Mapstruct mapper using mockito returning null object inspite being mocked

I have a mapper which converts one type to another type. I am writing a unit test forbusiness (service layer) where I am autowiring mapper. In the corresponding Junit of service class I am mocking mapper using Mock annotation. While debugging I could see that mapper is getting mocked but after debugging goes to service layer the mocked mapper method is returning null object

Mapper class

@Mapper(componentModel = "spring")
public interface EmployeeHolidayMapper {

	@Mapping(target = "employeeEntity.empId",source = "employeeHolidayDto.empId")
	@Mapping(target = "leaveStartDate",expression = "java(LocalDate.parse(employeeHolidayDto"
			+ ".getEmpLeaveStartDate().toString(),dateTimeFormatter))")
	@Mapping(target = "leaveEndDate",expression = "java(LocalDate.parse(employeeHolidayDto"
			+ ".getEmpLeaveEndDate().toString(),dateTimeFormatter))")
	public EmployeeHolidayEntity employeeHoliDayDtoToEmployeeHolidayEntity(EmployeeHolidayDto 
			employeeHolidayDto,LocalDate localDate,DateTimeFormatter dateTimeFormatter);

Service class

@Service
@RequiredArgsConstructor
public class EmployeeHolidayService {

	
	private final HolidayRepo holidayRepo;
	
	private final EmployeeHolidayRepo employeeHolidayRepo;
	
	private final EmployeeHolidayMapper employeeHolidayMapper;
	

	public Mono<Integer> saveEmployeeHolidayData(EmployeeHolidayDto employeeHolidayDto){
		return	Optional.ofNullable(employeeHolidayDto)
		.filter(dto->dto.getEmpLeaveEndDate()!=null && dto.getEmpLeaveStartDate() != null 
		   && dto.getEmpLeaveStartDate() > 0 && dto.getEmpLeaveEndDate() > 0 && dto.getEmpLeaveEndDate()
		   .compareTo(dto.getEmpLeaveStartDate()) >=0)
		.map(dto->{
				EmployeeHolidayEntity employeeHolidayEntity = employeeHolidayMapper
						.employeeHoliDayDtoToEmployeeHolidayEntity(dto, LocalDate.now(),
								DateTimeFormatter.BASIC_ISO_DATE);
				int diffOfDays = employeeHolidayEntity.getLeaveEndDate().getDayOfYear()
						- employeeHolidayEntity.getLeaveStartDate().getDayOfYear();
				int noOfLeavesLeft = holidayRepo.getNoofLeavesLeft(dto.getEmpId()) - (diffOfDays);
				if (noOfLeavesLeft > 0) {
					int updatedRows = holidayRepo.updateNoLeaveDays(dto.getEmpId(), noOfLeavesLeft);
					if (updatedRows > 0) {
						EmployeeHolidayEntity employeeHolidayEntitySaved = employeeHolidayRepo
								.saveAndFlush(employeeHolidayEntity);
						if (employeeHolidayEntitySaved.getEmployeeHolidayId() > 0) {
							return Mono.just(updatedRows);
						} else {
							return Mono.just(0);
						}
					} else {
						return Mono.just(0);
					}
				}
				else {
					return Mono.just(0);
				}
			
		})
		.orElse(null);
	}

Junit class using mockito

@ExtendWith(MockitoExtension.class)
class HolidaymanagementApplicationTests {

	@Mock
	HolidayRepo holidayRepo;

	@Mock
	EmployeeHolidayRepo employeeHolidayRepo;

	@Mock
	EmployeeHolidayMapper employeeHolidayMapper;

	@InjectMocks
	EmployeeHolidayService employeeHolidayService;

	@Test
	void testPostrequest() {
		String startDate = LocalDate.now().format(DateTimeFormatter.BASIC_ISO_DATE);
		String endDate = LocalDate.now().plusDays(2).format(DateTimeFormatter.BASIC_ISO_DATE);
		EmployeeHolidayDto employeeHolidayDto = new EmployeeHolidayDto(1, Integer.parseInt(startDate),
				Integer.parseInt(endDate));
		EmployeeEntity employeeEntity = new EmployeeEntity(1, null, null, null, null);
		EmployeeHolidayEntity employeeHolidayEntity = new EmployeeHolidayEntity(1,
				LocalDate.parse(startDate, DateTimeFormatter.BASIC_ISO_DATE),
				LocalDate.parse(endDate, DateTimeFormatter.BASIC_ISO_DATE), employeeEntity);
		when(employeeHolidayMapper.employeeHoliDayDtoToEmployeeHolidayEntity(employeeHolidayDto, LocalDate.now(),
				DateTimeFormatter.BASIC_ISO_DATE)).thenReturn(employeeHolidayEntity);
		HolidayEntity holidayEntity = new HolidayEntity(1, 10, employeeEntity);
		when(holidayRepo.getNoofLeavesLeft(1)).thenReturn(10);
		when(holidayRepo.updateNoLeaveDays(1, 8)).thenReturn(1);
		EmployeeHolidayEntity employeeHolidayDbEntity = new EmployeeHolidayEntity(1,
				employeeHolidayEntity.getLeaveStartDate(), employeeHolidayEntity.getLeaveEndDate(), employeeEntity);
		when(employeeHolidayRepo.saveAndFlush(employeeHolidayEntity)).thenReturn(employeeHolidayDbEntity);
		assertEquals(employeeHolidayService.saveEmployeeHolidayData(employeeHolidayDto).block(), 
				1);

In the above code while debugging I could see that mapper is mocked. Now when implementation goes to service class from asserEquals statement. The EmployeeHolidayEntity object fetched through mapper is returning null.

When running code a spring boot application mapper is working fine and is returning the object.

Below is my entity class.

@Entity
@Table(name = "empl_holiday_tbl")
@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@EqualsAndHashCode
public class EmployeeHolidayEntity implements Serializable{

	/**
	 * 
	 */
	private static final long serialVersionUID = 2429371741034836686L;
	
	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	@Column(name = "id", columnDefinition = "INT UNSIGNED AUTO_INCREMENT")
	private Integer employeeHolidayId;
	
	@Column(name = "startDate", columnDefinition = "DATE NOT NULL")
	private LocalDate leaveStartDate;
	
	@Column(name = "endDate", columnDefinition = "DATE NOT NULL")
	private LocalDate leaveEndDate;
	
	@ManyToOne
	@JoinColumn(name = "empId",nullable = false,columnDefinition = "INT UNSIGNED")
	private EmployeeEntity employeeEntity;

}

Your problem is that your mock when( never matches the execution.

 when(employeeHolidayMapper.employeeHoliDayDtoToEmployeeHolidayEntity(employeeHolidayDto, LocalDate.now(),
            DateTimeFormatter.BASIC_ISO_DATE)).thenReturn(employeeHolidayEntity);

contains a "LocalDate.now()" your actuall execution is a few milisenconds later. It will therefore be different and the when( never is used. The fallback of a mocked class is to return null which is your result.

To verify my suspicion you could try to change your code to

when(employeeHolidayMapper.employeeHoliDayDtoToEmployeeHolidayEntity(any(), any(),
            any())).thenReturn(employeeHolidayEntity);

your return value should now be correct.

Furthermore i would suggest changing your test setup. For Mockito tests with Mapstruct I usually use

@Spy
EmployeeHolidayMapper employeeHolidayMapper = Mappers.get(EmployeeHolidayMapper .class);

@InjectMocks
EmployeeHolidayService employeeHolidayService;

this way you get the real mapper since mapping is something that usually dont need mocking. You then don´t have to call when( on mappers in your tests.