What is an interface client? Why is it useful and why should you care? In this session, we’ll talk about the history of options available through Spring Cloud OpenFeign and Spring Cloud Square and also dive into upcoming options for Spring Framework 6 for HTTP and RSocket. We’ll discuss the advantages that the new offerings bring to the developer experience.

1. │©2022 VMware, Inc.
Declarative Clients in Spring
Rossen Stoyanchev
Olga Maciaszek-Sharma

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

3. About Rossen
Spring Framework and Spring for GraphQL teams
Spring MVC, WebFlux, WebSocket, RSocket

4. What is a declarative client?

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

6. Direct use of HTTP client

7. Client interface

8. Implementation

9. Implementation with helper methods

10. Client interface usage

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

12. Sample applications
Client interface method
calls HTTP calls
Proxy

13. Feign

14. OpenFeign/Feign

15. 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

16. Spring Cloud OpenFeign

17. 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

18. A declarative/ interface client

19. A declarative/ interface client use

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

23. 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

24. RetroSocket

25. Interface Clients
in Spring Framework 6.0

26. Interface clients are not new in the Spring Framework
Same proxy mechanism, different protocols…

27. We already had plans for RSocket Interface client

28. And Previous Request for an HTTP Interface Client

29. … deferred to Spring Cloud OpenFeign
And Previous Request for an HTTP Interface Client

30. Underlying mechanism is not specific to Spring Cloud
Spring Cloud just adds extensions on top
What should programming model look like, is re-using @RequestMapping
the right approach?
Opportunity to Revisit

31. Programming Model for an HTTP Interface Client
Define a single, concrete request
Avoid server-isms

32. What about @RequestMapping?
Maps 1 controller method to N requests
Any HTTP method Any URL path

33. Method parameters for @RequestMapping
HttpMethod
@RequestHeader
@RequestParam
@RequestBody
@RequestAttribute
@ModelAttribute
ServletRequest
…

34. Return Values from @RequestMapping
ResponseEntity
HttpHeaders
Body
void
Model
View
SseEmitter
…

35. New Method Annotation
@HttpExchange – declare, rather than map, a request

36. @HttpExchange Annotation Attributes
url
method
contentType
accept

37. Same as for @RequestMapping, but only a subset
Those that are not server specific
Method Parameters for @HttpExchange

38. URI
HttpMethod
@RequestHeader
@PathVariable
@RequestBody
@RequestParam
@CookieValue
Method Parameters for @HttpExchange

39. Return Values for @HttpExchange
Same as for @RequestMapping, but only a subset
Those that are not server specific

46. RSocket interface client
@RSocketExchange

47. @RSocketExchange parameters
@DestinationVariable
@Payload
Metadata Object, if followed by MimeType
MimeType

49. 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

50. Demo

51. 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