[jax-rs-spec users] [jsr339-experts] Re: Re: HEADS-UP: Revise MBR selection mechanism?

From: Sergey Beryozkin <>
Date: Thu, 24 May 2012 10:43:34 +0100

On 23/05/12 18:14, Marek Potociar wrote:
> On May 23, 2012, at 6:43 PM, Bill Burke wrote:
>> On 5/23/12 12:04 PM, Marek Potociar wrote:
>>> On May 23, 2012, at 4:49 PM, Sergey Beryozkin wrote:
>>>> On 23/05/12 15:37, Marek Potociar wrote:
>>>>> Hello experts,
>>>>> please review the following issue
>>>>> and provide your feedback.
>>>>> Apparently this may be considered a BW-breaking change. If we make it,
>>>>> the implementations may need to provide a BW compatibility switch if
>>>>> necessary.
>>>> I do not understand that issue at all. The resource method returns InputStream therefore the MBW supporting InputStream must win
>>> Not necessarily - see the section of the spec referenced from the issue - 4.2.2, step 4.
>>> The response media types takes precedence over the generic MBW type, so if the response is "application/json" InputStream and a custom Jackson JSON provider claims to support any java type (Object) for "application/json",

>>>it will win over the default InputStream provider that supports InputStream for any media type.

No, it will only win over default InputStream provider in case of
Accept: application/json, for all other types the default one wins.

That is however most likely not what the user wanted.
>>> The issue suggests to reverse this ordering.
>>> Also, the spec mandates that custom providers should always take precedence over the default ones, but the spec is not clear whether this should really be a primary or tertiary key. For MBW the issue suggests that the right thing would be to explicitly define it as a tertiary key.

I'm nervous this is going to break the fundamental expectation that the
custom provider wins.

I think what is suggested is a workaround to manage catch-all providers
that return true in readFrom or writeTo.

At this moment of time I believe that the right course of action is to
recommend the custom provider implementations to be more selective, ex,
providers managing POJOs should exclude InputStream.class, but...

>> To have to separate implementations (Jersey and Resteasy) come to the same separate conclusion I think gives pretty good reason to change the algorithm.
>> Beyond InputStream, really any "low-level" type would have to be treated as a special case in the current matching algorithm: InputStream, Reader, File, StreamingOutput, String, char[], and byte[]. Plus, if a user has a specific MBR/MBW for a specific type, then they probably want that MBR/MBW called no matter what. You can't get any more specific than the Java type.

I don't have a good answer to it.
Still, I believe that these are not main-stream cases at all.
Having a situation where a custom JSON/etc provider helping the service
dealing with POJOS, where the service also return byte[], etc of the
same media type is rare and seems unusual.

Why one would have a service with two methods, one returning Book &
another - InputStream ? Just get two separate endpoints if that is
really needed. I guess that can be tricky to do in cases where the
scanning gets the providers loaded and visible to all the web
application, but the scanning is an optional feature.

IMHO, The following course of action:
- recommend the custom providers to be as specific as possible in
- get a user to register a custom provider for InputStream, byte[],
String supporting a specific media type when really needed - it's very
very easy.

needs to be reviewed alongside the proposed change of the algorithm.

In this case the control is for the user to assert. In the proposed
change we are making an assumption that this is user will not want, etc,
based on the case centered around InputStream or primitive classes.

Consider the case where a custom provider supports reading both Document
and DOMSource, example, in order to consolidate the way DOM is
processed. Its generic type is Object.

Having a default JAXP or Document provider selected in front of it is
going to a big mistake, right ? Sorry if I did not get the proposed
change, please be patient if so

Thanks, Sergey

> All right, so I'd give Sergey some more time to review the issue but I'm glad to see that we are on the same side here.
> Marek
>> --
>> Bill Burke
>> JBoss, a division of Red Hat

Sergey Beryozkin
Talend Community Coders