code: public static Constraint url() {
  		return new Constraint("url", simplePayload("url")) {
  			public boolean isValid(Object actualValue) {
  				if (actualValue != null) {
  					if (!Utils.isValidURL(actualValue.toString())) {
  						return false;
  					}
  				}
  				return true;
  			}
  		};
  	}
docs: The 'url' constraint - field value must be a valid URL.
  @return constraint