Building RESTful APIs in Java? Getting tired of the annotation bloat forced upon you by your API specification framework which haunts your RESTful service controllers and model objects? Finding yourself in the situation where your code isn’t always in synch with your specification or vice versa? RAML and Swagger / OpenAPI are great specifications with powerful and shiny tooling, but they do also really have their downsides when it comes to messing with our code. Fear not! There are other alternatives worth investigating. In this talk, we’ll take a look at how we recently build, designed and documented a public REST API by doing Test Driven Documentation with some help of AsciiDoc and Spring (Auto) REST Docs.

1. Test-Driven Documentation
for your RESTful service
by Jeroen Reijn

2. JEROEN REIJN
Software Architect at Luminis /Amsterdam
• Focus on productivity and Developer Experience
• Been building APIs for the Web since 2010
• Writes most of his code in Java
• Tweets @jreijn

3. Let’s talk about Web APIs

4. source: https://www.programmableweb.com/

5. Streaming
1%
RPC
10%
REST
89%
REST RPC Streaming GraphQL
source: https://www.programmableweb.com/

6. REST

7. Source: http://geek-and-poke.com/geekandpoke/2013/6/14/insulting-made-easy

8. Richardson Maturity Model
Source: https://www.martinfowler.com/articles/richardsonMaturityModel.html
HATEOAS
(Hypermedia as the Engine of Application State)

9. REST
• Uniform interface
• Resources
• URIs
• Representations
• Hypermedia

10. Hypermedia
• Links in resource representations
• State navigations discoverable

11. Hypertext link specifications
• HAL
• JSON-LD
• JSON API
• Collection+JSON
• Siren

12. Hypertext link specifications
{
"_links" : {
"self" : {
"href" : "http://localhost:8080/"
},
"planets" : {
"href" : "http://localhost:8080/planets"
},
"people" : {
"href" : "http://localhost:8080/people"
}
}
}

13. Building an API
Design
BuildDocument

14. Let’s talk about API design

15. Specification driven
vs
Code driven

16. Specification Driven Design
Contract Driven Design
a.k.a

17. Manual spec

18. API design

19. RAML
• RESTful API Modelling Language (RAML)
• Based on YAML file format
• Powerful designer api with auto completion
• Code generation for Java
• No native support for HATEOAS

21. API Blueprint / Apiary
• Markdown flavoured syntax
• Powerful designer UI
• Documentation while designing
• Java code generator support is *very* minimal

23. Postman
• Postman client is great for testing APIs
• Supports designing & documenting APIs
• Has support for Api Mocks
• Enables fast prototyping of APIs
• Tools for different phases in the API lifecycle

25. Open API
• Widely adopted community-driven specification for REST APIs
• YAML and JSON based format
• Programming language-agnostic interface
• Allows both code first and contract first approaches to API design
• Evolving specification (version 3.0 released in the summer of 2017)

26. Swagger
• Tools around the OpenAPI specification
• Swagger editor
• Swagger-UI
• Swagger Codegen (server and client libs and many languages)
• Swagger Inspector

29. Swagger Codegen
/**
* Unique identifier representing a specific planet.
* @return id
**/
@ApiModelProperty(value = "Unique identifier representing a specific planet.")
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public Planet name(String name) {
this.name = name;
return this;
}

30. Retrospect
• API Design tools and platform are getting really powerful
• OpenAPI seems to be the most widely adopted api spec
• Generating code from your API specification is up to you

31. Code Driven Design

33. “ An API is only as good
as its documentation! ”

34. Let’s talk about API documentation

36. Manual documentation

37. Documentation based on specs
HTML / PDF

38. Docs based on code
@GetMapping(value = "/films/{id}")
ResponseEntity<Film> getFilmById(@PathVariable(“id") String id);

39. Docs based on code
@ApiOperation(value = "",
nickname = “getfilmbyid",
notes = "Get a specific film by id.",
response = Film.class,
authorizations = {
@Authorization(value = "apikeyQuery")
}, tags = {})
@ApiResponses(value = {
@ApiResponse(code = 200,
message = "A film.",
response = Film.class)})
@GetMapping(value = “/films/{id}")
ResponseEntity<Film> getFilmById(@ApiParam(value = "Id of the film.", required = true)
@PathVariable("id") String id);

41. Swagger drawbacks
• Hypermedia support not ‘final’
• Documentation is done through several annotations which are added in your
implementation classes
• API Implementation code overtime, gets overwhelmed with annotations
• Documentation is URI centric
• Documentation and API could become out-of-sync overtime

42. “ API documentation needs to be
more then just endpoints and
resources ”

43. Spring REST Docs
“Document RESTful services by combining
hand-written documentation with
auto-generated snippets
produced with Spring MVC Test.”

44. Spring REST Docs
• Documentation is built from tests
• Supports hypermedia (HATEOAS) APIs
• Immediate reaction to API changes
• Keeps your code clean
• Produces AsciiDoc snippets

45. Basic Spring MVC Test
@Test
public void testGetAllPlanets() throws Exception {
mockMvc.perform(get("/planets").accept(MediaType.APPLICATION_JSON))
.andExpect(status().isOk())
.andExpect(jsonPath("$.length()",is(2)));
}

46. With Spring REST Doc
@Test
public void testGetAllPlanets() throws Exception {
mockMvc.perform(get("/planets").accept(MediaType.APPLICATION_JSON))
.andExpect(status().isOk())
.andExpect(jsonPath("$.length()",is(2)))
.andDo(document("planets/get"));
}

47. AsciiDoc snippets
|===
|Path|Type|Description
|`id`
|`Number`
|The id of the planet
|`name`
|`String`
|The name of the planet
|`diameter`
|`Number`
|The diameter of the planet
|===

48. AsciiDoc
Plain-text writing format. AsciiDoc documents can be translated into various formats, including
HTML, DocBook, PDF and ePub.
= Hello, DevCon!
Jeroen Reijn <jeroen.reijn@luminis.eu>
An introduction to documenting Hypermedia APIs with
https://projects.spring.io/spring-restdocs/[Spring REST Docs].
== First Section
* item 1
* item 2

49. Generating documentation
Test
Generated
snippets
AsciiDoctor pluginHand-written
docs
Code
Document

50. Let’s build an API!

51. Spring Auto REST Docs
• Extension for Spring REST Docs
• Response field documentation generated by means of introspection
• Partial support for documenting Hypermedia ‘links’ out of the box
• DRY - Uses JavaDoc for generating documentation

52. Spring REST Docs advantages
• Ability to focus on domain model rather than URI’s
• ‘Guarantees’ that API documentation and implementation code are in-sync
• Forces you to update documentation for any implementation changes
• Ability to document with different payloads and explain different test scenarios
• Ability to document remote APIs (with WebTestClient)

53. In retrospect
• Documenting Hypermedia API is getting easier
• Validating APIs against their documentation is still ‘hard’
• Spring REST Docs can help documenting and validating the API
• Good documentation is key to working with an API

54. Those who stand for nothing,
fall for anything - Alexander
Hamilton
?Thank you
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