How to mock ObjectMapper.readValue() using mockito

Question

I'm testing a service layer and not sure how to mock ObjectMapper().readValue in that class. I'm fairly new to mockito and could figure out how to do it.

The following is my code,

service.java

private configDetail fetchConfigDetail(String configId) throws IOException {
    final String response = restTemplate.getForObject(config.getUrl(), String.class);
    return new ObjectMapper().readValue(response, ConfigDetail.class);
}

ServiceTest.java

@Test
public void testgetConfigDetailReturnsNull() throws Exception {

    restTemplate = Mockito.mock(restTemplate.class);
    Service service = new Service();
    Config config = Mockito.mock(Config.class);
    ObjectMapper objMapper = Mockito.mock(ObjectMapper.class);
            Mockito.doReturn("").when(restTemplate).getForObject(anyString(), eq(String.class));
    Mockito.doReturn(configDetail).when(objMapper).readValue(anyString(),eq(ConfigDetail.class));
    assertEquals(configDetail, service.getConfigDetail("1234"));
}

I get the following results when I run this test,

com.fasterxml.jackson.databind.exc.MismatchedInputException: No content to map due to end-of-input
 at [Source: (String)""; line: 1, column: 0]

Posting ServiceTest.Java here

@RunWith(MockitoJUnitRunner.class)
public class ConfigServiceTest {

    @Mock
    private ConfigPersistenceService persistenceService;

    @InjectMocks
    private ConfigService configService;

    @Mock
    ConfigDetail configDetail;

    @Mock
    private RestTemplate restTemplate;

    @Mock
    private ObjectMapper objMapper;

    @Mock
    private Config config;

    @Test
    public void testgetConfigDetailReturnsNull() throws Exception {

        ObjectMapper objMapper = Mockito.mock(ObjectMapper.class);
        Mockito.doReturn(ucpConfig).when(persistenceService).findById("1234");

        Mockito.doReturn("").when(restTemplate).getForObject(anyString(), eq(String.class));

        Mockito.when((objMapper).readValue(“”,ConfigDetail.class)).thenReturn(configDetail);
        assertEquals(ConfigDetail, ConfigService.getConfigDetail("1234"));
    }
}

Answer 1

With your current Service class it would be difficult to mock ObjectMapper, ObjectMapper is tightly coupled to fetchConfigDetail method.

You have to change your service class as follows to mock ObjectMapper.

@Service
public class MyServiceImpl {

    @Autowired
    private ObjectMapper objectMapper;

    private configDetail fetchConfigDetail(String configId) throws IOException {
        final String response = restTemplate.getForObject(config.getUrl(), String.class);
        return objectMapper.readValue(response, ConfigDetail.class);
    }
}

Here what I did is instead of creating objectMapper inside the method I am injecting that from outside (objectMapper will be created by Spring in this case)

Once you change your service class, you can mock the objectMapper as follows.

ObjectMapper mockObjectMapper = Mockito.mock(ObjectMapper.class);
Mockito.when(mockObjectMapper.readValue(anyString(), any(ConfigDetail.class)).thenReturn(configDetail);