WebClient synchronous call does not return from within a filter stack trace - spring-webflux

Question

In a Spring Gateway API I have a filter which calls a class to make a call to another API using WebClient. If I make the same call from say a controller the call returns. However when this webclient call is made from within the Filter stack it never returns. I am trying to make this call synchronously. I cannot use the block() method because Reactive classes error.
Here is the method in question:
public void doPost() {
ApiResponse<Void> response = webClientBuilder.build().post()
.uri("http://localhost:8080")
.retrieve()
.bodyToMono(new ParameterizedTypeReference<ApiResponse<Void>>() {})
.block();
}
I am very new to WebClient and need someone to tell me how I can synchronously make this call. I have tried another variation which is toFuture().get() instead of the last line but this also does not return.
It get the below error:
java.lang.IllegalStateException: block()/blockFirst()/blockLast() are blocking, which is not supported in thread reactor-http-nio-4
My mistake it is an authentication filter that this is being run from:
public class AuthServiceAuthenticationManager implements ReactiveAuthenticationManager {
private final MyClient myClient;
#Override
public Mono<Authentication> authenticate(Authentication authentication) {
//Below line does not return using my webclient
myClient.post();
UsernamePasswordAuthenticationToken token = new UsernamePasswordAuthenticationToken(authentication.getPrincipal(), authentication.getCredentials(), new ArrayList<GrantedAuthority>());
return Mono.just(token);
}
}

Answer 1

As I mentioned in comment, the reason is simple - you and blocking doPost is called from the reactive flow. WebClient is a non-blocking client and as you are using it from the ReactiveAuthenticationManager you could keep the whole flow reactive.
Solution:
Remove block() from the doPost and return Mono.
public Mono<ApiResponse<Void>> doPost() {
return webClientBuilder.build().post()
.uri("http://localhost:8080")
.retrieve()
.bodyToMono(new ParameterizedTypeReference<ApiResponse<Void>>() {})
}
Construct reactive flow in AuthServiceAuthenticationManager.
Logic of authenticate is not really clear but based on your example it could look like
public Mono<Authentication> authenticate(Authentication authentication) {
UsernamePasswordAuthenticationToken token = new UsernamePasswordAuthenticationToken(authentication.getPrincipal(), authentication.getCredentials(), new ArrayList<>());
return doPost()
.thenReturn(token);
}