Reactor Feign cannot deserialize Flux response - json

Question

I have the following controller
#RestController
public void MyController {
#GetMapping("/foo")
public Flux<Foo> getFoos() { /* return a flux of Foos*/ }
}
And a Feign client
public interface MyFeignClient {
#RequestLine("GET /foo")
Mono<Foo> getFoos();
}
public class MyClients {
public static MyFeignClient myFeignClient() {
return ReactorFeign.builder().target(MyFeignClient.class, "http://localhost:8080");
}
}
But when I call
StepVerifier.create(myFeignClient.foo())
.consumeNextWith(foo -> println(foo))
.verifyCompleted();
I got this error
java.lang.AssertionError: expectation "consumeNextWith" failed
(expected: onNext(); actual: onError(feign.FeignException: Cannot
deserialize instance of
com.example.Foo out of
START_ARRAY token at [Source: (BufferedReader); line: 1, column: 1]
reading GET
http://localhost:8080/foo))
at
reactor.test.ErrorFormatter.assertionError(ErrorFormatter.java:105)
at reactor.test.ErrorFormatter.failPrefix(ErrorFormatter.java:94) at
reactor.test.ErrorFormatter.fail(ErrorFormatter.java:64) at
reactor.test.ErrorFormatter.failOptional(ErrorFormatter.java:79) at
reactor.test.DefaultStepVerifierBuilder.lambda$consumeNextWith$1(DefaultStepVerifierBuilder.java:256)
at
reactor.test.DefaultStepVerifierBuilder$SignalEvent.test(DefaultStepVerifierBuilder.java:2112)
at
reactor.test.DefaultStepVerifierBuilder$DefaultVerifySubscriber.onSignal(DefaultStepVerifierBuilder.java:1408)
at
reactor.test.DefaultStepVerifierBuilder$DefaultVerifySubscriber.onExpectation(DefaultStepVerifierBuilder.java:1356)
at
reactor.test.DefaultStepVerifierBuilder$DefaultVerifySubscriber.onError(DefaultStepVerifierBuilder.java:1030)
at
reactor.core.publisher.FluxTake$TakeSubscriber.onError(FluxTake.java:138)
at
reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onError(FluxSubscribeOn.java:157)
at
feign.reactive.ReactiveInvocationHandler$1.request(ReactiveInvocationHandler.java:115)
at
reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.requestUpstream(FluxSubscribeOn.java:131)
at
reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.onSubscribe(FluxSubscribeOn.java:124)
at
feign.reactive.ReactiveInvocationHandler.lambda$invokeMethod$0(ReactiveInvocationHandler.java:99)
at reactor.core.publisher.FluxSource.subscribe(FluxSource.java:52)
at reactor.core.publisher.Flux.subscribe(Flux.java:7777) at
reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84) at
reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37) at
java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264) at
java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
at
java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at
java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:834) Suppressed:
feign.FeignException: Cannot deserialize instance of
com.example.Foo out of
START_ARRAY token at [Source: (BufferedReader); line: 1, column: 1]
reading GET
http://localhost:8080/foo at
feign.FeignException.errorReading(FeignException.java:130) at
feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:162)
at
feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:80)
at
feign.reactive.ReactiveInvocationHandler$1.request(ReactiveInvocationHandler.java:109)
... 13 more Caused by:
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot
deserialize instance of
com.example.Foo out of
START_ARRAY token at [Source: (BufferedReader); line: 1, column: 1]
at
com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
at
com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1343)
at
com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1139)
at
com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1093)
at
com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromArray(BeanDeserializerBase.java:1461)
at
com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:185)
at
com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:161)
at
com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4013)
at
com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3063)
at feign.jackson.JacksonDecoder.decode(JacksonDecoder.java:61) at
feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:183)
at
feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:147)
... 15 more
What have I done wrong here? How can I fix it?

Answer 1

A simple google search shows that the feign client does not support reactive. They have an incubating project trying to create support
Add Webflux support issue
Feign core
So to answer your questions
"What have i done wrong?"
You have chosen to use an HttpClient that does not support producers/consumers.
"How can i fix it?"
By not using the feign client and instead use an HttpClient that supports non blocking operations, like for instance, the spring WebClient.