ModelMapper throws NPE using JUnit Mockito

I'm having a NPE using ModelMapper

CatalogServiceTest

@RunWith(SpringJUnit4ClassRunner.class)
@SpringBootTest
public class CatalogServiceTest {
@Rule
public ExpectedException thrown = ExpectedException.none();
@InjectMocks private CatalogService service;
@Mock ModelMapper modelMapper;
@Mock CatalogMapper catalogMapper;
@Mock CatalogRepository catalogRepository;
@Before
public void setUp() throws Exception {
//        MockitoAnnotations.initMocks(this);
CatalogEntity catalogEntity = new CatalogEntity();
catalogEntity.setId("id");
catalogEntity.setCode("code");
catalogEntity.setType("type");
catalogEntity.setValue("value");
//        Optional<CatalogEntity> optionalCatalog = Optional.of(catalogEntity);
when(catalogRepository.findByCode(any(String.class))).thenReturn(catalogEntity);
}
@Test
public void whenFindByCode() {
//Act
CatalogDto myCatalogDto = service.findByCode("code");
//Assert
assertTrue(myCatalogDto.getCode().equals("code"));
}
}

CatalogService

@Service
public class CatalogService {
private static final Logger LOGGER = LoggerFactory.getLogger(CatalogService.class);
@Autowired
CatalogRepository catalogRepository;
@Autowired
CatalogMapper catalogMapper;
/**
* 
* @param type
* @return catalog objects which type is type
*/
public List<CatalogDto> findByType(String type) {
LOGGER.info("Getting catalogs by type {}", type);
List<CatalogEntity> catalogsEntityList = catalogRepository.findByType(type);
List<CatalogDto> catalogDtoList = new ArrayList<>();
catalogsEntityList.forEach(catalogEntity -> {
catalogDtoList.add(catalogMapper.convertCatalogEntityToCatalogDto(catalogEntity));
});
return catalogDtoList;
}
/**
* Find catalog by code.
* @param code
* @return catalog
*/
public CatalogDto findByCode(String code) {
LOGGER.info("Getting catalogs by code {}", code);
return catalogMapper.convertCatalogEntityToCatalogDto(catalogRepository.findByCode(code));
}
}

CatalogMapper

@Component
public class CatalogMapper {
@Autowired
private ModelMapper modelMapper;
/**
* Converts CatalogEntity object to CatalogDto object
* @param catalogEntity
* @return converted CatalogDto object
*/
public CatalogDto convertCatalogEntityToCatalogDto(CatalogEntity catalogEntity) {
return modelMapper.map(catalogEntity, CatalogDto.class);
}
}

CatalogRepository

@Repository
public interface CatalogRepository extends MongoRepository<CatalogEntity, String> {
List<CatalogEntity> findByType(String type);
CatalogEntity findByCode(String code);
}

The problem

The catalogRepository.findByCode(code) is returning a CatalogEntity object as expected and the problem comes after executing catalogMapper.convertCatalogEntityToCatalogDto(catalogRepository.findByCode(code)); that return null.

I'm using Intellij and this is the breakpoint just right before execute catalogMapper.convertCatalogEntityToCatalogDto function.

The catalogMapper is a mock with no stubbed methods.

There are a few ways in which you can fix your test:

Option 1: only test interaction with CatalogMapper

In this option, you stub a call to catalogMapper.convertCatalogEntityToCatalogDto. This is a thin unit test, you only test interactions with collaborating services.

As you said you want to test real implementation of mapper, there are two options:

Option 2: use SpringBootTest

In this option, you rely on SpringBootTest to set up your entire application context.

You need following changes:

• use @Autowired instead of @InjectMock to get you object under test
• use @MockBean instead of @Mock for repository. SpringBootTest ignores @Mock.
• get rid of other mocks
• as it creates entire application context, I would exclude this option, unless a full integration test is your goal (you started with @SpringBootTest in your code)

@SpringBootTest
public class CatalogServiceTest {

    @Rule
    public ExpectedException thrown = ExpectedException.none();
    @Autowired
    private CatalogService service;
    @MockBean
    CatalogRepository catalogRepository;

}

Option 3: construct services you need yourself

• get rid of @SpringBootTest
• mock only objects you want to mock - the repository
• create real objects for other services
• you may need to change field injection to constructor injection in your services, which is a good idea anyway

@Service
public class CatalogService {
final CatalogRepository catalogRepository;
final CatalogMapper catalogMapper;
@Autowired
public CatalogService(CatalogRepository catalogRepository, CatalogMapper catalogMapper) {
this.catalogRepository = catalogRepository;
this.catalogMapper = catalogMapper;
}
}

This approach creates only objects that are used by your test, not entire application context, so will likely result in leaner test that option 2.

@RunWith(MockitoJUnitRunner.class)
public class CatalogServiceTest {

    @Rule
    public ExpectedException thrown = ExpectedException.none();

    private CatalogService service;
    @Mock
    CatalogRepository catalogRepository;


    @Before
    public void setUp() throws Exception {
        var modelMapper = new ModelMapper();
        var catalogMapper =new CatalogMapper(modelMapper);
        service = new CatalogService(catalogRepository, catalogMapper);

        CatalogEntity catalogEntity = new CatalogEntity();
        catalogEntity.setId("id");
        catalogEntity.setCode("code");
        when(catalogRepository.findByCode(any(String.class))).thenReturn(catalogEntity);
    }

}