The following 'local file' test exists in multiple places in the JDK and can be consolidated in a shared utility method in sun.net.www.ParseUtil:

String host = url.getHost();
if (host == null || host.isEmpty() || host.equals("~") ||
        host.equalsIgnoreCase("localhost"))
    return true;

Also, we should refer to RFC 8089, Section 2 here for the definition of "local file" in a file: URL