Return value doesn't serialize to JSON when return type is Mono of Interface in WebFlux

[freddieyyh]

In WebMVC type application, Spring's message converter can convert the return value to a proper message(like a JSON) when controller return type declared by interface.

But, Webflux's behavior is different.

For example, we have Car class that implements Vehicle interface.

public interface Vehicle {
}

public class Car implements Vehicle {
    private String gearType;
}

And then there is a controller method like that.

@GetMapping("vehicle")
public Vehicle getVehicle() {
    return new Car();
}

I can get a json response of Car class with content-type: application/json header.

But, webflux's result is defferent.

@GetMapping("vehicle")
public Mono<Vehicle> getVehicle() {
    return Mono.just(new Car());
}

It only supports a 'content-type: text/event-stream' when return type is a mono of interface.
Is it a bug?

[rstoyanchev]

This odd. The signature with Mono<Vehicle> should work in both Spring MVC and WebFlux as expected. Do you have a sample?

[freddieyyh]

Here is my simple example.

Webflux : https://github.com/freddieyyh/demo/tree/webflux
WebMVC : https://github.com/freddieyyh/demo/tree/webmvc

Command curl -X GET -H "Accept: application/json" "localhost:8080/interface-a"
works on WebMVC demo, but doesn't work on Webflux demo.

[rstoyanchev]

The Result interface from the sample has no methods, so Jackson returns false for getObjectMapper().canSerialize(Result.class). Adding at least one accessor makes it work.

Spring MVC works is because it uses the actual type ResultA. We can make an improvement in WebFlux so that a concrete return value works the same, but for Mono<Result> it would still rely on the declared type, so it's best to have some sort of accessor on the interface.