如何实现秒杀令牌？

Controller

    @RequestMapping(value = "/generatetoken",method = RequestMethod.POST,consumes = CONTENT_TYPE_FORMED)
    @ResponseBody
    public CommonRetureType generateToken()  {
        String killToken = promoService.generateSecondKillToken(itemId, promoId,userModel.getId());

        if (killToken == null){
            throw new BusinessException(EmBusinessError.UNKOWN_ERRORR,"生成秒杀令牌失败");
        }
        return CommonRetureType.create(killToken);
    }

Service

    @Override
    public String generateSecondKillToken(Integer promoId,Integer itemId,Integer userId) {

        //判断是否库存已售罄，若对应的售罄key存在，则直接返回下单失败
        //在下单减库存的过程中，如果库存为0，则redisTemplate.opsForValue().set("promo_item_stock_invalid_"+itemId,"true");
        if(redisTemplate.hasKey("promo_item_stock_invalid_"+itemId)){
            return null;
        }
        
        PromoDO promoDO = promoDOMapper.selectByPrimaryKey(promoId);
        //dataobject->model
        PromoModel promoModel = convertFromDataObject(promoDO);
        if(promoModel == null){
            return null;
        }

        //判断当前时间是否秒杀活动即将开始或正在进行
        if(promoModel.getStartDate().isAfterNow()){
            promoModel.setStatus(1);
        }else if(promoModel.getEndDate().isBeforeNow()){
            promoModel.setStatus(3);
        }else{
            promoModel.setStatus(2);
        }
        //判断活动是否正在进行
        if(promoModel.getStatus().intValue() != 2){
            return null;
        }
        //判断item信息是否存在
        ItemModel itemModel = itemService.getItemByIdInCache(itemId);
        if(itemModel == null){
            return null;
        }
        //判断用户信息是否存在
        UserModel userModel = userService.getUserByIdInCache(userId);
        if(userModel == null){
            return null;
        }

        //获取秒杀大闸的count数量
        //会有一个发布接口，去发布库大闸的数量，一般为秒杀数量的3到5倍
        long result = redisTemplate.opsForValue().increment("promo_door_count_"+promoId,-1);
        if(result < 0){
            return null;
        }
        //生成token并且存入redis内并给一个5分钟的有效期
        String token = UUID.randomUUID().toString().replace("-","");

        redisTemplate.opsForValue().set("promo_token_"+promoId+"_userid_"+userId+"_itemid_"+itemId,token);
        redisTemplate.expire("promo_token_"+promoId+"_userid_"+userId+"_itemid_"+itemId,5, TimeUnit.MINUTES);

        return token;
    }