Junit to test a Class with Constructor Injection.

@InjectMocks doesn’t work to test a class with Constructor Injection.

Lets see the class to test:

@Component
public class MyService {

    private final MyDependency myFirstDependency;
    private final MyDependency mySecondDependency;

    MyService(@Qualifier("MyFirstDependencyBean") final MyDependency myFirstDependency,
              @Qualifier("MySecondDependencyBean") final MyDependency mySecondDependency){
        this.myFirstDependency = myFirstDependency;
        this.mySecondDependency = mySecondDependency;
    }

    public String myOperation(){
        if(myFirstDependency.toString().contains("First")){
            System.out.println("First Bean available");
            return "First";
        }
        if(mySecondDependency.toString().contains("Second")){
            System.out.println("Second Bean available");
            return "Second";
        }
        return "None";
    }

}

Junit that will fail with @InjectMocks is provided below. The Junits will fail randomly and Mockito when will not be applied properly.

@RunWith(SpringRunner.class)
public class MyServiceTest {

    @Mock
    private MyDependency myFirstDependency;

    @Mock
    private MyDependency mySecondDependency;

    @InjectMocks
    MyService myService;

    @Test
    public void testMyOperationFirstBean(){
        Mockito.when(myFirstDependency.toString()).thenReturn("First");
        Assert.assertEquals("First", myService.myOperation());
    }

    @Test
    public void testMyOperationSecondBean(){
        Mockito.when(myFirstDependency.toString()).thenReturn("Second");
        Mockito.when(mySecondDependency.toString()).thenReturn("Second");
        Assert.assertEquals("Second", myService.myOperation());
    }
}

Fix:

The fix is to not use @InjectMocks but to use the myService Constructor as shown below.

   MyService myService;

    @Before
    public void Setup(){
        myService = new MyService(myFirstDependency, mySecondDependency);
    }

The full Junit class is provided below.

@RunWith(SpringRunner.class)
public class MyServiceTest {

    @Mock
    private MyDependency myFirstDependency;

    @Mock
    private MyDependency mySecondDependency;

    MyService myService;

    @Before
    public void Setup(){
        myService = new MyService(myFirstDependency, mySecondDependency);
    }

    @Test
    public void testMyOperationFirstBean(){
        Mockito.when(myFirstDependency.toString()).thenReturn("First");
        Assert.assertEquals("First", myService.myOperation());
    }

    @Test
    public void testMyOperationSecondBean(){
        Mockito.when(myFirstDependency.toString()).thenReturn("Second");
        Mockito.when(mySecondDependency.toString()).thenReturn("Second");
        Assert.assertEquals("Second", myService.myOperation());
    }

}