mock.method call returns null after stubbing - junit

Question

I am trying to test using Mockito
my class under test is
#Service
public class DynatraceAPIServiceImpl implements DynatraceAPIService {
private String apiUrl = "someurl";
private String apiToken = "sometoken";
#Override
public CreateCustomMetricResponse createCustomMetric(CreateCustomMetricRequest request) throws MonitoringException {
logger.info("Inside create custom metric");
if (request == null) {
logger.error("create metric request is null");
throw new MonitoringException("Create metric request is null");
}
String metricId = DynatraceConstants.METRIC_ID;
String displayName = request.getDisplayName();
CreateCustomMetricResponse response = httpUtils.postCustomMetric(apiUrl + "/v1/timeseries/" + metricId, apiToken, request);
if (response == null) {
logger.error("Error in creating custom metric with name : " + displayName);
throw new MonitoringException("Error in creating custom metric with name : " + displayName);
}
logger.info("Custom metric : " + displayName + " is created successfully.");
return response;
}
}
and my Test class is :
#RunWith(MockitoJUnitRunner.class)
public class DynatraceAPIServiceImplTest {
#InjectMocks
DynatraceAPIServiceImpl dynatraceAPIServiceImpl;
#Mock
DynatraceHttpUtils httpUtilsMock;
#Mock
DynatraceMonitoringUtils monitoringUtilsMock;
#Test(expected = MonitoringException.class)
public void createCustomMetricGetsNonNullResponse() throws MonitoringException {
CreateCustomMetricRequest mockRequest = CreateCustomMetricRequest.builder()
.displayName(DISPLAY_NAME)
.types(new String[] {"test-type"})
.build();
CreateCustomMetricResponse response = CreateCustomMetricResponse.builder()
.displayName(DISPLAY_NAME)
.types(new String[] {"test-type"})
.timeseriesId(TIMESERIES_ID)
.build();
boolean val = true;
when(monitoringUtilsMock.isValidMetricIdValue(anyString())).thenReturn(val);
when(httpUtilsMock.postCustomMetric(API_URL + "/v1/timeseries/" + METRIC_ID, API_TOKEN, mockRequest)).thenReturn(response);
CreateCustomMetricResponse actualRespnose = dynatraceAPIServiceImpl.createCustomMetric(mockRequest);
//verify(httpUtilsMock, times(1)).postCustomMetric(anyString(), anyString(), any(CreateCustomMetricRequest.class));
//assertEquals(actualRespnose.getDisplayName(), DISPLAY_NAME);
}
}
Here, when I execute the tests, it always end up having the response value to be null in line
CreateCustomMetricResponse response = httpUtils.postCustomMetric(apiUrl + "/v1/timeseries/" + metricId, apiToken, request);
Even if I have used when() statement to return response as I have created, it is returning null.
Really appreciate if someone can let me know what is wrong here. Thanks.

Answer 1

That normally happens when the params your production code uses differ from the ones that you stubbed the call with, an easy way to find out is to write the test like this
when(httpUtilsMock.postCustomMetric(any(), any(), any())).thenReturn(response);
CreateCustomMetricResponse actualRespnose = dynatraceAPIServiceImpl.createCustomMetric(mockRequest);
verify(httpUtilsMock).postCustomMetric(API_URL + "/v1/timeseries/" + METRIC_ID, API_TOKEN, mockRequest);
If you do that, you'll get a nicer error showing the difference between what your code did and what you verified it for
A better approach in general is to use 'strict stubs' so if your code does anything different to what you stubbed the mock for you'll get a nice error telling you what, where and why