Autowired fields of mocked object returns null

有一个如下的StudentService类：

@Component
@EnableAutoConfiguration
public class StudentService {

@Autowired
StudentInstitutionMapper studentInstitutionMapper;

public Integer getPresentStudentCount(StudentParams studentParam) {
    // TODO Auto-generated method stub
    StudentInstitutionExample example = new StudentInstitutionExample();
    StudentInstitutionExample.Criteria criteria = example.createCriteria();
    criteria.andActiveYnEqualTo("Y");
    criteria.andDeleteYnEqualTo("N");
    criteria.andIsPresentEqualTo("Y");
    criteria.andInstitutionTestIdEqualTo(studentParam.getInstitutionTestId());
    List<StudentInstitution> studentInstitutionList = studentInstitutionMapper.selectByExample(example);//line 8 in method
    return studentInstitutionList.size();
}

}

@Test
    public void testStudentService_getPresentStudentCount1()
    {



        StudentService service=new StudentService();
        StudentParams studentParam=mock(StudentParams.class);

        Integer institutionTestId=3539;
        when(studentParam.getInstitutionTestId()).thenReturn(institutionTestId);


        int i=service.getPresentStudentCount(studentParam);
        assertEquals(0,i);

    }

然后，当我在使用时出现了NullPointerException异常，异常原因是因为studentInstitutionMapper是null。这在我只有使用mock时候才会导致的。

原因分析是因为在使用Mock时候，studentInstitutionMapper不能够被自动装配。解决方法如下：

@ExtendWith(MockitoExtension.class)
class StudentServiceTest {

    @Mock
    private StudentInstitutionMapper studentInstitutionMapper;

    @InjectMocks
    private StudentService studentService;

    @Test
    public void testStudentService_getPresentStudentCount1() {


        StudentParams studentParam = mock(StudentParams.class);

        Integer institutionTestId = 3539;
        when(studentParam.getInstitutionTestId()).thenReturn(institutionTestId);


        int i = studentService.getPresentStudentCount(studentParam);
        assertEquals(0, i);

    }
}

我们需要用@Mock标签去注释studentInstitutionMapper，这样子mock也能够使用了。也不会抛异常了。