Java读写锁ReadWriteLock

1.多个读锁不互斥，读锁与写锁互斥，写锁与写锁互斥

package com.ronbay.thread.timer;

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

/**
 * @author Ronbay
 * 一个面试题  缓存系统
 *
 */
public class CacheDemo {

	private Map<String,Object> cache = new HashMap<String,Object>();
	
	public static void main(String[] args) {
		String name = (String)new CacheDemo().getData("key");
		System.out.println("name=" + name);
	}

	ReadWriteLock rwLock = new ReentrantReadWriteLock();
	public  Object getData(String key){
		Object value = null;
		try {
			rwLock.readLock().lock();
			//若缓存有数据直接返回
			value  = cache.get(key);
			//若缓存没有再查数据库，再返回
			if(value == null){
				rwLock.readLock().unlock();
				rwLock.writeLock().lock();
				try {
					if(value == null){
						value="aaa";  //实际是去queryDB()
					}
					
				} catch (Exception e) {
					e.printStackTrace();
				}finally{
					rwLock.writeLock().unlock();
				}
				//再加上读锁
				rwLock.readLock().lock();
			}
		} catch (Exception e) {
			e.printStackTrace();
		}finally{
			rwLock.readLock().unlock();
		}
		return value;
		
	}
}