读写锁实现缓存系统demo

package uses;

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

public class CacheDemo {

	private Map<String,Object> cache=new HashMap<String,Object>();
	private ReadWriteLock rwl=new ReentrantReadWriteLock();
	public  Object getData(String key)
	{
		rwl.readLock();//如果都是读的话 可以让他进去这段代码，所以上读锁
		Object value=null;
		try {
			value=cache.get(key);
			if(value==null)
			{
				rwl.readLock().unlock();//开始写的话，加写锁，释放读锁
				rwl.writeLock().lock();
				try {
					if(value!=null)
					{
						value="aaaa";//实际是去数据库取数据
					}
				} catch (Exception e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}finally{
				rwl.writeLock().unlock();
				}
				rwl.readLock().lock();
			}
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}finally{
		rwl.readLock().unlock();
		}
		return value; 
	}
	public static void main(String[] args) {
 		 
	}
}