Mocked repository returns null

I believe you missed the whole idea of unit testing and mocking.

1. When you are unit testing your UserService, you DO NOT want to use the real UserRepository implementation.
2. You mocked a UserRepository, you do not expect the mock object to immediately behave as the real one. You need to make up its behavior (aka stubbing).
3. You should rarely need to use Spring Runner in unit test.

In order to decide the behavior of the mock object, you gotta know the expected interaction of your system-under-test (SUT, which is the UserService in your case) and its dependencies (UserRepository)

In your case, the test should looks like (haven't compiled, just show you the idea)

public class UserServiceTest {

    @InjectMocks
    UserService userService;

    @Mock
    UserDao mockUserRepository;

    @Before
    public void setUp(){
        MockitoAnnotations.initMocks(this);
    }

    @Test
    public void testFindUser(){
        // Given
        User dummyUser = new User();
        when(mockUserRepository.findByLoginAndPassword(anyString(), anyString()).thenReturn(dummyUser);

        // When
        User result = userService.findUser("first", "teste");

        // Then
        // you are expecting service to return whatever returned by repo
        assertThat("result", result, is(sameInstance(dummUser)));

        // you are expecting repo to be called once with correct param
        verify(mockUserRepository).findByLoginAndPassword("first", "teste");
    }
}

Also you can pass name parameter like ,

 @MockBean(name="userRepository")
 UserDao userRepository;

You need to revise your order of mocking first you should use @InjectMocks and then @Mock and before your test class name add @ExtendWith(MockitoExtension.class)

I was facing the same issue but after implementing the above steps it worked for me