Java如何实现五分钟内重复获取返回同一个短信验证码

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		try {
			//发送手机号码
			String mobile = request.getParameter("mobile");
			
			//6位验证码
			String verifyCode = null;
			
			HttpSession session = request.getSession();
			JSONObject json = (JSONObject)session.getAttribute("json");
			if(json == null){
				json = new JSONObject();
				json.put("mobile", mobile);
				verifyCode = String.valueOf(new Random().nextInt(899999) + 100000);
				json.put("verifyCode", verifyCode);
				json.put("createTime", System.currentTimeMillis());
				session.setAttribute("json", json);
			}else{
				verifyCode = json.getString("verifyCode");
				long createTime = json.getLong("createTime");
				if((System.currentTimeMillis() - createTime) > 1000 * 60 * 1){//超过5分钟重新生成验证码
					verifyCode = String.valueOf(new Random().nextInt(899999) + 100000);
					json.put("verifyCode", verifyCode);
					json.put("createTime", System.currentTimeMillis());
				}
			}
			//发送短信
			//使用的是榛子云短信(http://smsow.zhenzikj.com/doc/sdk.html)
			ZhenziSmsClient client = new ZhenziSmsClient("https://sms_developer.zhenzikj.com", "appId", "appSecret");
			String result = client.send(mobile, "您的验证码为:" + verifyCode + "，该码有效期为5分钟，该码只能使用一次!");
			System.out.println(verifyCode);
			
			response.getWriter().append("success");
		} catch (Exception e) {
			e.printStackTrace();
			response.getWriter().append("fail");
		}
	}