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When there is a ClientResponse from WebClient, in the simplest case we use clientResponse.bodyToMono(MyResponseModel.class) to serialize the response body.

But it is not clear what is the way to go when responses' format can differ depending on situation.

For instance, when response could be one of the two types

type 1: {"a": <number>, "b": <string>}
type 2: {"c": <date>, "d": {"items": <array>}}

I suppose the basic algorithm should be like this

1. try to serialize to type 1
2. if OK return model of type 1
3. else try to serialize to type 2
4. if OK return model of type 2
5. else error of serialization

What is the proper way to handle this serialization scenario with Spring WebFlux?

解决方案

You can deserialize the JSON response yourself. First get the response as json String and then do the deserialization yourself with the provided and configured Jackson ObjectMapper and if there is a JsonMappingException try it with model 2. This might not be as efficient as spring does it internally with Jackson2JsonDecoder as it processes the InputStream directly without creating a copy but this seems to be the only way to do it with the try/catch approach currently.

@RestController
class Controller {

    private ObjectMapper objectMapper;

    public Controller(ObjectMapper objectMapper) {
        this.objectMapper = objectMapper;
    }

    @GetMapping("/handler")
    public Mono<Object> getHandler() {
        return WebClient
            .create("https://baseurl.com")
            .get()
            .uri("/someUri")
            .retrieve()
            .bodyToMono(String.class)
            .flatMap(json ->
                Mono.<Object>fromCallable(() -> objectMapper.readValue(json, Model1.class))
                .onErrorResume(JsonMappingException.class, e ->
                        Mono.fromCallable(() -> objectMapper.readValue(json, Model2.class))
                )
            );
    }
}

Another solution would be to create a base class or interface for the response types and then implement a custom Jackson JsonDeserializer for it. This could detect which of the types should be used and then perform deserialization.