JUnit testing a method that returns Flux<String>

Question

The method in the service layer that is being tested below...:

@Service 
public class Service{

public Flux<String> methodBeingTested ()  {

Flux<String> fluxOfTypeString = Flux.just("test");

return fluxOfTypeString;
}
}

The JUnit Test:

@RunWith(MockitoJUnitRunner.class)
public class ServiceTest {

@InjectMocks
Service service;

@Mock
Utility Util;     
      
 @Before
    public void setUp() {
        MockitoAnnotations.initMocks(this);
    }

   @Test
    public void test() {

        Flux<String> fluxOfTypeString = Flux.just("test"); 

       when(service.methodToTest()).thenReturn(fluxOfTypeString);
}

In Debug mode I get the following error:

org.mockito.exceptions.misusing.WrongTypeOfReturnValue: 
FluxJust cannot be returned by toString()
toString() should return String

And when not in debug mode I get the following error:

org.mockito.exceptions.misusing.MissingMethodInvocationException: 
when() requires an argument which has to be 'a method call on a mock'.

Any ideas on what a possible workaround for this is? any help would be much appreciated.

Answer 1

You don't need Mockito at all to test methodToTest(), since no third party classes (like Utility, which you mock in your ServiceTest) are called:

import org.junit.jupiter.api.Test;
import reactor.core.publisher.Flux;
import reactor.test.StepVerifier;

class MyTest {

    private Service service = new Service();

    @Test
    void name() {    
        Flux<String> actual = service.methodToTest();

        StepVerifier.create(actual)
                .expectNext("test")
                .verifyComplete();
    }
}

More information about testing reactive streams can be found in this article: https://www.baeldung.com/reactive-streams-step-verifier-test-publisher