学习多线程2---缓存模拟

package com.test;

import java.util.HashMap;
import java.util.Map;

public class CachDemo {
    private Map<String ,Object> cache = new HashMap<String,Object>();
    public Object getData(String key){
		Object value = cache.get(key);
		
		if(value == null){
			value = "aaaa";//去数据库查
		}
		
		return value;
    	
    }
}

package com.test;

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>();
	public static void main(String[] args) {
		// TODO Auto-generated method stub

	}

	private ReadWriteLock rwl = new ReentrantReadWriteLock();
	public  Object getData(String key){
		rwl.readLock().lock();
		Object value = null;
		try{
			value = cache.get(key);
			if(value == null){
				rwl.readLock().unlock();
				rwl.writeLock().lock();
				try{
					//检测堵塞在writeLock后唤醒继续执行的线程
			        //在read锁释放，write锁加上的瞬间有可能有线程把value付了值
					if(value==null){
						value = "aaaa";//实际是去queryDB();
					}
				}finally{
					rwl.writeLock().unlock();
				}
				rwl.readLock().lock();
			}
		}finally{
			rwl.readLock().unlock();
		}
		return value;
	}
}