Java并发核心浅谈（二）-白红宇

回顾

在上一篇我们大概了解到了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;复制代码

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;            }        }复制代码

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。