From: Ron Sigal <rsigal_at_redhat.com>
Date: Thu, 05 Dec 2013 13:13:17 -0400

We've had a lengthy discussion about media type negotiation on
https://issues.jboss.org/browse/RESTEASY-994, and I have a question.

The discussion started with the observation by a user, Erik Mattheis,
that, given

   Accept: application/json, */*

the result returned by

    @GET
    @Path("automatic")
    @Produces({ APPLICATION_JSON, APPLICATION_XML })
    public Holder automatic()
    {
       return ...
    }

is in JSON format, but

    @GET
    @Path("variant")
    @Produces({ APPLICATION_JSON, APPLICATION_XML })
    public Holder variant()
    {
       List<Variant> XML_OR_JSON =
Variant.mediaTypes(APPLICATION_XML_TYPE, APPLICATION_JSON_TYPE).build();
       System.out.println("selected variant: " +
request.selectVariant(XML_OR_JSON));
    }

prints

    selected variant: Variant[mediaType=application/xml, language=null,
encoding=null]

Now, assuming that Request.selectVariant() is governed by
http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1,
according to which "application/json" and "application/xml" both have an
implicit quality factor of 1.0, and given that
http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1 has no
discussion of the breaking of ties, I believe that Resteasy isn't
violating the spec when Request.selectVariant() chooses
"application/xml". On the other hand, it seems REASONABLE to select
"application/json" instead. Does that make sense?

Pursuing the matter further, I have a concern about the algorithm given
in Section 3.8 "Determining the media type of responses". Given

Accept: application/*;q=1.0, application/json;q=0.5

and

    @Produces({"application/json","application/xml" })

in Step 6 we get

   M = { S("application/*;q=1.0","application/json"), S("application/*;q=1.0","application/xml"), S("application/json;q=0.5', "application/json") }
     = {"application/json;q=1.0","application/xml;q=1.0","application/json;q=0.5" }

which can be ordered

   "application/json;q=1.0" >="application/xml;q=1.0" >="application/json;q=0.5"

or

   "application/xml;q=1.0" >="application/json;q=1.0" >="application/json;q=0.5"

so that either "application/json;q=1.0" or "application/xml;q=1.0" could
be returned legally. That seems to be in contradiction to the mandate
in http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.1 to use
quality factors to select a media type. In particular,
"application.json" should have a quality factory of 0.5.

Any comments?

Thanks,
Ron