我找到了答案。我应该更改struts配置如下：

I've found the answer. I should change the struts configuration as following:

<struts>



    <constant name="struts.action.extension" value=""/>



    <package name="default" extends="struts-default">

        <action name="register">
            <result type="tiles">register</result>
        </action>
    </package>


</struts>