java读写锁是什么

public class CacheDemo {

    private Map<String, Object> cache = new HashMap<>();

    private ReadWriteLock readWriteLock = new ReentrantReadWriteLock();

    public static void main(String[] args) {

    }

    public Object getData(String key) {

        Object value = null;

        //首先开启读锁，从缓存中去取

        readWriteLock.readLock().lock();

        try {

            value = cache.get(key);

            //如果缓存中没有释放读锁，上写锁

            if (value == null) {

                //对应queryDB()

                readWriteLock.readLock().unlock();

                //读锁必须unlock之后才能获取写锁

                readWriteLock.writeLock().lock();

                try {

                    //对应queryDB()

                    value = queryDB();

                } finally {

                    //释放写锁

                    readWriteLock.writeLock().unlock();

                }

                //然后再上读锁

                readWriteLock.readLock().lock();

            }

        } finally {

            //最后释放读锁

            readWriteLock.readLock().unlock();

        }

        return value;

    }

    public Object queryDB() {

        return "aaaa";

    }

}