Clean code - Where should @Autowired be applied?

Question

I\'ll start with a simple example. You have a Spring boot application that runs a CommandLineRunner class on initialization.

// MyCommandLineRunner.         


        

Answer 1

Consider making the field ds final, then you don't need @Autowired. See more about dependency injection http://docs.spring.io/spring-boot/docs/current/reference/html/using-boot-spring-beans-and-dependency-injection.html#using-boot-spring-beans-and-dependency-injection

To keep the code clean, have you considered using Lombok annotations? @RequiredArgsConstructor(onConstructor = @__(@Autowired)) would generate the constructor with @Autowired annotations. See more here https://projectlombok.org/features/Constructor.html

Your code could look like this:

@Slf4j
@RequiredArgsConstructor
// MyCommandLineRunner.java
public class MyCommandLineRunner implements CommandLineRunner {

    //final fields are included in the constructor generated by Lombok
    private final DataSource ds;

    @Override
    public void run(String... args) throws Exception {
        log.info("DataSource: {} ", ds.toString());
    }
}

// Application.java
@SpringBootApplication
@RequiredArgsConstructor(onConstructor_={@Autowired}) // from JDK 8
// @RequiredArgsConstructor(onConstructor = @__(@Autowired)) // up to JDK 7
public class Application {

    private final Datasource ds;

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

    @Bean 
    public MyCommandLineRunner schedulerRunner() {
        return new MyCommandLineRunner(ds);
    }
}

Later edit

Solution without Lombok relies on Spring to inject dependency when the bean is created

@SpringBootApplication
public class Application {

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

    @Bean
    /**
     * dependency ds is injected by Spring
     */
    public MyCommandLineRunner schedulerRunner(DataSource ds) {
        return new MyCommandLineRunner(ds);
    }
}

// MyCommandLineRunner.java
public class MyCommandLineRunner implements CommandLineRunner {
    private final Log logger = LogFactory.getLog(getClass());

    private final DataSource ds;

    public MyCommandLineRunner(DataSource ds){
        this.ds = ds;
    }

    @Override
    public void run(String... args) throws Exception {
        logger.info("DataSource: "+ ds.toString());
    }
}