code: public static String captchaNumber(int length) {
          StringBuilder sb   = new StringBuilder();
          Random        rand = new Random();
          for (int i = 0; i < length; i++) {
              sb.append(rand.nextInt(10));
          }
          return sb.toString();
      }
docs: 返回指定长度随机数字组成的字符串
  
  @param length 指定长度
  @return 随机字符串