code: public String index() throws IOException {
      String userName = get("user.name");
      if (Strings.isEmpty(userName)) { return null; }
      Avatar avatar = avatarBase.getAvatar(userName);
      if (null == avatar) {
        avatar = avatarBase.getDefaultAvatar();
      }
      AvatarUtil.copyTo(avatar, ServletActionContext.getResponse());
      return null;
    }
docs: 只显示头像
  
  @throws IOException