[求助]Struts2+Spring+ibatis 动态更新实现不了，固定更新就可以

Users.xml
<sqlMap>
   <typeAlias alias="user" type="com.feng.pojo.Users" />
 <update id="updateUser" parameterClass="user">
        <![CDATA[
            update users set
             <dynamic prepend=" ">
                 <isNotNull prepend="," property="userpassword" removeFirstPrepend="true">
                  userpassword=#userpassword#
                 </isNotNull>               
                 <isNotNull prepend="," property="userprivileges">
                  userprivileges=#userprivileges#
                 </isNotNull>
             </dynamic>
             where username=#username#
        ]]>
    </update>
</sqlMap>userUpdate.jsp
<%@ page language="java"  pageEncoding="utf-8"%>
<%@ taglib prefix="s" uri="/struts-tags" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>更新</title>
  </head>  <body>    <s:property value="tips"/>
   
    <s:form action="user/updateUser.action">
     <s:textfield name="user.username" label="姓名"/>
     <s:textfield name="user.userpassword" label="新密码"/>
     <s:textfield name="user.userprivileges" label="权限"/>
     <s:submit value="修改" />
    </s:form>
  </body>
</html>
UserAction.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE struts PUBLIC
    "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
    "http://struts.apache.org/dtds/struts-2.0.dtd">
<struts>
 <package name="user" extends="struts-default" namespace="/user">
     <action name="updateUser" class="com.feng.action.UserAction" method="userUpdate">
      <result name="success">/userUpdate.jsp</result> 
      <result name="loginfail">/userUpdate.jsp</result>
     </action>
  </package>  
</struts>
UserAction.java
package com.feng.action;import com.feng.pojo.Users;
import com.feng.services.IsUserServices;public class UserAction { private IsUserServices userServices;
 private Users user;
 private String username;
 private String userpassword;
 private String tips;
 
 public void setUserServices(IsUserServices userServices) {
  this.userServices = userServices;
 }
 //省略user等SET和GET方法
 public String userUpdate()
 {
  String result="loginfail";
  try{
   userServices.updateUser(user);
   this.setTips("修改成功！");
   result="success";
  }catch (Exception e) {
   e.getStackTrace();
   this.setTips("修改失败！");
  }
  return result;
 }