I want to test a @Service class that is normally called with SpringApplication.run().
The service class is:
@Service
@EnableConfigurationProperties(AppProperties.class)
public class MongoService {
    private static final Logger logger = LoggerFactory.getLogger(MongoService.class);
    private MongoClient mongoClient;
    private final AppProperties properties;
    @Autowired
    public MongoService(AppProperties properties) {
        this.properties = properties;
    }
    /**
     * Open connection
     */
    public void openConnection() {
        try {
            mongoClient = new MongoClient(new MongoClientURI(properties.getMongoConnectionString()));
        } catch (Exception e) {
            logger.error("Cannot create connection to Search&Browse database", e);
            throw new BackendException("Cannot create connection to Search&Browse database");
        }
    }
}
When it is called by the controller started with SpringApplication.run(), the MongoService is not null but, when I try from a jUnit it's not working.
So, I'm trying this:
@RunWith(SpringRunner.class)
@SpringBootTest
@ContextConfiguration(classes = AppProperties.class)
public class MongoServiceTest {
    private static final Logger logger = LoggerFactory.getLogger(MongoServiceTest.class);
    @Autowired
    MongoService mongoService;
    @Test
    public void MongoServiceAutowired() {   
        assertNotNull(mongoService);
    }
}
but I'm getting this exception:
org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'mypackage.MongoServiceTest': Unsatisfied dependency expressed through field 'mongoService'; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'mypackage.services.mongo.MongoService' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
Any clue? Where am I failing?
To check the Service class, we need to have an instance of the Service class created and available as a @Bean so that we can @Autowire it in our test class. We can achieve this configuration using the @TestConfiguration annotation.
It is used to mark the class as a service provider. So overall @Service annotation is used with classes that provide some business functionalities. Spring context will autodetect these classes when annotation-based configuration and classpath scanning is used.
I assume that your AppProperties and MongoService is not in the same package
If not you can inject MongoService in that way : 
create another class named TestConfiguration
@ComponentScan(basePackageClasses = {
        MongoService.class,
        AppProperties.class
})
@SpringBootApplication
public class TestConfiguration {
    public static void main(String[] args) {
        SpringApplication.run(TestConfiguration.class, args);
    }
}
And in the test just change to :
@RunWith(SpringRunner.class)
@SpringBootTest(classes = TestConfiguration.class) 
public class MongoServiceTest {
    private static final Logger logger = LoggerFactory.getLogger(MongoServiceTest.class);
    @Autowired
    MongoService mongoService;
    @Test
    public void MongoServiceAutowired() {   
        assertNotNull(mongoService);
    }
}
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