RouterFunctions Builders do not support ServerResponse subtypes

[krezovic]

org.springframework.web.reactive.function.server.RouterFunctions builder methods use HandlerFunction<ServerResponse> as their input types, rather than something like HandlerFunction<? extends ServerResponse> or equivalent generic signature similar to RouterFunctions::route(RequestPredicate,HandlerFunction).

This makes it infeasible to use other ServerResponse sub types such as EntityResponse or RenderingResponse.

Example code that does not compile

import org.springframework.context.annotation.Bean;
import org.springframework.stereotype.Component;
import org.springframework.web.reactive.function.server.RenderingResponse;
import org.springframework.web.reactive.function.server.RouterFunction;
import org.springframework.web.reactive.function.server.RouterFunctions;
import org.springframework.web.reactive.function.server.ServerResponse;
import reactor.core.publisher.Mono;

@Component
public class TestRouter {
    @Bean
    RouterFunction<? extends ServerResponse> myRoutes() {
        return RouterFunctions.route()
                .GET("/example", request -> renderSomething())
                .GET("/example2", request -> renderSomething2())
                .build();
    }

    Mono<RenderingResponse> renderSomething() {
        return RenderingResponse.create("example").build();
    }

    Mono<ServerResponse> renderSomething2() {
        return RenderingResponse.create("example").build().map(ServerResponse.class::cast);
    }
}

[rstoyanchev]

Indeed, it looks like the generics on the Builder are too restrictive. Both HandlerFunction and HandlerFilterFunction allow subtypes of ServerResponse, and it should be possible to pass those as is.

This is an issue on the WebFlux side only given the generic type is on the Mono (container) type. The same examples works fine for WebMvc where the generic type is the return type, and it doesn't require casting.

[rstoyanchev]

Re-opening to apply on WebMvc side as well for consistency not only with WebFlux, but within WebMvc.fn as well with HandlerFunction, HandlerFilterFunction, and RouterFunction all of which are declared to allow subtypes.