I got a problem might not be possible to resolve but I still want to see if anyone has any suggestion.

Please see the MyService and MyServiceTest Java classes below.

I tried to make the restTemplate mock persist as there is a line of code in my actual method (demo using the callExternalService()). Meaning, when I run the test, I want to have the restTemplate.exchange to always return the responseEntityTest dummy values.

Is it possible? right now, there always a null pointer exception since the “this restTemplate = null” will erase the mock.

Thank you!

*/imports....*

@Service
public class MyService {

    private RestTemplate restTemplate;

    @Autowired
    public MyService(RestTemplate restTemplate) {
        this.restTemplate = restTemplate;
    }

    public String callExternalService() {
        String url = "https://api.example.com/resource";
        this.restTemplate = null; //just for demo the problem
        HttpEntity<String> requestEntity = new HttpEntity<>(null);
        ResponseEntity<String> response = restTemplate.exchange(url, HttpMethod.GET, requestEntity, String.class);
        return response.getBody();
    }
}

*/some imports*

public class MyServiceTest {

    @Mock
    private RestTemplate restTemplateMock;

    @InjectMocks
    private MyService myService;

    @BeforeEach
    public void setUp() {
        MockitoAnnotations.openMocks(this);
    }

    @Test
    public void testCallExternalService() {
        // Mocking the RestTemplate's behavior
        ResponseEntity<String> responseEntityTest = new ResponseEntity<>("Mocked Response", HttpStatus.OK);

        when(restTemplateMock.exchange(
                anyString(),
                any(HttpMethod.class),
                any(HttpEntity.class),
                any(Class.class)
        )).thenReturn(responseEntityTest);

        // Calling the method under test
        String response = myService.callExternalService();

        // Verifying the response
        assertEquals("Mocked Response", response);
    }


}

I tried to make the restTemplate mock persist as there is a line of code in my actual method (demo using the callExternalService()). Meaning, when I run the test, I want to have the restTemplate.exchange to always return the responseEntityTest dummy values.

Is it possible? right now, there always a null pointer exception since the “this restTemplate = null” will erase the mock.