What is a declarative HTTP client vs. an imperative one? Why are they useful and why should I care? We’ll talk about options that are available in the Spring portfolio and what’s coming on the horizon. In this session, we’ll look at the history of declarative clients in Spring via Spring Cloud OpenFeign. We’ll also dive into upcoming options that are coming to Spring and the advantages that these new technologies bring to the developer experience. Spring One Tour Tel-Aviv 2022.

1. Olga Maciaszek-Sharma
Declarative Clients in Spring
Copyright © 2020 VMware, Inc. or its affiliates.

2. About Olga
Software Engineer in Spring Cloud team
Spring Cloud LoadBalancer, Spring Cloud OpenFeign, Spring
Cloud Netflix, Spring Cloud Contract, Spring Cloud Square,
RSocket HTTP Bridge
TT/ Mastodon: @olga_maciaszek

3. What is a declarative client?

4. Sample applications
Customer Service Verification Service
HTTP POST /verify
Customer Verification Result

5. Direct use of HTTP client

6. Client interface

7. Implementation

8. Implementation with helper methods

9. Client interface usage

10. What if we could just create an annotated
interface and have a library/ framework
translate it into actual HTTP client requests?

11. Declarative/ interface client
Client interface
method calls
HTTP calls
Proxy

12. Feign

13. OpenFeign/Feign

14. Feign
Java to HTTP client binder
● Core part of a former Spring Cloud Netflix module, passed over to the community
● Supports various HTTP clients, including Apache HTTP, Apache HC5, Ok HTTP
● Contracts - support for custom annotations and argument resolution
● Support for a broad range of encoders/ decoders, including Jackson and GSON
● Metrics, including Micrometer support

15. Spring Cloud OpenFeign

16. Spring Cloud OpenFeign
Spring MVC and Spring Cloud support for OpenFeign
● Spring MVC annotations support
● Auto-configurations for Feign-specific beans
● Spring Cloud LoadBalancer support
● Spring Cloud CircuitBreaker support
● Tracing support

17. A declarative/ interface client

18. A declarative/ interface client use

19. Spring Cloud OpenFeign Demo
Applications communicating over HTTP
Customer Service
SC OpenFeign Client
SC LoadBalancer
SC Sleuth
Verification Service
POST /verify
Customer Verification Result
Eureka Service Registry

20. Spring Cloud OpenFeign - popularity
Highly-readable interface clients with robust integrations
● 906 GitHub stars
● 8670549 Maven downloads in 2021
● Nearly flat learning curve for Spring MVC users

21. Spring Cloud OpenFeign - issues
● Issues with reusing client interfaces as controllers
○ @RequestMapping at class level user issues (GH-547, GH-678)
○ CVE-2021-22044
● Maintenance issues caused by dependence on third party upstream project
● Lack of non-blocking support

22. Spring Cloud Square
Spring Boot and Spring Cloud support for Retrofit and OkHttp
● WebClient support for Retrofit interface clients
● Auto-configurations
● Load-balancing
● Tracing
● Only Retrofit annotations
● Incubator project

23. RetroSocket
Interface client for RSocket
● Incubator project

24. HTTP & RSocket Interface Clients
in Spring Framework 6.0

25. Interface clients are not new in
the Spring Framework
Same mechanism, different protocols

26. Why HTTP and RSocket now?
Discussions with Spring Cloud around
OpenFeign, Retrofit, and RSocket integrations

27. Spring Framework 6.0 already had plans
for RSocket interface client

28. Old request for HTTP, deferred to…
Spring Cloud OpenFeign

29. Old request for HTTP, deferred to…
Spring Cloud OpenFeign

30. Opportunity to revisit
Underlying mechanism is not specific to Spring Cloud
Cloud features are extensions on top

31. Some issues can only be addressed
in Spring Framework
programming model vs reusing @RequestMapping

32. Decided to explore
for Spring Framework 6.0

33. HTTP interface client programming model
Relationship to server @RequestMapping?

34. Two goals
Define a single request (no wildcards of any kind!)
Avoid outright server-specific

35. @RequestMapping attributes
For mapping of 1..N requests
Any HTTP method Any URL path

36. @RequestMapping method parameters
ServletRequest
HttpMethod
@RequestHeader
@RequestParam
@RequestBody
@RequestAttribute
@ModelAttribute
…

37. @RequestMapping return values
ResponseEntity
HttpHeaders
Body
void
Model
View
SseEmitter
…

38. New method annotation
to declare rather than map requests
@RequestMapping
@HttpExchange

39. @HttpExchange attributes
url
method
contentType
accept

40. @HttpExchange method parameters
Subset of those for @RequestMapping
i.e. reusing method parameter annotations

41. @HttpExchange method parameters
URI
HttpMethod
@RequestHeader
@PathVariable
@RequestBody
@RequestParam
@CookieValue

42. @HttpExchange return values
Subset of those for @RequestMapping
i.e. also using return value types

44. Blocking

45. Reactive

46. status, headers, body

47. headers

48. execute only

50. Spring Interface Clients Demo
Applications communicating over HTTP
Customer Service
Spring HTTP Interface Client
SC LoadBalancer
Verification Service
POST /verify
Customer Verification Result
Eureka Service Registry

51. RSocket interface client
@RSocketExchange

52. @RSocketExchange attributes
value

53. @RSocketExchange method parameters
@DestinationVariable
@Payload
Object, if followed by MimeType
MimeType

54. @RSocketExchange return values
concrete values
producer of values

56. Blocking

57. Reactive

59. Demo

60. Resources
● Demo code - reactive HTTP client
● Demo code - non-reactive HTTP client with load-balancing
● Demo-code - RSocket client
● Spring Cloud OpenFeign
● Spring Framework documentation for HTTP interface client
● Spring Framework documentation for RSocket interface client
● Spring Cloud Square
● RetroSocket

61. Thank you
Twitter: @olga_maciaszek
© 2020 Spring. A VMware-backed project.