I have:

Java Spring WEB app. Unit-testing MapStruct with JPA repository injected (JUnit, Mockito).

My problem:

I made my tests work, but I can’t understand how they work and I have doubts whether it is a good practice in unit-testing.

Long detailed story:

I am writing unit-tests for the MapStruct class (ExchangeRateMapper, or rather ExchangeRateMapperImpl).
This mapper has injected dependency – JPA repository (CurrencyRepository).

This is simplified view of mapper:

@Mapper(componentModel = "spring")
public abstract class ExchangeRateMapper {
    @Autowired
    private CurrencyRepository currencyRepository;

    // ...
}

I tried to write tests, mocking JPA repository.
That way it doesn’t work:

@ExtendWith(SpringExtension.class)
@ExtendWith(MockitoExtension.class)
@SpringBootTest(classes = {ExchangeRateMapperImpl.class})
public class ExchangeRateMapperTests {
    @Mock
    CurrencyRepository currencyRepository;

    @InjectMocks
    @Autowired
    ExchangeRateMapper exchangeRateMapper;

    // ...

    @Test
    void exchangeRateXMLOToExchangeRateDOTest() {
        Mockito.when(currencyRepository.findByCode("EUR")).thenReturn(Optional.ofNullable(currencyEUR));
        Mockito.when(currencyRepository.findByCode("USD")).thenReturn(Optional.ofNullable(currencyUSD));
        ExchangeRateDO result = exchangeRateMapper.exchangeRateXMLOToExchangeRateDO(exchangeRateXMLO);
        Assertions.assertEquals(exchangeRateDO, result);
    }
}

However, when I changed @Mock to @MockBean (org.mockito.Mock to org.springframework.boot.test.mock.mockito.MockBean)

and @InjectMocks to @Inject (org.mockito.InjectMocks to jakarta.inject.Inject),

then it works:

@ExtendWith(SpringExtension.class)
@ExtendWith(MockitoExtension.class)
@SpringBootTest(classes = {ExchangeRateMapperImpl.class})
public class ExchangeRateMapperTests {
    @MockBean
    CurrencyRepository currencyRepository;

    @Inject
    @Autowired
    ExchangeRateMapper exchangeRateMapper;

    // ...

    @Test
    void exchangeRateXMLOToExchangeRateDOTest() {
        Mockito.when(currencyRepository.findByCode("EUR")).thenReturn(Optional.ofNullable(currencyEUR));
        Mockito.when(currencyRepository.findByCode("USD")).thenReturn(Optional.ofNullable(currencyUSD));
        ExchangeRateDO result = exchangeRateMapper.exchangeRateXMLOToExchangeRateDO(exchangeRateXMLO);
        Assertions.assertEquals(exchangeRateDO, result);
    }
}

Can somebody explain, please, why it works only that strange way?

Is this approach acceptable or, if not, what would be better solution to test such mapper?

I expected tests to work with @Mock and @InjectMocks. I guess that problem is in ExchangeRateMapper being abstract class and CurrencyRepository being interface, withal me wanting dependencies be injected automatically within Spring context. Still, I don’t get where exactly is problem.