Java多线程--读写锁

读-读不互斥：读读之间不阻塞
读-写互斥：读阻塞写，写也会阻塞读。
写-写互斥：写写阻塞。
jdk源中对ReentrantReadWriteLock提供了例子

public class CachedData {
    Object data;
    volatile boolean cacheValid;
    final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

    void processCachedData() {
        rwl.readLock().lock();
        if (!cacheValid) {
            // Must release read lock before acquiring write lock
            rwl.readLock().unlock();
            rwl.writeLock().lock();
            try {
                // Recheck state because another thread might have
                // acquired write lock and changed state before we did.
                if (!cacheValid) {
                    // data = ...
                    cacheValid = true;
                }
                // Downgrade by acquiring read lock before releasing write lock
                rwl.readLock().lock();
            } finally {
                rwl.writeLock().unlock(); // Unlock write, still hold read
            }
        }

        try {
            // use(data);
        } finally {
            rwl.readLock().unlock();
        }
    }
}

class RWDictionary {
    private final TreeMap<String, Data> m = new TreeMap<String, Data>();
    private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
    private final Lock r = rwl.readLock();
    private final Lock w = rwl.writeLock();

    public Data get(String key) {
        r.lock();
        try {
            return m.get(key);
        } finally {
            r.unlock();
        }
    }

    public Object[] allKeys() {
        r.lock();
        try {
            return m.keySet().toArray();
        } finally {
            r.unlock();
        }
    }

    public Data put(String key, Data value) {
        w.lock();
        try {
            return m.put(key, value);
        } finally {
            w.unlock();
        }
    }

    public void clear() {
        w.lock();
        try {
            m.clear();
        } finally {
            w.unlock();
        }
    }
}

未完待续。。。