Mockito测试UserService时UserMapper空指针问题求助

问题原因 #

@InjectMocks只会自动注入被@Mock或@Spy标记的依赖，你的UserMapper既没有被Mockito注解标记，也没有手动注入，所以测试时会出现null。

解决方案 #

方案1：手动实例化真实UserMapper并构造Service #

MapStruct会为UserMapper接口生成对应的实现类（比如UserMapperImpl），你可以直接创建真实的Mapper实例，通过构造函数手动注入到UserServiceImpl中：

@ExtendWith(MockitoExtension.class)
class UserServiceTest {
    private UserServiceImpl userService;
    @Mock
    private UserRepository userRepository;
    // 实例化真实的UserMapper
    private final UserMapper userMapper = UserMapper.INSTANCE; // 如果Mapper定义了INSTANCE常量，否则用new UserMapperImpl()

    @BeforeEach
    void setUp() {
        // 手动构造Service，注入Mock的Repository和真实的Mapper
        userService = new UserServiceImpl(userRepository, userMapper);
    }

    @Test
    public void findById_withValidUserId_returnsUser() {
        Integer userId = 1;
        User expectedUser = User.builder().id(userId).build();
        Mockito.when(userRepository.findById(Mockito.argThat(argument -> argument != null && argument >= 1)))
                .thenReturn(Optional.of(expectedUser));
        UserResponse userResponse = userService.findById(userId);

        Assertions.assertNotNull(userResponse);
        Assertions.assertEquals(expectedUser.getId(), userResponse.id());
    }
}

方案2：使用Spring Boot Test容器管理真实Mapper #

如果是Spring Boot项目，可借助Spring容器自动装配真实的UserMapper，同时用@MockBean替换UserRepository为Mock对象：

@SpringBootTest
class UserServiceTest {
    @Autowired
    private UserServiceImpl userService;
    @MockBean
    private UserRepository userRepository;

    @Test
    public void findById_withValidUserId_returnsUser() {
        Integer userId = 1;
        User expectedUser = User.builder().id(userId).build();
        Mockito.when(userRepository.findById(Mockito.argThat(argument -> argument != null && argument >= 1)))
                .thenReturn(Optional.of(expectedUser));
        UserResponse userResponse = userService.findById(userId);

        Assertions.assertNotNull(userResponse);
        Assertions.assertEquals(expectedUser.getId(), userResponse.id());
    }
}

这种方式无需手动处理Mapper实例，但会加载Spring容器，测试启动速度稍慢。

方案3：用@Spy注入真实UserMapper #

通过@Spy标记真实的Mapper实例，让Mockito自动注入到Service中：

@ExtendWith(MockitoExtension.class)
class UserServiceTest {
    @InjectMocks
    private UserServiceImpl userService;
    @Mock
    private UserRepository userRepository;
    @Spy
    private final UserMapper userMapper = UserMapper.INSTANCE; // 或new UserMapperImpl()

    @Test
    public void findById_withValidUserId_returnsUser() {
        Integer userId = 1;
        User expectedUser = User.builder().id(userId).build();
        Mockito.when(userRepository.findById(Mockito.argThat(argument -> argument != null && argument >= 1)))
                .thenReturn(Optional.of(expectedUser));
        UserResponse userResponse = userService.findById(userId);

        Assertions.assertNotNull(userResponse);
        Assertions.assertEquals(expectedUser.getId(), userResponse.id());
    }
}

内容的提问来源于stack exchange，提问作者IvanDx