With InjectMocks, one of my mock become null, without it, an other Mock is null

Question

So in this version, repository is null (and metric is not null) :

@Mock
OperationRepository repository;
@Mock
Metric metric;

SQLStrategy sqlStrategy;

@BeforeEach
public void setup() {
    MockitoAnnotations.initMocks(this);
    sqlStrategy = new SQLStrategy(metric);
}

In this version, metric is null (and repository is not null) :

@Mock
OperationRepository repository;
@Mock
ReneMetric reneMetric;

@BeforeEach
public void setup() {
    MockitoAnnotations.initMocks(this);
}

@InjectMocks
private SQLStrategy sqlStrategy = new SQLStrategy(reneMetric);

This is how they are defined in the class :

@Slf4j
@RequiredArgsConstructor
@Component("sqlDb")
public class SQLStrategy extends StorageStrategy {

@Autowired
private OperationRepository repository;

private final Metric metric;

I was wondering what could be doing this behavior?

I can understand the need for @InjectMocks for repository to not be null since its an @autowired But I really dont understand why metric become null when I use @InjectMocks.

Answer 1

First of all , @InjectMocks is a Mockito stuff and is nothing to do with Spring. It has no ideas about @Autowired and its existence will not affect @InjectMocks 's behaviour.

Normally you do not need to manually create the instance that you want to inject mocks into. If that instance has non no-arg constructor , Mockito will choose biggest constructors to create instance and inject the mocks through this constructor.

But in case the instance has a no-arg constructor or you instantiate it manually, Mockito will first use property setter injection , and then field injection to inject mocks.

And one caveat is that for the final field , field injection will not happen . You can refer to the docs for more information.

So change to the following should inject both mocks into SQLStrategy :

@ExtendWith(MockitoExtension.class)
public class SQLStrategyTest {
    
    @Mock
    OperationRepository repository;

    @Mock
    ReneMetric reneMetric;


    @InjectMocks
    private SQLStrategy sqlStrategy;


    @Test
    public void test(){
    }


}

@Slf4j
@RequiredArgsConstructor
@Component("sqlDb")
public class SQLStrategy extends StorageStrategy {

    public SQLStrategy(OperationRepository repo , Metric metric){
        this.repo = repo;
        this.metric = metric;
    }
}

And the following explains why your example get such result :

1. You manually instantiate SQLStrategy . But at the moment when it is instantiated , Mockito still not create a ReneMetric mock , so new SQLStrategy(null) is called to create SQLStrategy

2. Mockito create a mocked instance of OperationRepository and ReneMetric

3. Since you manually create SQLStrategy , Mockito will try to use property setter injection , and then field injection to inject the mocks into it. For the OperationRepository , it is injected because of the field injection. For the Metric , it is not injected because it is a final field and SQLStrategy does not have any setter for it.

4. So OperationRepository is not null and Metric is null