Mockito Inject mock into Spy object

Mockito cannot perform such a tricky injections as it's not an injection framework. So, you need to refactor your code to make it more testable. It's easy done by using constructor injection:

public class Engine{
    private Configuration configuration;

    @Inject 
    public Engine(Configuration configuration) {
        this.configuration = configuration;
    }
    ........
}

public class Car{
    private Engine engine;

    @Inject    
    public Car(Engine engine) {
        this.engine = engine;
    }
}

In this case you have to handle the mocking and injection manually:

public class CarTestCase{

    private Configuration configuration;

    private Engine engine;

    private Car car;

    @Before
    public void setUp(){
        configuration = mock(Configuration.class);
        engine = spy(new Engine(configuration));
        car = new Car(engine);
    }

    @Test
    public void test(){

       Mockito.when(configuration.getProperties("")).return("Something");
       car.drive();
    }

}

The (simplest) solution that worked for me.

@InjectMocks
private MySpy spy = Mockito.spy(new MySpy());

No need for MockitoAnnotations.initMocks(this) in this case, as long as test class is annotated with @RunWith(MockitoJUnitRunner.class).

I've also wandered how to inject a mock into a spy.

The following approach will not work:

@Spy
@InjectMocks
private MySpy spy;

But the desired behavior can be achieved by a "hybrid" approach, when using both annotation and manual mocking. The following works perfectly:

@Mock
private NeedToBeMocked needToBeMocked;

@InjectMocks
private MySpy mySpy;

@InjectMocks
private SubjectUnderTest sut;

@BeforeMethod
public void setUp() {
    mySpy = Mockito.spy(new MySpy());
    MockitoAnnotations.initMocks(this);
}

(SubjectUnderTest here depends on MySpy, and MySpy in its turn depends on NeedToBeMocked).

UPD: Personally, I think that if you have to do such a magic too often, it might be a sign that there is something wrong with dependenicies between your classes and it is worth to perform a little bit of refactoring to improve your code.

I also met this issue during the unit testing with Spring boot framework, but I found one solution for using both @Spy and @InjectMocks

Previous answer from Yoory N.

@Spy
@InjectMocks
private MySpy spy;

Because InjectMocks need to have instance created, so the solution works for me is at below,

@Spy
@InjectMocks
private MySpy spy = new MySpy();