写一个缓存系统的伪代码

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;


public class CatchDemo {
private Map cache = new HashMap();


public static void main(String[] args) {


}
//ReadWriteLock
//读锁与读锁不互斥，读锁与写锁，写所欲写锁互斥


// 线程安全
// 向数据库查询数据时，先查缓存系统，若有，则返回
    //    若没有，则从数据库查询数据写入缓存，再读取
private ReadWriteLock rwl = new ReentrantReadWriteLock();


public /* synchronized */ Object getData(String key) {
rwl.readLock().unlock();
Object obj = null;
try {
obj = cache.get(key);
if (obj == null) {
rwl.readLock().unlock();
rwl.writeLock().lock();
try {
if(obj == null)
obj = "aaa";// 实际上是queryDB
} finally {
rwl.writeLock().unlock();
}
rwl.readLock().lock();
}
} catch (Exception e) {
e.printStackTrace();
} finally {
rwl.readLock().unlock();
}
return obj;
}
}