Clarify behaviour for container managed HTTP headers

[markt-asf]

Clarify behaviour for container managed HTTP headers

[gregw]

They key question I think we should answer, is how and when are server generated response fields visible to the application?

The Date header is a MUST send in RFC9110, so the server MUST send it (java has a clock). So should the application be able to see that header in the response? should the app be able to modify or delete that header? Jetty has it visible, allows it to be modified, but not deleted.

Jetty treats the Server header field the same way, but it is less important as the protocol is still correct without it.

The Content-Length and Content-Encoding header fields are typically set by the server when the response is committed. Should they be visible to the application after a flush or are they private to how the server actually sent the data. Jetty makes these visible to the app after a commit, but I don't see there is actually reason or need to do so. So perhaps we should say they may be visible.

Finally, there are partial field values derived from methods like setLocale and setCharacterEncoding. Should these be visible in the response headers immediately after these methods are called, or only after setContentType is called or the response is committed or never?

If we don't want definitive answers to any of these questions, then verbage saying result may differ would be good.

cheers