You should test the controller for successful business logic execution and response generation. The following test case is written for MovieController, which is annotated with @WebFluxTest and has an autowired WebTestClient, which will be used to invoke the endpoints:

@RunWith(SpringRunner::class)
@WebFluxTest(MovieController::class)
@ActiveProfiles("dev")
class MovieControllerTest {

    @MockBean
    lateinit var movieService: MovieService;

    @MockBean
    lateinit var movieRepository: MovieRepository;

    @Autowired
    lateinit var webTestClient: WebTestClient;

The preceding code snippet performs creates mocking beans and the testing utility:

    @Test
    fun `List Movies - Happy Path`() {
        // Given
        var movieFlux: Flux<Movie> = Flux.fromIterable(listOf(getMovie()));
        `when`(movieService.findAll()).thenReturn(movieFlux);

        // When
        webTestClient.get().uri("/movies")
                .header(HttpHeaders.CONTENT_TYPE, "application/json")
                .exchange()
                .expectStatus().isOk()
                .expectBody()
                .json("""
          [{"id":1,"title":"Avengers","year":2018,"genre":
        {"name":"Action","description":"Action"},"ratings":[],
        "cast":[]}]""");

        // Then
        verify(movieService, times(1)).findAll();
    }

The preceding code snippet performs the controller testing for the List Movies scenario's happy path:

    @Test
    fun `Rate Movie - Happy Path`() {
        // Given
        var movie = getMovie();
        movie.ratings.add(MovieRating("Great", 5, Date()))
        `when`(movieService.rate(1, "Great", 
         5)).thenReturn(Mono.just(movie));

        // When
        webTestClient.put().uri("/movies/1/rate?
        comment=Great&rating=5")
                .exchange()
                .expectStatus().isOk()
                .expectBody()
                .json("""
         {"id":1,"title":"Avengers","year":2018,"genre":
         {"name":"Action","description":"Action"},"ratings":
          [{"comment":"Great","rating":5}],"cast":[]}""")

        // Then
        verify(movieService, times(1)).rate(1, "Great", 5)
    }    

The preceding code snippet performs the controller testing for the Rate Movie scenario's happy path:

    @Test
    fun `Rate Movie - Failure Path`() {
        // Given
        `when`(movieService.rate(2, "Great", 
         5)).thenReturn(Mono.error(MovieNotFoundException.create(2)));

        // When
        webTestClient.put().uri("/movies/2/rate?
        comment=Great&rating=5")
                .exchange()
                .expectStatus().isBadRequest()
                .expectBody()
                .json("""{"code":400,"message":"Movie by id 2, not 
                 found"}""")

        // Then
        verify(movieService, times(1)).rate(2, "Great", 5)
    }

The preceding code snippet performs the controller testing of the Rate Movie scenario's failure path:

    @Test
    fun `Get Movie by Id - Happy Path`() {
        // Given
        `when`(movieService.findOne(1))
        .thenReturn(Mono.just(getMovie()));

        // When
        webTestClient.get().uri("/movies/1")
                .exchange()
                .expectStatus().isOk()
                .expectBody()
                .json("""
               {"id":1,"title":"Avengers","year":2018,"genre":
              {"name":"Action","description":"Action"},
              "ratings":[],"cast":[]}""")

        // Then
        verify(movieService, times(1)).findOne(1);
    }

The preceding code snippet does the controller testing of the Get Movie by Id scenario's happy path:

    fun getMovie() : Movie {
        return Movie(1, "Avengers", 2018, MovieGenre("Action", 
        "Action"), ArrayList<MovieRating>(), ArrayList<Actor>())
    }
}

The preceding code snippet creates mock Movie data for testing.

In the preceding controller test case, WebTestClient is used to hit particular endpoints of the controller and verify whether it returns the correct status codes, bodies, and so on.