JUnit testing a method that returns Flux<String>

Issue

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.

Solution

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

Answered By - Georgii Lvov