[jax-rs-spec users] Re: Hypermedia API

From: Santiago Pericas-Geertsen <>
Date: Thu, 11 Dec 2014 14:46:37 -0500


 Yes, structural links in entities is not something that JAX-RS provides any "special" support for (in some cases, people have included them in headers, but it is arguably less clean).

 The reason why JAX-RS hasn't done much (other than the JAXB Link serialization bit) is that JAX-RS has not been, and likely never will be, in the business of (structured) entity serialization; it delegates to specific JSON and XML libraries for that. Clearly this is an issue for link processing, but the architectural decision of not duplicating existing APIs is certainly sound.
 As you point out, JAX-RS would need some additional meta-data to "learn" about these links in representations. However, this needs to be done without introducing unnecessary coupling and in a standard manner --that is, not in a way that would require using a JAX-RS implementation for it to work. This is a difficult problem to solve.

 JSON-LD [1] is a step in the right direction, especially since becoming a W3C recommendation. Not having full control of the serialization is still an issue for us, but perhaps there's something we can do working with the new JSON-B EG.

-- Santiago


On Dec 11, 2014, at 1:43 PM, Casey Lee <> wrote:

> I agree, Markus that the issue here is the technical infrastructure
> (or lack of) around HATEOAS has limited its adoption and
> understanding. Specifically, the limitation is with the fact that the
> links we currently have with JAX-RS 2.0 are only transitional links in
> the HTTP header, but no support for structural links in the Entity.
> I feel the issue is that the current API is all about RESOURCES, which
> causes server side developers to focus more on the URIs than on the
> REPRESENTATIONS. Additionally, this has leaked into the client API,
> causing the client side developer to also have an awareness of the
> resources, which limits the need to think about links or at best makes
> the links optional.
> Is there an opportunity to evolve the API to enable adding (server
> side) and retrieving (client side) Links from the Entity? One
> approach would be by annotating your Entity POJOs where Links would be
> added. This would allow some declaration of the structural (and
> possibly transitional) links for the representation.
> This would cause developers (client and server side) to begin to think
> about links and the structure/relationships of the representations.
> At our organization, we've developed our own sets of annotations for
> declaring the structure of your representations and associating them
> to a media type. All of our documentation and the API that the client
> uses is based on the following:
> * Follow a link
> * Get back a representation
> * Find a link in the entity
> * Repeat
> This causes us to spend most of our effort describing the media types,
> and very little if any effort describing the URIs.
> -Casey
> On Wed, Dec 10, 2014 at 1:36 PM, Markus KARG <> wrote:
>> Santiago,
>> you asked for statements on the field of hypermedia and reactive. I will
>> take the chance to start discussion hereby on the field of hypermedia and
>> provide a starter for reactive in a separate thread.
>> Some weeks back I gave a lecture on JAX-RS 2.0 big picture at JUG Stuttgart,
>> just as I did infrequently at other events before. I'd like to describe the
>> reactions as those were stereotypical, independent of audience, location and
>> date. People where convinced by the very clean separation of concerns
>> (application made up from pure domain objects, technical aspects separated
>> from domain model), and the mostly declarative programming style (simply
>> adding annotations to declare needs, engine solves the needs "under the
>> hood" using a sophisticated and extensible technical infrastructure). But
>> when the presentation came to hypermedia support, they were some kind of
>> shocked by the mostly algorithmic coding style needed to make it work,
>> voiding the aforementioned separation of concerns and declarative code
>> style. While the existing API clearly is a foundation to achieve at least
>> "something", real HATEOAS becomes a hack with the existing low level support
>> only. Code gets cluttered with old-style techno-punk, which is hard to read
>> and understand. This is due to the lack of a declarative way to tell the
>> infrastructure how to make up the links from application domain state, and
>> how to provide the links to an entitiy provider so he can merge them into
>> the wire-level representation. Certainly everbody would vote for a
>> declarative kind of solution fitting into the existing infrastructure. On
>> the other hand, nobody (yes, really zero) people wanted to agree that they
>> have a REAL NEED for HATEOAS (hence, neither for an explicit HATEOAS API) as
>> 100% of all attendees admitted that their recent and current RESTful
>> projects are on level 1 or 2 of the REST Maturity Model only, and that the
>> largest obstacle to level 3 is not a techical issue (hence not a missing
>> explicit HATEOAS API) but the fact that HATEOS as a paradigm simply is not
>> well understood by most of them and / or they do not see the actual benefit
>> of HATEOAS in the real world: It wouldn't pay off, but it would be cool, to
>> sum it up.
>> So the question is: Is HATEOAS commonly understood well enough that it makes
>> actual sense to provide an explicit API for it, or does it make sense to
>> make an API even when it is not? And if we define an API, do we all agree
>> that it should support the separation of concerns and declarative style that
>> is typical for JAX-RS?
>> I think without an agreement on that general topics, it wouldn't be a good
>> idea to discuss any kind of details of API proposals in the area of HATEOAS.
>> Bill and Sergey, what's your opinion on that?
>> Regards
>> -Markus
>> -----Original Message-----
>> From: Santiago Pericas-Geertsen []
>> Sent: Mittwoch, 10. Dezember 2014 20:03
>> To:
>> Cc: Marek Potociar
>> Subject: Welcome to the JAX-RS 2.1 EG
>> Hello Experts,
>> Welcome to the JAX-RS 2.1 (JSR 370) expert group!
>> This is the official mailing list for the JSR. Note that the old mailing
>> list for JAX-RS 2.0 (JSR 339) is still available for 2.0 matters.
>> Before we start any discussions, I would like everyone to take a couple of
>> minutes and read the JSR description one more time to make sure we are all
>> on the same page ;)
>> ===
>> 2.1 Please describe the proposed Specification:
>> Server-Sent Events (SSE) is a new technology defined as part of the HTML5
>> set of recommendations for a client (e.g., a browser) to automatically get
>> updates from a server via HTTP. It is commonly employed for one-way
>> streaming data transmissions in which a server updates a client periodically
>> or every time an event takes place.
>> JAX-RS 2.0 introduced the notion of asynchronous processing for both the
>> client and the server APIs. However, asynchronous processing alone cannot
>> deliver on all the promises of a modern architecture without the help of
>> non-blocking I/O. If only blocking I/O is available, asynchronous processing
>> simply pushes the problem from one thread to the next --this is akin to
>> borrowing from a person to pay another, the problem is not really solved,
>> only deferred. Thus, support for non-blocking I/O is necessary to achieve
>> high throughput and efficiently manage resources like threads.
>> In summary, the following is a list of the tasks in scope for JAX-RS 2.1:
>> * Adding support for SSE.
>> * Improving integration with CDI.
>> * Exploring support for non-blocking I/O in providers (filters,
>> interceptors, etc.).
>> * Evaluating ways in which declarative security can be supported either
>> directly in this JSR or by leveraging other EE-platform JSRs.
>> * Making JAXB conditional on runtimes where it is available.
>> * Providing integration with JSON-B.
>> * Building upon the hypermedia API added in version 2.0.
>> * Investigating the reactive programming paradigm as a way to improve the
>> JAX-RS asynchronous client API.
>> * Evaluating any requirements necessary to support the use of JAX-RS
>> resource classes as controllers in the MVC 1.0 JSR.
>> ===
>> Some useful links:
>> [JSR]
>> [JAX-RS Spec] [JIRA for 2.1]
>> [RI]
>> [E-mail Archives]
>> As before, all of our discussions will be conducted using the expert's
>> alias and (automatically) CCed to the user's alias.
>> Some of the 2.1 tasks above require coordination with other specifications
>> (JSON-B, Security), so these tasks will tackled later on in the process.
>> We have tentatively selected 2 topics to start our discussions, both of
>> which require some investigation, these are: hypermedia improvements and
>> reactive programming. If you have any suggestions/comments/concerns about
>> these two topics, feel free to start a discussion about them. We will be
>> sending some more info as well in the upcoming weeks.
>> Looking forward to working with all of you!
>> --
>> Santiago Pericas-Geertsen
>> Marek Potociar
>> JSR 370 Spec Leads