Using @InjectMocks to replace @Autowired field with a mocked implementation

I want to replace an autowired class of a service in my spring boot app with a mocked implementation of that class that I created specifically for testing.

I chose to create this mocked implementation because the behaviour of this class is too complicated to mock using mockito as it requires multiple other mocks itself.

I am not able to work out how to inject this mocked implementation into the service.

Here is a minimal example of the situation:

@Service
public class ServiceIWantToTestImpl implements ServiceIWantToTest{

    @Autowired
    ComplicatedDependency complicatedDependency;

    @Override
    public void methodUsingDependency(){
        String string = complicatedDependency.doSomething();
        System.out.println(string);
    }

}


public class MockComplicatedDependency implements ComplicatedDepencency{

    public MockComplicatedDependency(...){
        // Inject other mocked objects into this mock
    }

    public String doSomthing(){
        // This would be a mocked version of this function for testing
        return "test";
    }

}

@RunWith(MockitoJUnitRunner.class)
public class TestingTheService(){
    @InjectMock
    private static ServiceIWantToTest serviceIWantToTest = new ServiceIWantToTestImpl();

    @Mock
    ComplicatedDependency mockComplicatedDependency;

    @BeforeClass
    public static void init(){
        mockComplicatedDependency = new MockComplicatedDependency(...);
    }

    @Test
    public void testAttempt(){
        serviceIWantToTest.methodUsingDependency();  // This method calls complicatedDependency.doSomething() which does not run the mocked version in MockComplicatedDependency which I wanted to inject, and would always return null instead of the "test" string I put in this example.
    }

}