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是共享的（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。