Problem deserialize flux/mono into Feign Spring Cloud - spring-webflux

Question

i develop a micro services application with Kotlin Webflux (Reactor3), Eureka, Zuul and Feign. Except that I always have an error when I make a call to an API via my micro service Feign. It looks like he can not deserialize the data. Could you please tell me if Feign is compatible with Flux and Monno?
thank you
{
"timestamp": "2019-05-29T07:39:43.998+0000",
"path": "/hobbies/",
"status": 500,
"error": "Internal Server Error",
"message": "Type definition error: [simple type, class reactor.core.publisher.Flux]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of reactor.core.publisher.Flux (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information\n at [Source: (PushbackInputStream); line: 1, column: 1]"
}

Answer 1

Feign doesn't provide support for Mono/Flux deserialiazation. There exists alternative feign library who fully support it: feign-reactive.
Note though, this is a rewrite of feign which fully use reactive code, differ from OpenFeign's Feign core.
Here's a snippet on how to use it, alongside with normal Feign, taken from the sample app.
#SpringBootApplication(exclude = ReactiveLoadBalancerAutoConfiguration.class)
#RestController
#EnableReactiveFeignClients
#EnableFeignClients
public class FeignApplication {
#Autowired
private GreetingReactive reactiveFeignClient;
#Autowired
private Greeting feignClient;
public static void main(String[] args) {
SpringApplication.run(FeignApplication.class, args);
}
#GetMapping("/greetingReactive")
public Mono<String> greetingReactive() {
return reactiveFeignClient.greeting().map(s -> "reactive feign! : " + s);
}
#GetMapping("/greeting")
public String greeting() {
return "feign! : " + feignClient.greeting();
}
}

Answer 2

In addition to Adhika Setya Pramudita response, I would like to mention that in order to return Mono in controller, you must use Spring WebFlux instead of Spring MVC

Answer 3

I was not able to make #Adhika Setya Pramudita solution working and gut tells me that it cannot even run due to mixing #EnableReactiveFeignClients and
#EnableFeignClients which are require corresponding #EnableWebFlux or #EnableWebMvc and thus defining both may compile but will fail in runtime.
Since op did not mention target language I'm feeling like to share Kotlin setup that works in my case:
build.gradle.kts
implementation("org.springframework.boot:spring-boot-starter-webflux")
implementation("com.playtika.reactivefeign:feign-reactor-core:2.0.22")
implementation("com.playtika.reactivefeign:feign-reactor-spring-configuration:2.0.22")
implementation("com.playtika.reactivefeign:feign-reactor-webclient:2.0.22")
Config.kt
#Configuration
#EnableWebFlux
#EnableReactiveFeignClients
class Config {
}
MyEntity.kt
class MyEntity #JsonCreator constructor(
#param:JsonProperty("my_value") val my_value: String
)
MyFeignClient.kt
#Component
#ReactiveFeignClient(
url = "\${package.service.my-service-url}",
name = "client"
)
interface MyFeignClient {
#GetMapping(value = ["/my/url?my_param={my_value}"], consumes = ["application/json"])
fun getValues(
#PathVariable(name = "my_value") myValue: String?,
): Mono<MyEntity?>?
}
Then here goes code in some service class:
val myClient: MyFeignClient = WebReactiveFeign.builder<MyFeignClient>()
.contract(ReactiveContract(SpringMvcContract()))
.target(MyFeignClient::class.java, "http://example.com")
// feel free to add .block() to get unpacked value or just chain your logic further
val response = myClient.getValues(param)