Services with business logic need to be tested for their correct functionality. In order to do this, Services like the following one can be used:

@RunWith(SpringRunner.class)
public class CommentServiceTest {

    @MockBean
    private CommentRepository commentRepository;

    private CommentService commentService;

    @Before
    public void init() {
        commentService = new CommentService(commentRepository);
    }

    @Test
    public void 
       getAllCommentsForToday_HappyPath_ShouldReturn1Comment() {
        // Given
        Comment comment = new Comment();
        comment.setComment("Test");
        comment.setType(CommentType.PLUS);
        comment.setCreatedDate(new 
            Timestamp(System.currentTimeMillis()));
        List<Comment> comments = Arrays.asList(comment);
        LocalDate now = LocalDate.now();
        

        when(commentRepository.findByCreatedYearAndMonthAndDay(now.getYear(), now.getMonth().getValue(), 
         now.getDayOfMonth())).thenReturn(comments);

        // When
        List<Comment> actualComments = 
         commentService.getAllCommentsForToday();

        // Then
        verify(commentRepository, 
          times(1)).findByCreatedYearAndMonthAndDay(now.getYear(), 
          now.getMonth().getValue(), now.getDayOfMonth());
        assertThat(comments).isEqualTo(actualComments);
    }

    @Test
    public void saveAll_HappyPath_ShouldSave2Comments() {
        // Given
        Comment comment = new Comment();
        comment.setComment("Test Plus");
        comment.setType(CommentType.PLUS);
        comment.setCreatedBy("Shazin");
        comment.setCreatedDate(new 
          Timestamp(System.currentTimeMillis()));

        Comment comment2 = new Comment();
        comment2.setComment("Test Star");
        comment2.setType(CommentType.STAR);
        comment2.setCreatedBy("Shahim");
        comment2.setCreatedDate(new  
          Timestamp(System.currentTimeMillis()));
        List<Comment> comments = Arrays.asList(comment, comment2);
        when(commentRepository.saveAll(comments)).thenReturn(comments);

        // When
        List<Comment> saved = commentService.saveAll(comments);

        // Then
        assertThat(saved).isNotEmpty();
        verify(commentRepository, times(1)).saveAll(comments);

    }
}

In the preceding test case for CommentService, CommentRepository is annotated with @MockBean, as testing of its functionality has already been done in the JPA repository testing. During Service, test mocking is done using the Mockito library to just mock repository method invocations and verify the correct invocation.

The following is the service test case for UserService:

@RunWith(SpringRunner.class)
public class UserServiceTest {

    @MockBean
    private UserRepository userRepository;

    private UserService userService;

    @Before
    public void init() {
        this.userService = new UserService(userRepository);
    }

    @Test
    public void getAllCommentsForToday_HappyPath_ShouldReturn1Comment() {
        // Given
        User user = new User();
        user.setUsername("shazin");
        user.setPassword("sha908");
        user.setRole("USER");

        when(userRepository.findByUsername("shazin")).thenReturn(user);

        // When
        UserDetails actual = userService.loadUserByUsername("shazin");

        // Then
        verify(userRepository, times(1)).findByUsername("shazin");
    }

}

In the preceding test case, the UserRepository.findByUsername method is mocked to return a given user and finally verify whether that method is invoked exactly once.