code: public static String generate(Integer length) {
          StringBuilder sb = new StringBuilder();
          for (int i = 0; i < length; i++) {
              int number = random.nextInt(seed.length());
              sb.append(seed.charAt(number));
          }
          return sb.toString();
      }
docs: 生成随机字符串
  @param length 长度
  @return 长度为length的字符串