A Url of the form "http://hostname" is not validated, when the hostname is simple, i.e does not have any dots.
I think this is a valid URL and it should pass validation.
The bug is in UrlValidator::isValidAuthority(), line 382
String topLevel = domainSegment[segmentCount - 1];
if (topLevel.length() < 2 || topLevel.length() > 4)
In this case, topLevel = "hostname"