Add IPv6 support in RestTemplate

[bclozel]

Prior to this commit, RestTemplate would not would
not accept IPv6 raw addresses in URLs because UriComponentsBuilder
would not parse/encode the Host part correctly.

The UriComponentsBuilder now parses and encode raw IPv6 addresses
in the "[1abc:2abc:3abc::5ABC:6abc]" format and also supports the
use of IPv6 scope_ids (see JDK8 java.net.Inet6Address),
like "[1abc:2abc:3abc::5ABC:6abc%eth0]".

Issue: SPR-10539

[rstoyanchev]

This looks very good overall. A couple of things I noticed while experimenting.

If an invalid IPv6 format is used (e.g. omit the closing ] or use non-alphanumeric char) the result is a UriComponents where the host is tacked on to the path, so it looks like it got parsed but the path is invalid. Since the IPv6 format is quite specific we should do something to guard against such cases. Maybe check if there is a scheme but no host, or if the path starts with [ could be another giveaway?

HierarchicalUriComponents.Type.HOST now allows [ ] : for any host value. Since HierarchicalUriComponents can only be created through UriComponentsBuilder, in theory a regular host should never contain [ : but it could end up containing ] (e.g. this is now allowed http://example.com] even if highly unlikely). Perhaps what we need is an additional HOST_IPV6 enum that would be used instead when the host is an IPv6 address?

[bclozel]

Good catch. I'll modify my PR and add more tests on edge cases.
There are also IPv6 compatible IPv4 addresses like ::192.168.1.1.

[bclozel]

I've changed a couple of things in this PR.

I've splitted the host regexp in 2; the match process is getting more complex and I wonder if we should use named groups to make it more readable/manageable.

On the first point, I've added a "scheme but no host" check in the fromHttpUrl method, because some URIs (emails) do verify that condition but are still valid.

I agree, IPv4 and IPv6 are different enough to create a separate HOST_IPV6 type.

I'm not really happy with the condition check to decide on wich HierarchicalUriComponents.Type we should use (checking if the host part starts with [); IMHO, the only alternative would be to use named groups in the regexp and some kind of flag saying if the host is a IPv6 address or not.

[rstoyanchev]

This is now merged.

I'm not really happy with the condition check to decide on wich HierarchicalUriComponents.Type we should use (checking if the host part starts with [); IMHO, the only alternative would be to use named groups in the regexp and some kind of flag saying if the host is a IPv6 address or not.

There are few ways to make it a little nicer but since it is a private matter within HierarchicalUriComponents, the current solution is fine. I did a little polish, just to cover the call to verify(), but the approach is the same.