struts2 i18n & ModelDriven

dear all
i have a problem about struts2

i use following example :
Struts 2 – i18n or localization example
to implements i18n
that's works OK

but when i modify the example
i use <s:text name="global.username"/> to get i18n text in properties files in login.jsp...

then use ModelDrive to get username & passwrod
(implements ModelDriven interface & use getModel() method)

but i found a problem
the getModel() method execute many times when user submit form then validate fail, redirect to login.jsp
that's strange..
i don't know why, thanks a lot:)