struts2+spring：validation.xml校验不显示fielderror?

struts2+spring：validation.xml校验不显示fielderror?但是能正确阻止错误数据。
 他咋就不显示错误信息呢？比如一个注册用户的ActionStruts.xml <action name="RegisterUser" class="com.action.UserAction"
method="registerUser">
<result name="success" type="redirect">/index.jsp</result>
<result name="input" type="redirect">
/user/register.jsp
</result>
</action>
Action类 package com.action; public String registerUser()
{
//省去是注册用户的实现
}
注册页面 register.jsp <s:form action="RegisterUser" >
<table border="1">
<tr>
<td>
用户名*
</td>
<td>
<s:textfield name="username"></s:textfield>
</td>
</tr>
<tr>
<td>
<s:submit value="同意服务条款并确认注册"></s:submit>
</td>
</tr>
</table>
</s:form>UserAction-RegisterUser-validation.xml<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE validators PUBLIC "-//OpenSymphony Group//XWork Validator 1.0.2//EN"
"http://www.opensymphony.com/xwork/xwork-validator-1.0.2.dtd"><validators> <field name="username">
<field-validator type="requiredstring">
<param name="trim">true</param>
<message>username不允许为空</message>
</field-validator> <field-validator type="stringlength">
<param name="minLength">5</param>
<param name="maxLength">16</param>
<message>length of username should between ${minLength} to ${maxLength}</message>
</field-validator>
</field></validators>