java锁ReentrantReadWriteLock源码分析

[diaosichengxuyuan]

ReentrantReadWriteLock是可重入的读写锁，支持公平和非公平模式，ReadLock是共享锁，WriteLock是独占锁。

ReadLock.lock()和unlock()

 public static class ReadLock implements Lock, java.io.Serializable {
        public void lock() {
            //共享的获取状态1
            sync.acquireShared(1);
        }
        
        public void unlock() {
            //共享的释放状态1
            sync.releaseShared(1);
        }
}

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    public final void acquireShared(int arg) {
        //如果尝试共享获取状态1不成功，加入同步队列
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }
    
    public final boolean releaseShared(int arg) {
        //尝试释放成功，唤醒下个节点
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
    
    private void doAcquireShared(int arg) {
        //加入同步队列
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            //自旋，如果前驱是首节点，就尝试获取状态；如果获取获取状态
            //失败或者前驱不是首节点，LockSupport.park(Thread)，
            //阻塞当前线程
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    
    //唤醒同步队列中的下一个线程
private void doReleaseShared() {
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;           
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
           !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue; 
            }
            if (h == head) 
                break;
        }
    }
    
}

abstract static class Sync extends AbstractQueuedSynchronizer {

        //获取读锁数量，高16位为读锁
        static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }
        //获取写锁数量，低16位为写锁
        static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
        static final class ThreadLocalHoldCounter
            extends ThreadLocal<HoldCounter> {
            public HoldCounter initialValue() {
                return new HoldCounter();
            }
        }

        protected final boolean tryReleaseShared(int unused) {
            Thread current = Thread.currentThread();
            //如果当前线程是第一个读线程
            if (firstReader == current) {
                if (firstReaderHoldCount == 1)
                    firstReader = null;
                else
                    firstReaderHoldCount--;
            }
            //否则，需要从ThreadLocal中取出当前线程中读锁个数，
            //然后减1
            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;
            }
            //死循环方式通过CAS释放状态
            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;
            }
        }


        protected final int tryAcquireShared(int unused) {
            //当前线程
            Thread current = Thread.currentThread();
            //状态值，包括读锁和写锁
            int c = getState();
            //如果写锁数量不为0，并且独占线程不是当前线程，表示有其他
            //线程获取了写锁，直接返回-1，当前线程无法获取读锁
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;
            //如果写锁数量为0或者当前线程自己获取到的写锁，则当前线程
            //可以继续获取读锁
            int r = sharedCount(c);
            //readerShouldBlock方法，对于公平锁，只要队列中有线程在        
            //等待，那么将会返回true，也就意味着读线程需要阻塞；对于非
            //公平锁，如果当前有线程获取了写锁，则返回true。
            //如果读线程不需要阻塞，且读锁个数小于最大值，且CAS设置状
            //态成功
            if (!readerShouldBlock() &&
                r < MAX_COUNT &&
                compareAndSetState(c, c + SHARED_UNIT)) {
                //如果读锁个数为0，设置当前线程为第一个读线程
                if (r == 0) {
                    firstReader = current;
                    firstReaderHoldCount = 1;
                }
                //如果读锁不为0，但是当前线程是第一个读线程，直接增加
                //firstReaderHoldCount值
                else if (firstReader == current) {
                    firstReaderHoldCount++;
                }
                //否则，需要从ThreadLocal中取出当前线程的读锁个数
                //这个地方在ThreadLocal中保存了每个线程持有的读锁数
                //量，因为读锁是可以共享的，每个线程都可以获取读锁并且
                //可以重入，如果不保存的话，就不知道这个线程是否获取了
                //读锁，也不知道它重入了几次
                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;
            }
            //如果读线程需要阻塞，或读锁个数大于最大值，或CAS设置状
            //态失败，进行死循环式设置。这个地方与排它锁不同，排它锁
            //需要进入同步队列，因为排它锁是其他线程长久占有锁，需要 
            //等待唤醒，而这里可能稍微等待即可获取锁，如果需要进入同步
            //队列，会直接返回-1
            return fullTryAcquireShared(current);
        }


        final int fullTryAcquireShared(Thread current) {
            HoldCounter rh = null;
            for (;;) {
                int c = getState();
                //一旦有别的线程获得了写锁，返回-1，失败
                if (exclusiveCount(c) != 0) {
                    if (getExclusiveOwnerThread() != current)
                        return -1;
                }
                //如果读线程需要阻塞
                else if (readerShouldBlock()) {
                    if (firstReader == current) {
                    }
                    //说明有别的读线程占有了锁
                    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;
                    }
                }
                //如果读锁达到了最大值，抛出异常
                if (sharedCount(c) == MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                //如果CAS设置状态成功
                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;
                }
            }
        }
    }

WriteLock.lock()和unlock()

 public static class WriteLock implements Lock, java.io.Serializable {

    public void lock() {
            //独占式获取状态1
            sync.acquire(1);
        }
        
    public void unlock() {
            //独占式释放状态1
            sync.release(1);
        }
}

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    
    public final void acquire(int arg) {
        //如果尝试获取状态1不成功，加入同步队列，然后自旋
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
    
    
    public final boolean release(int arg) {
        //尝试释放状态1成功，唤醒后继节点
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
}

abstract static class Sync extends AbstractQueuedSynchronizer {

        //获取读锁数量，高16位为读锁
        static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }
        //获取写锁数量，低16位为写锁
        static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
        static final class ThreadLocalHoldCounter
            extends ThreadLocal<HoldCounter> {
            public HoldCounter initialValue() {
                return new HoldCounter();
            }
        }


        protected final boolean tryRelease(int releases) {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            //状态变更
            int nextc = getState() - releases;
            //如果写锁数量变为0，则其他线程可获取锁；如果不为0，表示
            //当前线程重入了几次，未释放完，其他线程不可获取锁
            boolean free = exclusiveCount(nextc) == 0;
            if (free)
                setExclusiveOwnerThread(null);
            setState(nextc);
            return free;
        }

        protected final boolean tryAcquire(int acquires) {
            Thread current = Thread.currentThread();
            //获取锁数量
            int c = getState();
            //获取写锁数量
            int w = exclusiveCount(c);
            //如果锁数量不为0
            if (c != 0) {
                //如果写锁为0--表示读锁不为0，那么任何线程(包括当前)
                //线程，都不可以获取写锁，返回-1
                //如果写锁不为0且独占线程不是当前线程--表示其他线程
                //获取了写锁，返回-1
                if (w == 0 || current != getExclusiveOwnerThread())
                    return false;
                //如果写锁数量超过最大值，报错
                if (w + exclusiveCount(acquires) > MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                //如果锁数量不为0且写锁数量不为0且独占线程是当前线程
                //，则表示是当前线程重入，不用CAS方式设置状态
                setState(c + acquires);
                return true;
            }
            if (writerShouldBlock() ||
                !compareAndSetState(c, c + acquires))
                return false;
            setExclusiveOwnerThread(current);
            return true;
        }

        final int getReadHoldCount() {
            if (getReadLockCount() == 0)
                return 0;

            Thread current = Thread.currentThread();
            if (firstReader == current)
                return firstReaderHoldCount;

            HoldCounter rh = cachedHoldCounter;
            if (rh != null && rh.tid == getThreadId(current))
                return rh.count;

            int count = readHolds.get().count;
            if (count == 0) readHolds.remove();
            return count;
        }
    }

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