Spring WebClient Error - reactor.core.publisher.FluxOnAssembly cannot be cast to class reactor.core.publisher.Mono - spring-boot

Question

Following is the rest service that I have implemented
#GetMapping(produces = MediaType.TEXT_EVENT_STREAM_VALUE, value = "/events")
Flux<Event> events() {
Flux<Event> eventFlux = Flux.fromStream(Stream.generate {new Event(System.currentTimeMillis(), LocalDate.now())})
Flux<Long> durationFlux = Flux.interval(Duration.ofSeconds(1))
return Flux.zip(eventFlux, durationFlux)
.map {it.t1}
}
public static void main(String[] args) {
SpringApplication.run(ReactiveServiceApplication)
}
The webclient to consume this looks as follows.
#Bean
WebClient client() {
return WebClient.create("http://localhost:8080")
}
#Bean
CommandLineRunner demo (WebClient client) {
return { args ->
client.get().uri("/events")
.accept(MediaType.TEXT_EVENT_STREAM)
.exchange()
.flatMap { response -> response.bodyToFlux(Event.class) }
.subscribe { println it }
}
}
public static void main(String[] args) {
new SpringApplicationBuilder(ReactiveClientApplication)
.properties(Collections.singletonMap("server.port","8081"))
.run(args)
}
The error I am getting is as follows.
reactor.core.Exceptions$ErrorCallbackNotImplemented: java.lang.ClassCastException: class reactor.core.publisher.FluxOnAssembly cannot be cast to class reactor.core.publisher.Mono (reactor.core.publisher.FluxOnAssembly and reactor.core.publisher.Mono are in unnamed module of loader 'app')
Caused by: java.lang.ClassCastException: class reactor.core.publisher.FluxOnAssembly cannot be cast to class reactor.core.publisher.Mono (reactor.core.publisher.FluxOnAssembly and reactor.core.publisher.Mono are in unnamed module of loader 'app')
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:118) ~[reactor-core-3.3.5.RELEASE.jar:3.3.5.RELEASE]
Why is the error happening ?

Answer 1

The reason was that using flatMap actualy returns a Mono. So the closure { response -> response.bodyToFlux(Event.class) } was actually returning a Flux where as the flatMap was expecting a Mono.
Changing to flatMapMany will fix this. Code below.
CommandLineRunner getFluxDemoExchange (WebClient client) {
return { args ->
client.get().uri("/events")
.accept(MediaType.TEXT_EVENT_STREAM)
.exchange()
.flatMapMany { response -> response.bodyToFlux(Event.class) }
.subscribe { println it }
}
}