struts对象绑定属性被覆盖，初学struts用法问题?

描述:action中有user对象，这个类中有add()和update() ，但是在add(){

    user =  new User();

user.setUserName("某个");

    //操纵属性

}

update(){

    user.getUserName();

}

这样加入多线程情况下(用户提交表单到update , 但是add被人先执行了，那么user被清空。属性被覆盖)，update中的getUsername就不是前台绑定并传递过来的，而是add中set的【属性丢了】

不知道我描述的清除吗

附上action一点代码

package com.hzyc.hzycpos.action;


import java.util.ArrayList;
import java.util.List;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.struts2.interceptor.ServletRequestAware;
import org.apache.struts2.interceptor.ServletResponseAware;
import org.springframework.stereotype.Controller;

import com.hzyc.hzycpos.domain.User;
import com.opensymphony.xwork2.ActionSupport;
import com.opensymphony.xwork2.ModelDriven;

/**
 * 用户管理控制层
 * 通过实现自动引入接口实现IOC方式的request注入
 * 
 * @author SHAOSHUAI
 * 使用ModelDriven<User>驱动方式，对于传递多个对象不方便
 * 改使用set对象方法传递
 */
@Controller
public class UserAction extends ActionSupport implements ServletRequestAware, ServletResponseAware {

    //model绑定
    private User user;
    //List<User> User对象集合
    private List<User> userList;
    

    private HttpServletRequest request;
    private HttpServletResponse response;
    
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    
    private String info = "";
    public String add() throws Exception{
        info = "添加用户信息";
        
        //如何能获取input中的值呢
        System.out.println(user.getUsername());
        request.setAttribute("name","是谁呢");
        
        user = new User();
        user.setPassword("123456");
        user.setUsername("邵帅");
        userList = new ArrayList<User>();
        userList.add(user);
        return SUCCESS;
    }
    
    public String update() throws Exception{
        info = "更新用户信息";
        System.out.println(user.getUsername()+"====update");
        return ERROR;
    }
    public String getInfo(){
        return info;
    }
    public void setInfo(String info){
        this.info = info;
    }
    public void setServletResponse(HttpServletResponse response) {
        this.response = response;
    }
    public void setServletRequest(HttpServletRequest request) {
        // TODO Auto-generated method stub
        this.request = request;
    }

    /**
     *  获取参数绑定model
     */
    
    public void setUser(User user){
        System.out.println("set User对象");
        this.user = user;
    }
    /**
     * 用于jsp的ognl标签获取
     * */
    public User getUser(){
        System.out.println("get User对象");
        return this.user;
    }
    /**
     * 用于jsp标签获取
     * */
    public List<User> getUserList() {
        return userList;
    }
    

    
}