You could put the validation.xml file into .../WEB-INF/validation.xml,which is the same directory as the struts-config.xml file,
and in the config file, you should:
<plug-in className="org.apache.struts.validator.ValidatorPlugIn">
<set-property property="pathnames"
value="/WEB-INF/validator-rules.xml,
/WEB-INF/validation.xml" />
</plug-in>