code: public static String getStrCode(int length) {
          StringBuffer str = new StringBuffer();
          while (str.length() <= length - 1)
              str.append(strArray[new Random().nextInt(strArray.length)]);
          return str.toString();
      }
docs: 自定义长度的字符串验证码
  @param length 验证码位数长度
  @return 返回生成的字符串验证码