Application.java

@SpringBootApplication
public class Application implements CommandLineRunner {
    
    // Objeto para escrever no console de log
    private final Logger logger = LoggerFactory.getLogger(Application.class);
   
    // Injeção, instância automática do objeto
    @Autowired
    private DummyService dummyService;

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

    @Override
    public void run(String... strings) throws Exception {
        String foo = dummyService.foo();
        logger.info("DummyService retornou {}", foo);
    }
}

DummyService.java

@Service
public class DummyService {
    private final Logger logger = LoggerFactory.getLogger(DummyService.class);

    public String foo() {
        logger.debug("Foo foi chamado");
        return "bar";
    }
}