缓存系统设计

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public class CacheSystemTest {

    public static void main(String[] args) {
        MyCache cache = new MyCache();
        ExecutorService threadpool = Executors.newCachedThreadPool();
        
        for(int i =0;i<10;i++) {
           Thread t = new Thread(new Runnable() {
                @Override
                public void run() {
                   cache.getData("testKey");
                }
            });
           threadpool.execute(t);
        }
        
        threadpool.shutdown();
    }
}

class MyCache {
    private Map<String,Object> cacheStore = new HashMap<String, Object>();
    private ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
    
    public Object getData(String key) {
        Object data = null;
        //获取读锁，开始在缓存中查找我们想要的data
        readWriteLock.readLock().lock();
        try {
            data = cacheStore.get(key);
            if(data==null) {//缓存中并未找到们想要的数据
                readWriteLock.readLock().unlock(); //释放读锁
                readWriteLock.writeLock().lock(); //开始查询数据库并且数据库中查询的数据写入缓存，获取写锁 
                try {
                    if(data==null) {
                        //query the data from the DB
                        data = "data query from db";
                        cacheStore.put(key, data);
                    }
                }finally {
                    //数据库中数据写入缓存成功，释放写锁
                    readWriteLock.writeLock().unlock();
                    //并且此时需要重新获取读锁，否则下方finally里面释放读锁报错
                    readWriteLock.readLock().lock();
                }
            }
        }finally {
            //缓存中就找到我们想要的数据，读取成功，释放读锁
            readWriteLock.readLock().unlock();
        }
        
        
        readWriteLock.writeLock().lock();
        System.out.println(Thread.currentThread().getName() + " obtain the data from cache system: " + (String)data);
        readWriteLock.writeLock().unlock();
        return data;
    }
}