struts2 json传递对象

今天在用struts2 异步请求从后台传一个对象到前台时遇到了一个小小的问题，现在此作一个标记，把主要的代码贴上以作备忘。

struts.xml

<action name="getSystemContactorInfo" class="userAction" method="getSystemContactorInfo">
            <result type="json">
                <param name="includeProperties">systemContactor\.userName,systemContactor\.cellPhone,
                    systemContactor\.notesMail
                param>


            result>
        action>

UserAction

public String getSystemContactorInfo() {
        UserModel systemContactor = new UserModel();
        systemContactor.setUserName("张三");
        systemContactor.setCellPhone("13240151465");
        systemContactor.setNotesMail("[email protected]");
        return Action.SUCCESS;
     }

UserModel

public class UserModel{
   private String userName;
   private String cellPhone;
   private String notesMail;
//省略get、set方法
}

jsp页面中

$.ajax({
            type:'post',
            url: 'getSystemContactorInfo.action',
            dataType: 'json',
            async: true,
            success: function showContent(json) {
                var userInfo = json.systemContactor;
                $("#userName").html(userInfo.userName);
                $("#cellPhone").html(userInfo.cellPhone);
                $("#notesMail").html(userInfo.notesMail);
            }
        });

systemContactor 当systemContactor为一个字串或数字时可以这样写
systemContactor.*当systemContactor为list时可这样传
到现在也不是太明白当systemContactor为一个对象时为什么不能直接写在里面