#冲刺创作新星#重入锁与读写锁 原创

protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (!hasQueuedPredecessors() &&
            compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0)
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}
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public class Cache {
    static Map<String, Object> map = new HashMap<String, Object>();
    static ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
    static Lock r = rwl.readLock();
    static Lock w = rwl.writeLock();
    // 获取一个key对应的value
    public static final Object get(String key) {
        r.lock();
        try {
            return map.get(key);
        } finally {
            r.unlock();
        }
    }
    // 设置key对应的value，并返回旧的value
    public static final Object put(String key, Object value) {
        w.lock();
        try {
            return map.put(key, value);
        } finally {
            w.unlock();
        }
    }// 清空所有的内容
    public static final void clear() {
        w.lock();
        try {
            map.clear();
        } finally {
            w.unlock();
        }
    }
}
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protected final boolean tryAcquire(int acquires) {
    /*
     * Walkthrough:
     * 1. If read count nonzero or write count nonzero
     *    and owner is a different thread, fail.
     * 2. If count would saturate, fail. (This can only
     *    happen if count is already nonzero.)
     * 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.
     */
    Thread current = Thread.currentThread();
    int c = getState();
    int w = exclusiveCount(c);
    if (c != 0) {
        // (Note: if c != 0 and w == 0 then shared count != 0)
        //存在读锁或者当前获取线程不是已经获取写锁的线程
        if (w == 0 || current != getExclusiveOwnerThread())
            return false;
        if (w + exclusiveCount(acquires) > MAX_COUNT)
            throw new Error("Maximum lock count exceeded");
        // Reentrant acquire
        setState(c + acquires);
        return true;
    }
    if (writerShouldBlock() ||
        !compareAndSetState(c, c + acquires))
        return false;
    setExclusiveOwnerThread(current);
    return true;
}
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final int fullTryAcquireShared(Thread current) {
            /*
             * 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.
             */
            HoldCounter rh = null;
            for (;;) {
                int c = getState();
                if (exclusiveCount(c) != 0) {
                    if (getExclusiveOwnerThread() != current)
                        return -1;
                    // else we hold the exclusive lock; blocking here
                    // would cause deadlock.
                } else if (readerShouldBlock()) {
                    // Make sure we're not acquiring read lock reentrantly
                    if (firstReader == current) {
                        // assert firstReaderHoldCount > 0;
                    } 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;
                    }
                }
                if (sharedCount(c) == MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                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;
                }
            }
        }
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public void processData() {
        readLock.lock();
        if (!update) {
            // 必须先释放读锁
            readLock.unlock();
            // 锁降级从写锁获取到开始
            writeLock.lock();
            try {
                if (!update) {
                    // 准备数据的流程（略）
                    update = true;
                }
                readLock.lock();
            } finally {
                writeLock.unlock();
            }
            // 锁降级完成，写锁降级为读锁
        }
        try {
            // 使用数据的流程（略）
        } finally {
            readLock.unlock();
        }
    }
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