Struts2 returns json, frontend jsonp and gets it as null?

the front end adopts vue+webpack, listening port 8000
the back end adopts the combination of java (struts2+hibernate+spring), and tomcat listening port 8080

the front end wants to get the json data returned from the backend, but gets it as null? Initially judged as a back-end problem, find a solution
tested with jquery"s ajax method and also obtained as null

the front end displays the jsonp code of the data component:

jsonp: function(url, data={}, callbackName = "callback") {
   return new Promise((resolve, reject) => {
       console.log(url)
       console.log(data)
       console.log(callbackName)
       window.__onGetData__ = data => resolve(data)
       console.log(window.__onGetData__)
       let queryString = Object.keys(data).map(k => `${k}=${data[k]}`).join(`&`)
       console.log(queryString)
       let script = document.createElement("script")
       document.head.appendChild(script)
       script.src = `${url}?${callbackName}=__onGetData__`
       script.addEventListener("error", ()=>{ reject("load error")})
       document.head.removeChild(script)
  })
},
  showData: function(){
       this.jsonp("http://localhost:8080/QAsys/user_getAll")
       .then(data =>{
       console.log(data)
          tableData = data;
       }).catch(err => {
          console.log(err)
       })
  }

returns the action code of json:

public class UserAction extends ActionSupport implements ServletRequestAware, ServletResponseAware {
    private HttpServletRequest request; //request-api
    private HttpServletResponse response;
    private ObjectMapper mapper = new ObjectMapper();//jackson 
    private String result = new String();//
    private Page page = new Page(); //

    public void setServletRequest(HttpServletRequest request) {
        this.request = request;
    }
    public void setServletResponse(HttpServletResponse response){ this.response = response;}
    public void setResult(String result){
        this.result = result;
    }

    @Autowired
    private UserService userService;

    public void setUserService(UserService userService) {
        this.userService = userService;
    }

    public String getAllUser(){
        //
        page.setOffset(0);
        page.setLength(10);

        response.setContentType("text/plain");
        response.setHeader("Pragma", "No-cache");
        response.setHeader("Cache-Control", "no-cache");
        response.setHeader("Access-Control-Allow-Origin", "*");//
        response.setDateHeader("Expires", 0);

        try{
            List<User> list = userService.getAllUser(page.getOffset(), page.getLength());
            page.setMax(list.size());//
            //json
            result = mapper.writeValueAsString(list);
            System.out.println(result);
            PrintWriter out = response.getWriter();
            String jsonpCallback = request.getParameter("callback");//
            out.println(jsonpCallback+"("+result+")");//jsonp
        }catch (Exception e){
            e.printStackTrace();
        }
        return SUCCESS;
    }

struts.xml

<struts>
    <constant name="struts.i18n.encoding" value="UTF-8"></constant>
    <!--<constant name="struts.action.excludePattern" value="../webapp/static/*/.*?" />-->

    <!-- Struts22.5strict-method-invocation()truefalse -->
    <package name="default" extends="struts-default" namespace="/" strict-method-invocation="false">
        <result-types>
            <result-type name="json" class="org.apache.struts2.json.JSONResult"/>
        </result-types>

        <!-- CRUD -->
        <action name="user_*" class="org.ssh.action.UserAction" method="{1}User">
            <result name="success" type="json">
                <param name="root">result</param>
                <param name="callbackParameter">callback</param>
            </result>
        </action>
    </package>

</struts>
Mar.16,2021

turns out that being unable to get data is related to the name of the callback function agreed upon by the front and back end, and the header file information that does not need to be added to response (the specific principle has not been studied)

MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3250c-2be0c.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3250c-2be0c.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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