web/src/main/java/org/springframework/security/web/util/UrlUtils.java
public static boolean isAbsoluteUrl(String url) {
		if (url == null) {
			return false;
		}
		final Pattern ABSOLUTE_URL = Pattern.compile("\\A[a-z0-9.+-]+://.*",
				Pattern.CASE_INSENSITIVE);

		return ABSOLUTE_URL.matcher(url).matches();
	}
public static boolean isAbsoluteUrl(String url) {
		if (url == null) {
			return false;
		}
		final Pattern ABSOLUTE_URL = Pattern.compile("\\A[a-z0-9.+-]+://.*",
				Pattern.CASE_INSENSITIVE);

		return ABSOLUTE_URL.matcher(url).matches();
	}
Decides if a URL is absolute based on whether it contains a valid scheme name, as
defined in RFC 1738.