5.页面优化技术（缓存）以及超卖问题

静态 ？jsp动态？静态可以缓存在客户端，通过ajax从服务端拉数据？

目前系统最大的瓶颈就在数据库访问。因此，系统优化的方案核心在于减少数据库的访问，而缓存就是一个好方法。

1.页面缓存

一、页面缓存

以商品列表为例，Controller方法改造如下

    @RequestMapping(value="/to_list", produces="text/html")
    @ResponseBody
    public String list(HttpServletRequest request, HttpServletResponse response, Model model,MiaoshaUser user) {
    	model.addAttribute("user", user);
    	//取缓存
    	String html = redisService.get(GoodsKey.getGoodsList, "", String.class);
    	if(!StringUtils.isEmpty(html)) {
    		return html;
    	}
    	List<GoodsVo> goodsList = goodsService.listGoodsVo();
    	model.addAttribute("goodsList", goodsList);
//    	 return "goods_list";
    	SpringWebContext ctx = new SpringWebContext(request,response,
    			request.getServletContext(),request.getLocale(), model.asMap(), applicationContext );
    	//手动渲染
    	html = thymeleafViewResolver.getTemplateEngine().process("goods_list", ctx);
    	if(!StringUtils.isEmpty(html)) {
    		redisService.set(GoodsKey.getGoodsList, "", html);
    	}
    	return html;
    }

二、URL缓存

跟页面缓存原理一样，只是根据不同的url参数从缓存中获取不同的页面数据

以查看商品详情的方法为例，Controller方法改造如下

    @RequestMapping(value="/to_detail2/{goodsId}",produces="text/html")
    @ResponseBody
    public String detail2(HttpServletRequest request, HttpServletResponse response, Model model,MiaoshaUser user,
    		@PathVariable("goodsId")long goodsId) {
    	model.addAttribute("user", user);
    	
    	//取缓存
    	String html = redisService.get(GoodsKey.getGoodsDetail, ""+goodsId, String.class);
    	if(!StringUtils.isEmpty(html)) {
    		return html;
    	}
    	//手动渲染
    	GoodsVo goods = goodsService.getGoodsVoByGoodsId(goodsId);
    	model.addAttribute("goods", goods);
    	
    	long startAt = goods.getStartDate().getTime();
    	long endAt = goods.getEndDate().getTime();
    	long now = System.currentTimeMillis();
    	
    	int miaoshaStatus = 0;
    	int remainSeconds = 0;
    	if(now < startAt ) {//秒杀还没开始，倒计时
    		miaoshaStatus = 0;
    		remainSeconds = (int)((startAt - now )/1000);
    	}else  if(now > endAt){//秒杀已经结束
    		miaoshaStatus = 2;
    		remainSeconds = -1;
    	}else {//秒杀进行中
    		miaoshaStatus = 1;
    		remainSeconds = 0;
    	}
    	model.addAttribute("miaoshaStatus", miaoshaStatus);
    	model.addAttribute("remainSeconds", remainSeconds);
//        return "goods_detail";
    	
    	SpringWebContext ctx = new SpringWebContext(request,response,
    			request.getServletContext(),request.getLocale(), model.asMap(), applicationContext );
    	html = thymeleafViewResolver.getTemplateEngine().process("goods_detail", ctx);
    	if(!StringUtils.isEmpty(html)) {
    		redisService.set(GoodsKey.getGoodsDetail, ""+goodsId, html);
    	}
    	return html;
    }

三、对象缓存

对象缓存控制粒度比页面缓存细，但要注意对象变更时缓存值的处理

MiaoshaUserService方法修改如下

public MiaoshaUser getById(long id) {
		//取缓存
		MiaoshaUser user = redisService.get(MiaoshaUserKey.getById, ""+id, MiaoshaUser.class);
		if(user != null) {
			return user;
		}
		//取数据库
		user = miaoshaUserDao.getById(id);
		if(user != null) {
			redisService.set(MiaoshaUserKey.getById, ""+id, user);
		}
		return user;
	}
	

public boolean updatePassword(String token, long id, String formPass) {
		//取user
		MiaoshaUser user = getById(id);
		if(user == null) {
			throw new GlobalException(CodeMsg.MOBILE_NOT_EXIST);
		}
		//更新数据库
		MiaoshaUser toBeUpdate = new MiaoshaUser();
		toBeUpdate.setId(id);
		toBeUpdate.setPassword(MD5Util.formPassToDBPass(formPass, user.getSalt()));
		miaoshaUserDao.update(toBeUpdate);
		//处理缓存
		redisService.delete(MiaoshaUserKey.getById, ""+id);
		user.setPassword(toBeUpdate.getPassword());
		redisService.set(MiaoshaUserKey.token, token, user);
		return true;
	}

*静态化（不做）

三、解决超卖

    @RequestMapping(value="/do_miaosha", method=RequestMethod.POST)
    @ResponseBody
    public Result<OrderInfo> miaosha(Model model,MiaoshaUser user,
    		@RequestParam("goodsId")long goodsId) {
    	model.addAttribute("user", user);
    	if(user == null) {
    		return Result.error(CodeMsg.SESSION_ERROR);
    	}
    	//判断库存
    	GoodsVo goods = goodsService.getGoodsVoByGoodsId(goodsId);//10个商品，req1 req2
    	int stock = goods.getStockCount();
    	if(stock <= 0) {
    		return Result.error(CodeMsg.MIAO_SHA_OVER);
    	}
    	//判断是否已经秒杀到了
    	MiaoshaOrder order = orderService.getMiaoshaOrderByUserIdGoodsId(user.getId(), goodsId);
    	if(order != null) {
    		return Result.error(CodeMsg.REPEATE_MIAOSHA);
    	}
    	//减库存 下订单 写入秒杀订单
    	OrderInfo orderInfo = miaoshaService.miaosha(user, goods);
        return Result.success(orderInfo);
    }

通过数据库的方式

1.SQL语句限制

GoodsDao 减库存方法更改如下（防止多个用户同时秒杀到最后一件商品，出现负）

@Update("update miaosha_goods set stock_count = stock_count - 1 
where goods_id = #{goodsId} and stock_count > 0")

public int reduceStock(MiaoshaGoods g);

2.建立唯一索引（防止一个用户发送多个请求，秒杀到多件同类商品）

给用户建立唯一索引，如果插入几个相同用户，则订单创建失败

	@Transactional
	public OrderInfo createOrder(MiaoshaUser user, GoodsVo goods) {
		OrderInfo orderInfo = new OrderInfo();
		orderInfo.setCreateDate(new Date());
		orderInfo.setDeliveryAddrId(0L);
		orderInfo.setGoodsCount(1);
		orderInfo.setGoodsId(goods.getId());
		orderInfo.setGoodsName(goods.getGoodsName());
		orderInfo.setGoodsPrice(goods.getMiaoshaPrice());
		orderInfo.setOrderChannel(1);
		orderInfo.setStatus(0);
		orderInfo.setUserId(user.getId());
		long orderId = orderDao.insert(orderInfo);
		MiaoshaOrder miaoshaOrder = new MiaoshaOrder();
		miaoshaOrder.setGoodsId(goods.getId());
		miaoshaOrder.setOrderId(orderId);
		miaoshaOrder.setUserId(user.getId());
		orderDao.insertMiaoshaOrder(miaoshaOrder);
		
		//将order存放到redis中
		redisService.set(OrderKey.getMiaoshaOrderByUidGid, ""+user.getId()+"_"+goods.getId(), miaoshaOrder);
		 
		return orderInfo;
	}

	public MiaoshaOrder getMiaoshaOrderByUserIdGoodsId(long userId, long goodsId) {
		//return orderDao.getMiaoshaOrderByUserIdGoodsId(userId, goodsId);
		return redisService.get(OrderKey.getMiaoshaOrderByUidGid, ""+userId+"_"+goodsId, MiaoshaOrder.class);
	}

前后性能差异

/**
 * QPS:1267 load:15 mysql
 * 5000 * 10
 * QPS:2884, load:5 
 * */