Extract Controller API to separate package

[rolaca11]

Affects: All

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There is a really nice consequence of how the official Jax-RS API is structured. Since the implementation of the Jax-RS API is not packaged with the API itself (obviously) there is a way to define REST APIs by creating a package, let's call it service-api so that not only the methods, but all the URI paths, the request methods, or even the accepted mime-types can be defined for every method in the service-api package.

Based on this package we can create the service-client and service-impl modules so that both packages instantly know how to communicate with eachother through the defined REST API and none of the API information needs to get duplicated.

A similar feature (although not necessarily bounded to REST APIs) could be achieved with the Spring framework by extracting the @controller, ResponseEntity and related annotations/classes to a different package.

It could be a nice addition to the way teams implement REST APIs. The reason a team might not want to use Jax-RS instead of Spring controller is that there are some nice features already in the Spring Controllers, for example, that the ResponseEntity can take a generic type, as opposed to the Jax-RS's Response, or the way Spring controllers handle exceptions.

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Maybe the above description is a little cluttered, so I'll try and show an example of what I'm thinking about:

We have 3 packages: service-api, service-impl, service-client:

service-api.jar:

@RequestMapping("/api/service")
public interface Service {
    @GetMapping
    ResponseEntity<Resp> getStuff();
}

service-impl.jar:

@RestController
public class ServiceImpl implements Service {
    // ...
}

service-client.jar:

@Configuration
public class Config {
    @Bean
    public Service client() {
        return RestClient.api(Service.class);
    }
}

@Component
public class SomeBean {
    @Autowired
    private Service client;

    // ...

    public void callService() {
        client.getStuff();
    }

    // ...

}

[rstoyanchev]

@rolaca11 I believe this is a duplicate of #16747. Do you agree? If I understand correctly, your goal is to have an automatic client simply by pointing to an interface with @Controller and @RequestMapping annotations on it. Have you seen the Feign client in Spring Cloud?

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[rolaca11]

I'm sorry for the late answer.
Yes, it is quite similar to what OpenFeign does. I'm not familiar with that library, but on first look, it is really similar. Can I use a standard Spring controller with an OpenFeign interface on the server-side?

[rstoyanchev]

Yes it does. I'm closing this as duplicate.