Java Reactor + Caffeine Caching + Mockito = Strict stubbing argument mismatch

I am using Java Reactor in a Spring-boot project, where I need to cache my data in one of the intermediate steps. It's on Spring Boot 3, Java 17, Junit5...

My caching service looks like this:

@Component
public class CachingService {
  private static final int DURATION = 60;

  public Mono<Resource> ofMono(final UUID guid, final Mono<Resource> mono) {
    final String cachingKey = "key";
    final Function<String, Mono<Resource>> monoFn = ofMono(key -> mono);
    return Mono.defer(() -> monoFn.apply(cachingKey));
  }

  private Function<String, Mono<Resource>> ofMono(final Function<String, Mono<Resource>> fn) {
    final AsyncLoadingCache<String, Resource> cache =
        Caffeine.newBuilder()
            .expireAfterWrite(Duration.ofSeconds(DURATION).multipliedBy(2))
            .refreshAfterWrite(Duration.ofSeconds(DURATION))
            .buildAsync((k, e) -> fn.apply(k).subscribeOn(Schedulers.fromExecutor(e)).toFuture());

    return (k) -> Mono.fromFuture(cache.get(k));
  }
}

So my purpose is caching an instance of 'Resource' record, which is published as Mono from another service, and will be consumed in another service again.

I am injecting cachingService in another service and calling it's method:

  @Override
  public Mono<Resource> getResourceWithAccess(final UUID guid) {
    final Mono<Boolean> viewAccess = accessService.getViewAccess(guid);

    return viewAccess
        .flatMap(hasAccess -> mapToResource(guid, hasAccess))
        .flatMap(resource -> cachingService.ofMono(guid, Mono.just(resource)));
  }

The accessService.getViewAccess and mapToResource used in the code:

  Mono<Boolean> accessService.getViewAccess(UUID fileGuid);

  Mono<Resource> mapToResource(
      final UUID guid, final Boolean hasAccess);

I have a Mockito test like this:

@Test
  void test() {
    // GIVEN
    ...
    private ResourceWithAccessService service
    @Mock private CachingService cachingService;

    Resource expectedResource = ...
    when(cachingService.ofMono(fileGuid, Mono.just(expectedResource ))).thenReturn(Mono.just(expectedResource ));

    // WHEN
    final Mono<Resource > actualResource =
        service.getResourceWithAccess(fileGuid);

    // THEN
    StepVerifier.create(actualResource).expectNext(expectedResource).verifyComplete();
  }

Unfortunately, I am getting this error:

java.lang.AssertionError: expectation ....
actual: onError(org.mockito.exceptions.misusing.PotentialStubbingProblem: 
Strict stubbing argument mismatch. Please check:
 - this invocation of 'ofMono' method:
    cachingService.ofMono(
    98f0b409-20b9-3674-a0e6-eb5048219f2c,
    MonoJust
);
    -> at com...lambda$getResourceWithAccess$1(ResourceWithAccessServiceImp.java:52)
 - has following stubbing(s) with different arguments:
    1. cachingService.ofMono(
    98f0b409-20b9-3674-a0e6-eb5048219f2c,
    MonoJust
);

I have found a workaround finally. By using ArgumentCaptor, I am still able to check if my method is called with correct parameters.

  private void verifyCachingService(final UUID fileGuid, final Resource resource) {
    final ArgumentCaptor<Mono> acMono = ArgumentCaptor.forClass(Mono.class);
    verify(cachingService).ofMono(eq(fileGuid), acMono.capture());
    StepVerifier.create(acMono.getValue()).expectNext(resource).verifyComplete();
  }