Java并发核心浅谈（二）

回顾

在上一篇 Java并发核心浅谈 我们大概了解到了Lock和synchronized的共同点，再简单总结下：

• Lock主要是自定义一个 counter，从而利用CAS对其实现原子操作，而synchronized是c++ hotspot实现的 monitor（具体的咱也没看，咱就不说）
• 二者都可重入（递归，互调，循环），其本质都是维护一个可计数的 counter，在其它线程访问加锁对象时会判断 counter 是否为 0
• 理论上讲二者都是阻塞式的，因为线程在拿锁时，如果拿不到，最终的结果只能等待（前提是线程的最终目的就是要获取锁）读写锁分离成两把锁了，所以不一样

举个例子：线程 A 持有了某个对象的 monitor，其它线程在访问该对象时，发现 monitor 不为 0，所以只能阻塞挂起或者加入等待队列，等着线程 A 处理完退出后将 monitor 置为 0。在线程 A 处理任务期间，其它线程要么循环访问 monitor，要么一直阻塞等着线程 A 唤醒，再不济就真的如我所说，放弃锁的竞争，去处理别的任务。但是应该做不到去处理别的任务后，任务处理到一半，被线程 A 通知后再回去抢锁

公平锁与非公平锁

不共享 counter

        // 非公平锁在第一次拿锁失败也会调用该方法
        public final void acquire(int arg) {
        // 没拿到锁就加入队列
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
        }
        
        // 非公平锁方法
        final void lock() {
            // 走来就尝试获取锁
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1); // 上面那个方法
        }
        
        // 公平锁 Acquire 计数
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            // 拿到计数
            int c = getState();
            if (c == 0) {
                // 公平锁会先尝试排队 非公平锁少个 !hasQueuedPredecessors() 其它代码一样
                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 // 当前线程前有线程等待，则排队
         *         current thread, and {@code false} if the current thread
         *         is at the head of the queue or the queue is empty // 队列为空不用排队
         * @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;
            // 当前线程处于头节点的下一个且不为空则不用排队
            // 或该线程就是当前持有锁的线程，即重入锁，也不用排队
            return h != t &&
                ((s = h.next) == null || s.thread != Thread.currentThread());
        }
        
        // 加入等待队列
        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;
                }
                // 获取失败会检查节点状态
                // 然后 park 线程
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
        /** waitStatus value to indicate thread has cancelled */
        static final int CANCELLED =  1; // 线程取消加锁
        /** waitStatus value to indicate successor's thread needs unparking */
        static final int SIGNAL    = -1;  // 解除线程 park
        /** waitStatus value to indicate thread is waiting on condition */ // 
        static final int CONDITION = -2; // 线程被阻塞
        /**
         * waitStatus value to indicate the next acquireShared should
         * unconditionally propagate
         */
        static final int PROPAGATE = -3; // 广播
        
        // 官方注释
        /**
         * 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;

读锁与写锁（共享锁与排他锁）

读锁：共享 counter

写锁：不共享 counter

        // 读写锁和线程池的类似之处
        // 高 16 位为读计数，低 16 位为写计数
        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. */ // 获取读计数
        static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }
        /** Returns the number of exclusive holds represented in count. */ // 获取写计数
        static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
        
        /**
         * A counter for per-thread read hold counts. 每个线程自己的读计数
         * 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()); // 线程 id
        }
        
    // 尝试获取读锁
    protected final int tryAcquireShared(int unused) {
            // ReentrantReadWriteLock ReadLock 读锁
            /*
             * 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();
            // 如果写锁计数不为零，且当前线程不是写锁持有线程，则可以获得读锁
            // 言外之意，获得写锁的线程不可以再获得读锁
            // 个人认为不用判断写计数也行
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;
            // 获得读计数
            int r = sharedCount(c);
            // 检查等待队列 读计数上限
            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 每个线程自己的读计数，非共享。但是锁计数与其它读操作共享，不与写操作共享
                        // readHolds 为ThreadLocalHoldCounter，继承于 ThreadLocal，存 cachedHoldCounter
                        cachedHoldCounter = rh = readHolds.get();
                    else if (rh.count == 0)
                        readHolds.set(rh);
                    rh.count++;
                }
                return 1;
            }
            // 说明在排队中，就一直遍历，直到队首，实际起作用的代码和上面代码差不多
            // 大师本人也说了代码有冗余
             /*
             * 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);
        }
        
    // 获得写锁  
    protected final boolean tryAcquire(int acquires) {
            /*
             * Walkthrough:
             * 1. If read count nonzero or write count nonzero
             *    and owner is a different thread, fail. 
             * 读锁不为零（读锁排除写锁，但是与读锁共享）
             * 写锁不为零且锁持有者不为当前线程，则获得锁失败
             * 2. If count would saturate, fail. (This can only
             *    happen if count is already nonzero.) // 计数器已达最大值，获得锁失败
             * 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；队列策略也是可以的，会在下面解释
             */
            Thread current = Thread.currentThread();
            int c = getState();
            // 获得写计数
            int w = exclusiveCount(c);
            if (c != 0) {
                // (Note: if c != 0 and w == 0 then shared count != 0)
                // 检查所持有线程
                if (w == 0 || current != getExclusiveOwnerThread())
                    return false;
                // 检查最大计数
                if (w + exclusiveCount(acquires) > MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                // Reentrant acquire 线程重入获得锁，直接更新计数
                setState(c + acquires);
                return true;
            }
            // 队列策略
            // state 为 0，检查是否需要排队
            // 针对公平锁：去排队，如果当前线程在队首或等待队列为空，则返回 false，自然会走后面的 CAS
            // 否则就返回 true，则进入 return false;
            // 针对非公平锁：写死为 false，直接 CAS
            if (writerShouldBlock() ||
                !compareAndSetState(c, c + acquires))
                return false;
            // 设置当前写锁持有线程
            setExclusiveOwnerThread(current);
            return true;
        }    
    
    // 因为读锁是多个线程共享读计数，各自维护了自己的读计数，所以释放的时候比写锁释放要多些操作
     protected final boolean tryReleaseShared(int unused) {
            Thread current = Thread.currentThread();
            // 当前线程是第一读线程的操作
            // firstReader 作为字段缓存，是考虑到第一次读的线程使用率高？
            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;
            }
        }

总结一下

公平锁和非公平锁的“锁”实现是基于CAS，公平性基于内部维护的Node链表

读写锁，可以粗略的理解为读和写两种状态，所以这儿的设计类似线程池的状态。只不过，读计数是可以多个读线程共享的（排除写锁），每个读的线程都会维护自己的读计数。写锁的话，独占写计数，排除一切其他线程。