code: public void setDomain(@Nullable String domain) {
          if (!StringUtils.isEmpty(domain)) {
              this.domain = domain.toLowerCase(Locale.ENGLISH); // IE allegedly needs this.
          } else {
              this.domain = null;
          }
      }
docs: Specifies the domain within which this cookie should be presented.
  
  <p>The form of the domain name is specified by RFC 2109. StandardCookieProcessor domain name begins with a dot
  ({@code .foo.com}) and means that the cookie is visible to servers in a specified Domain Name System (DNS) zone
  (e.g. {@code www.foo.com}, but not {@code a.b.foo.com}). By default, cookies are only returned to the server that
  sent them.
  
  @param domain the domain name within which this cookie is visible; form is according to RFC 2109.
  
  @see #getDomain()