block()/blockFirst()/blockLast() are blocking, which is not supported in thread reactor-http-server-epoll-18

I am trying to post data using WebTestClient But unfortunately such message is thrown:

java.lang.AssertionError: Status expected:<200> but was:<500>

> POST http://localhost:41087/movie
> Accept: [application/json;charset=UTF-8]
> WebTestClient-Request-Id: [1]
> Content-Type: [application/json;charset=UTF-8]
> Content-Length: [101]

{"movieId":1,"title":"Title for movie 1","genre":"Genre for movie 1","address":"Address for movie 1"}

< 500 INTERNAL_SERVER_ERROR Internal Server Error
< Content-Type: [application/json;charset=UTF-8]
< Content-Length: [226]

{"timestamp":"2020-01-03T10:02:18.467+0000","path":"/movie","status":500,"error":"Internal Server Error","message":"block()/blockFirst()/blockLast() are blocking, which is not supported in thread reactor-http-server-epoll-18"}


	at org.springframework.test.web.reactive.server.ExchangeResult.assertWithDiagnostics(ExchangeResult.java:200)
	at org.springframework.test.web.reactive.server.StatusAssertions.isEqualTo(StatusAssertions.java:59)
	at org.springframework.test.web.reactive.server.StatusAssertions.isEqualTo(StatusAssertions.java:51)
	at com.geborskimateusz.microservices.core.movie.MovieServiceApplicationTests.postAndVerify(MovieServiceApplicationTests.java:111)
	at com.geborskimateusz.microservices.core.movie.MovieServiceApplicationTests.getMovieNotFound(MovieServiceApplicationTests.java:58)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:515)
	at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:115)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:171)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:72)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:167)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:114)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:59)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:105)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:72)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:95)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:71)
	at java.util.ArrayList.forEach(ArrayList.java:1257)
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:110)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:72)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:95)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:71)
	at java.util.ArrayList.forEach(ArrayList.java:1257)
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:110)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:72)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:95)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:71)
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
	at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
	at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:220)
	at org.junit.platform.launcher.core.DefaultLauncher.lambda$execute$6(DefaultLauncher.java:188)
	at org.junit.platform.launcher.core.DefaultLauncher.withInterceptedStreams(DefaultLauncher.java:202)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:181)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:128)
	at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:69)
	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: java.lang.AssertionError: Status expected:<200> but was:<500>
	at org.springframework.test.util.AssertionErrors.fail(AssertionErrors.java:55)
	at org.springframework.test.util.AssertionErrors.assertEquals(AssertionErrors.java:82)
	at org.springframework.test.web.reactive.server.StatusAssertions.lambda$isEqualTo$0(StatusAssertions.java:59)
	at org.springframework.test.web.reactive.server.ExchangeResult.assertWithDiagnostics(ExchangeResult.java:197)
	... 43 more

Why is that happening?, cant I block value or is it something wrong with post code?
I am totally new to this reactive abstracion, such implementation of WebTestClient post worked with non reactive approach. Here is code that fails:

Here postAndVerify fails:

@ExtendWith(SpringExtension.class)
@SpringBootTest(webEnvironment = RANDOM_PORT, properties = {"spring.data.mongodb.port: 0"})
public class MovieServiceApplicationTests {

    @Autowired
    WebTestClient webTestClient;

    @Autowired
    MovieRepository movieRepository;


    @Test
    public void getMovie() {
        Integer given = 1;

        postAndVerify(given, HttpStatus.OK);


        getAndVerify(given, HttpStatus.OK)
                .jsonPath("$.movieId").isEqualTo(given)
                .jsonPath("$.genre").isNotEmpty()
                .jsonPath("$.title").isNotEmpty()
                .jsonPath("$.address").isNotEmpty();

    }

    private WebTestClient.BodyContentSpec postAndVerify(Integer id, HttpStatus httpStatus) {
        Movie movie = Movie.builder()
                .movieId(id)
                .title("Title for movie " + id)
                .genre("Genre for movie " + id)
                .address("Address for movie " + id)
                .build();

        return webTestClient.post()
                .uri("/movie")
                .body(Mono.just(movie), Movie.class)
                .accept(MediaType.APPLICATION_JSON_UTF8)
                .exchange()
                .expectStatus().isEqualTo(httpStatus)
                .expectHeader().contentType(MediaType.APPLICATION_JSON_UTF8)
                .expectBody();
    }

}

And implementation of create method looks like this:

@Override
public Movie createMovie(Movie movie) {
    MovieEntity movieEntity = movieMapper.apiToEntity(movie);

    Mono<Movie> movieMono = movieRepository.save(movieEntity)
            .onErrorMap(DuplicateKeyException.class, ex -> new InvalidInputException("Duplicate key for movieId: " + movie.getMovieId()))
            .log()
            .map(movieMapper::entityToApi);

    return movieMono.block();
}

The reactive approach is used in non-blocking applications. You choose to use reactive because you don't want to block, and still you do it.

The solution is simple. You should never call block in a reactive application.

return movieMono.block();

From your createMovie you should return a Mono<Movie> and then keep chaining on. But since you have not provided your implementation of who is calling createMovie i can't help you more than this.

Please, read this answer. It should help.

As Thomas mentioned, blocking a non-blocking API is't a good idea.