HATEOAS is without a doubt, the least understood pillar of REST. It seems difficult to implement and shows no immediate reward for it, so many developers don't even bother. The truth is, it just has some bad PR and a horrible acronym that sounds like a breakfast cereal. Join me to take a look at the theory and practice behind using hypermedia by examining both web services and web clients. Along the way we will look at some exciting upcoming Spring HATEOAS features, like the Affordances API, and talk about what the future holds for hypermedia in your web services.

1. Spring HATEOAS hypermedia
driven web services
and clients that understand them
Matija Dujmović

2. Inspiration

3. The Web
• URI
• http://bbc.com/news/technology-44351214
• HTTP
• GET, POST, etc.
• HTML

4. Hypermedia APIs
• URI
• http://api.example.com/employees/123
• HTTP
• GET, POST, etc.
• Hypermedia Type
• JSON?

5. Hypermedia types?
Media Type
+
(hyper)link support
=
Hypermedia Type

6. Static HTML -> hypermedia APIs

7. Static HTML
• links
<a href="http://example.com/registration" >Register</a>
• forms
<form action="http://example.com/registration" method="post">
Name: <input name="name" type="text" required="true" />
Email: <input name="email" type="text" required="true" />
<input type="submit" value="Register" />
</form>

8. JSON-based CRUD APIs
• JSON
{
"name": "John",
"email": "john@example.com"
}
• no more links and forms
• API changes -> client changes & redeploys

9. All about the links (and forms)
• links define resource metadata
• objects
• defining properties, not just their values
• e.g. "humidityStatus": "OVER"
• addresses
• actions

10. Using hypermedia

11. Hypermedia types
• HAL
• HAL-FORMS
• Collection+JSON
• Siren
• UBER

12. HAL
• Hypermedia Application Language
• supports address metadata

13. HAL _links hash
JSON
HAL
GET http://localhost:8080/attendants/1

14. HAL _links hash cont.
GET http://localhost:8080/attendants
JSON
HAL

15. HAL _embedded hash
GET http://localhost:8080/attendants
JSON
HAL

16. HAL-FORMS
• extends HAL
• adds support for action metadata
• supported by the Affordances API
• DISCLAIMER
• Affordances API is only available in new SNAPSHOT builds

17. HAL-FORMS _templates hash
JSON
HAL-FORMS
GET http://localhost:8080/attendants/1

18. HAL-FORMS _templates hash cont.
JSON
HAL-FORMSGET http://localhost:8080/attendants

19. HATEOAS
• Hypermedia As The Engine Of Application State
• use resource metadata
• support evolvable API interactions
• avoid tight coupling

20. Using Spring HATEOAS

21. Configuration
• @EnableHypermediaSupport(type = HypermediaType.HAL_FORMS)

22. LinkBuilder
• wrap objects/collections in Resource/Resources wrappers
• linkTo() -> define values in _links hash
• afford() -> define values in _templates hash

23. Demo time

24. Possibilities
• expose validations to clients
• e.g. OIB

25. Conclusion

26. Recommended materials
• books
• „RESTful Web APIs”- Mike Amundsen, Leonard Richardson
• „RESTful Web Clients: Enabling Reuse Through Hypermedia”- Mike Amundsen
• code examples
• https://github.com/spring-projects/spring-hateoas-examples
• https://github.com/RWCBook

27. Questions?