I have a method in a service that I want to test and which uses MongoTemplate as follows:

@Service
public class MongoUpdateUtilityImpl implements MongoUpdateUtility {
    private final MongoTemplate mongoTemplate;
    
    @Autowired
    MongoUpdateUtilityImpl (final MongoTemplate mongoTemplate) {
        this.mongoTemplate = mongoTemplate;
    }

    @Override
    public Object update(final String id, final Map<String, Object> fields, final Class<?> classType) {
        ...
        this.mongoTemplate.updateFirst(query, update, classType);
        return this.mongoTemplate.findById(id, classType);
    }       
}

Then I'm trying to test this method with mocked methods for mongo template:

@SpringBootTest
@RunWith(SpringJUnit4ClassRunner.class)
@ActiveProfiles("test")
public class MongoUpdateUtilityTest {
    @MockBean
    private MongoTemplate mongoTemplate;

    @Autowired
    private MongoUpdateUtility mongoUpdateUtility;

    /**
     * Test en el que se realiza una actualización correctamente.
     */
    @Test
    public void updateOK1() {
        final Map<String, Object> map = new HashMap<>();

        map.put("test", null);
        map.put("test2", "value");

        when(mongoTemplate.updateFirst(Mockito.any(Query.class), Mockito.any(Update.class), Mockito.any(Class.class)))
                .thenReturn(null);
        when(mongoTemplate.findById(Mockito.anyString(), Mockito.any(Class.class))).thenReturn(null);

        assertNull(this.mongoUpdateUtility.update("2", map, Map.class));
    }
}

I have read this question but when I try the answer marked as solution, it says that MongoTemplate cannot be initialized. I'd prefer to mock this than to use and embebded database since I'm limited on the libraries that I can use.

My error:

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'operacionesPendientesRepository' defined in es.santander.gdopen.operpdtes.repository.OperacionesPendientesRepository defined in @EnableMongoRepositories declared on MongoRepositoriesRegistrar.EnableMongoRepositoriesConfiguration: Invocation of init method failed; nested exception is java.lang.NullPointerException