A project we're working on has recently ran into a need to access the HTTP request headers in order they were received. This does not seem currently possible as HeaderMap is just a hash table.

One way I see to get this functionality would be to make HeaderMap non-final and provide a mechanism for configuring the class used for request headers, but I don't particularly like this as the HeaderMap API is fairly wide and some of the methods are semantically tricky for this particular purpose.

Obviously we'd prefer a solution where the modifications to Undertow itself are acceptable for upstreaming so we don't have to maintain a custom fork of it. Any suggestions would be appreciated.