code: protected void initBinder(final HttpServletRequest request, final ServletRequestDataBinder binder) {
          if (serviceValidateConfigurationContext.isRenewEnabled()) {
              binder.setRequiredFields(CasProtocolConstants.PARAMETER_RENEW);
          }
      }
docs: Initialize the binder with the required fields.
  
  @param request the request
  @param binder  the binder