为什么我这个map没有序列化成json，前台一直返回错误

Question

在后台都能正确输出map，我猜是map没有正确序列化成json，求高人指点

 package com.lhw.action;

import com.lhw.service.news.GetNewsTitleIndexService;
import com.opensymphony.xwork2.ActionSupport;
import org.apache.struts2.json.annotations.JSON;
import org.springframework.stereotype.Controller;

import javax.annotation.Resource;
import java.util.HashMap;
import java.util.Map;

/**
 * Created by lhw on 2016/10/27.
 */
@Controller("getNewsTitle")
public class GetNewsTitleAction extends ActionSupport{

    private Map<String,String> map = new HashMap<>();

    private transient GetNewsTitleIndexService getNewsTitleIndexService;

    @Override
    public String execute() throws Exception {
        //调用service读取数据放到map中
        map = getNewsTitleIndexService.getNewsTitleIndex(0,5);
        System.out.println(map);
        return SUCCESS;
    }

    public GetNewsTitleIndexService getNewsTitleIndexService() {
        return getNewsTitleIndexService;
    }

    @Resource(name = "getNewsTitleIndexService")
    public void setGetNewsTitleIndexService(GetNewsTitleIndexService getNewsTitleIndexService) {
        this.getNewsTitleIndexService = getNewsTitleIndexService;
    }

    public Map<String, String> getMap() {
        return map;
    }

    public void setMap(Map<String, String> map) {
        this.map = map;
    }
}

Answer 1

 <?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE struts PUBLIC
        "-//Apache Software Foundation//DTD Struts Configuration 2.3//EN"
        "http://struts.apache.org/dtds/struts-2.3.dtd">
<struts>
    <package name="Registration" extends="struts-default">

        <action name="user*" class="com.lhw.action.User{1}Action">
            <result name="success">/index.jsp</result>
            <result name="registFail">/regist.jsp</result>
            <result name="loginSuccess">${loginPreUrl}</result>
            <result name="loginFail">/login.jsp</result>
            <result name="logoutSuccess">${preUrl}</result>
        </action>
    </package>

    <constant name="struts.i18n.encoding" value="UTF-8"/>

    <package name="example" extends="json-default">
        <action name="getNewsTitle" class="com.lhw.action.GetNewsTitleAction">
            <!-- 配置类型的json的Result -->
            <result type="json">
                <!-- 为该Result指定参数 -->
                <param name="noCache">true</param>
                <param name="contentType">text/html</param>
                <!-- 设置只序列Action的map属性 -->
                <param name="includeProperties">map</param >
                <param name="root">map</param >
            </result>
        </action>
        <action name="*">
            <result>{1}.jsp</result>
        </action>
    </package>

</struts>