@Mock doesn't do anything by itself. You need to have something to activate it:
* In JUnit 4, you would annotate your test class with @RunWith(MockitoJUnitRunner.class).
* In JUnit 5, you would annotate your test class with @ExtendWith(MockitoExtension.class). You need this additional dependency.
* If you can't use either, explicitly call MockitoAnnotations.initMocks(this) in your test class. This would preferably be done in an @Before / @BeforeEach method.