java.net.URI.create("localhost:8080/foo") // Works
java.net.URI.create("127.0.0.1:8080/foo") // Throws exception
java.net.URI.create("//127.0.0.1:8080/foo") // Works
Is double slash required for when you have the host as an IP Address? I glanced through the RFC for URI - https://www.rfc-editor.org/rfc/rfc3986. But could not find anything pertaining to this.
java.net.URI.create uses the syntax described in RFC 2396.
java.net.URI.create("localhost:8080/foo")
This doesn't produce an exception, but the URI is parsed in a way which you probably don't expect. Its scheme (not host!) is set to localhost, and the 8080/foo isn't port + path, but a scheme-specific part. So this doesn't really work.
java.net.URI.create("//localhost:8080/foo")
parses the URL without scheme, as a net_path grammar element (see RFC 2396 for details).
Here's the relevant grammar excerpt from the RFC 2396:
URI-reference = [ absoluteURI | relativeURI ] [ "#" fragment ]
// This is how 'localhost:8080/foo' is parsed:
absoluteURI = scheme ":" ( hier_part | opaque_part )
relativeURI = ( net_path | abs_path | rel_path ) [ "?" query ]
...
// This is how '//127.0.0.1:8080/foo' is parsed:
net_path = "//" authority [ abs_path ]
...
// Scheme must start with a letter,
// hence 'localhost' is parsed as a scheme, but '127' isn't:
scheme = alpha *( alpha | digit | "+" | "-" | "." )
One proper way would be:
java.net.URI.create("http://localhost:8080/foo")