package com.cn.gbx;

import java.util.HashMap;

import java.util.Map;

import java.util.concurrent.locks.ReadWriteLock;

import java.util.concurrent.locks.ReentrantReadWriteLock;

public class CacheDesign {

	private Map<String, Object> cache = new HashMap<String, Object>();

	//对象锁的设计

//	public synchronized Object getData(String key){

//		Object value = null;

//		value = cache.get(key);

//		if (value == null) {

//			value = "queryDao";

//			cache.put(key, value);

//		}

//		return value;

//	}

	//可重入锁的设计

	static ReadWriteLock rwl = new ReentrantReadWriteLock();

	public synchronized Object getData(String key){

		Object value = null;

		rwl.readLock().lock();

		try {

			value = cache.get(key);

			if (value == null) {

				rwl.readLock().unlock();

				rwl.writeLock().lock();

				try{

					if (value == null) {  //确保后边的线程不会重读写

						value = "queryDao";

						cache.put(key, value);

					}

				}finally{

					rwl.writeLock().unlock();

				}

				rwl.readLock().lock();

			}

		}finally{

			rwl.readLock().unlock();

		}

		return value;

	}

	public static void main(String[] args) {

	}

}