How to Extract Domain Names from URLs Reliably: Alternative Techniques and Edge Cases

Extracting Domain Names from URLs

This question seeks to extract domain names from URLs effectively. While the provided Java code appears to function reasonably, the author inquires about any superior techniques or potential edge cases that may undermine its reliability.

Alternative Implementation Using java.net.URI

A cleaner approach to parsing URLs is recommended, utilizing the java.net.URI class instead of java.net.URL. The latter has known issues, such as its problematic equals method, which can expose code to vulnerabilities in scenarios involving untrusted inputs:

public static String getDomainName(String url) throws URISyntaxException {
    URI uri = new URI(url);
    String domain = uri.getHost();
    return domain.startsWith("www.") ? domain.substring(4) : domain;
}

This improved method should effectively retrieve domain names while handling edge cases involving incorrect casing or variations in the "www" prefix.

Additional Edge Cases to Consider

The original code fails to account for specific URL formats:

• httpfoo/bar (relative URL with path starting with "http")
• HTTP://example.com (case-insensitive protocol)
• //example.com (protocol-relative URL with host)
• www/foo (relative URL with path starting with "www")
• wwwexample.com (domain starting with "www", but not the "www" prefix)

To address these edge cases, consider referencing Appendix B of RFC 3986, which provides a comprehensive regular expression for parsing URI references and breaking them down into individual components.

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