请教：关于struts2.0 服务器端验证Validator

    public ActionErrors validate(ActionMapping mapping,HttpServletRequest request){        ActionErrors errors=new ActionErrors();        if(username.equals("")||username==null){            errors.add("username",new ActionMessage("username"));        }        if(password.equals("")||password==null){            errors.add("password",new ActionMessage("password"));        }        return errors;    }
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    @Override
    public void validate()  {
       System.out.println( " Calling validate() " );
        if ( ! (loc.equals(Locale.US) || loc.equals(Locale.CHINA)))  {
                   addFieldError( " loc " , getText( " validation.loc " ));
       }  
   }