗余
             /*
             * 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.
             */
            return fullTryAcquireShared(current);
        }
        
    // 獲得寫鎖  
    protected final boolean tryAcquire(int acquires) {
            /*
             * Walkthrough:
             * 1. If read count nonzero or write count nonzero
             *    and owner is a different thread, fail. 
             * 讀鎖不為零（讀鎖排除寫鎖，但是與讀鎖共享）
             * 寫鎖不為零且鎖持有者不為當前線程，則獲得鎖失敗
             * 2. If count would saturate, fail. (This can only
             *    happen if count is already nonzero.) // 計數器已達最大值，獲得鎖失敗
             * 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. // 重入是可以的；隊列策略也是可以的，會在下面解釋
             */
            Thread current = Thread.currentThread();
            int c = getState();
            // 獲得寫計數
            int w = exclusiveCount(c);
            if (c != 0) {
                // (Note: if c != 0 and w == 0 then shared count != 0)
                // 檢查所持有線程
                if (w == 0 || current != getExclusiveOwnerThread())
                    return false;
                // 檢查最大計數
                if (w + exclusiveCount(acquires) > MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                // Reentrant acquire 線程重入獲得鎖，直接更新計數
                setState(c + acquires);
                return true;
            }
            // 隊列策略
            // state 為 0，檢查是否需要排隊
            // 針對公平鎖：去排隊，如果當前線程在隊首或等待隊列為空，則返回 false，自然會走后面的 CAS
            // 否則就返回 true，則進入 return false;
            // 針對非公平鎖：寫死為 false，直接 CAS
            if (writerShouldBlock() ||
                !compareAndSetState(c, c + acquires))
                return false;
            // 設置當前寫鎖持有線程
            setExclusiveOwnerThread(current);
            return true;
        }    
    
    // 因為讀鎖是多個線程共享讀計數，各自維護了自己的讀計數，所以釋放的時候比寫鎖釋放要多些操作
     protected final boolean tryReleaseShared(int unused) {
            Thread current = Thread.currentThread();
            // 當前線程是第一讀線程的操作
            // firstReader 作為字段緩存，是考慮到第一次讀的線程使用率高？
            if (firstReader == current) {
                // assert firstReaderHoldCount > 0;
                if (firstReaderHoldCount == 1)
                    firstReader = null;
                else
                    firstReaderHoldCount--;
            } else {
                HoldCounter rh = cachedHoldCounter;
                if (rh == null ||
                    rh.tid != LockSupport.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;
                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;
            }
        }

公平鎖和非公平鎖的“鎖”實現是基于CAS，公平性基于內部維護的Node鏈表

讀寫鎖，可以粗略的理解為讀和寫兩種狀態，所以這兒的設計類似線程池的狀態。只不過，讀計數是可以多個讀線程共享的（排除寫鎖），每個讀的線程都會維護自己的讀計數。寫鎖的話，獨占寫計數，排除一切其他線程。