Struts2 验证框架 xml配置验证两个字段相等

<validators>
<field name="password"
<field-validator type="requiredString">
<param name="trim">true</param>
<message>不能为空</message>
</field-validator>
</field>
<field name="againpassword"
<field-validator type="fieldexpression">
<param name="expression"><![CDATA[againpassword==password]]></param>
<message>My credit limit should be MORE than my girlfriend</message>
</field-validator>
</field>
</vaidators>