Spring @Autowired Annotation with Example - RameshMF/spring-boot-developers-guide GitHub Wiki

In this article, we will discuss a very important Spring dependency injection annotation that is a @Autowired annotation.

We can use the @Autowired to mark a dependency which Spring is going to resolve and inject. We can use this annotation with a constructor, setter, or field injection.

Constructor Injection

@RestController
public class CustomerController {
	private CustomerService customerService;
	
	@Autowired
	public CustomerController(CustomerService customerService) {
		this.customerService = customerService;
	}
}

Setter Injection

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class CustomerController {
	private CustomerService customerService;
	
	@Autowired
	public void setCustomerService(CustomerService customerService) {
		this.customerService = customerService;
	}
}

Field Injection

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class CustomerController {
	@Autowired
	private CustomerService customerService;
}

Apply @Autowired Annotation to Different Scenarios

@Autowired annotation can be applied to different scenarios. Below are the few examples for the same.

Example: Apply the annotation to methods with arbitrary names and/or multiple arguments:

public class MovieRecommender {

    private MovieCatalog movieCatalog;

    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public void prepare(MovieCatalog movieCatalog,
            CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }
    // ...
}

Example: Apply @Autowired to fields as well and even mix it with constructors:

public class MovieRecommender {

    private final CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    private MovieCatalog movieCatalog;

    @Autowired
    public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
        this.customerPreferenceDao = customerPreferenceDao;
    }
    // ...
}

Example: Apply @Autowired annotation to a field or method that expects an array of that type:

public class MovieRecommender {

    @Autowired
    private MovieCatalog[] movieCatalogs;
    // ...
}

Example: Apply @Autowired annotation to the same applies for typed collections:

public class MovieRecommender {

    private Set<MovieCatalog> movieCatalogs;

    @Autowired
    public void setMovieCatalogs(Set<MovieCatalog> movieCatalogs) {
        this.movieCatalogs = movieCatalogs;
    }
    // ...
}

You may express the non-required nature of a particular dependency through Java 8’s java.util.Optional:

public class SimpleMovieLister {
    @Autowired
    public void setMovieFinder(Optional<MovieFinder> movieFinder) {
        ...
    }
}

We can also use @Autowired for interfaces that are well-known resolvable dependencies: BeanFactory, ApplicationContext, Environment, ResourceLoader, ApplicationEventPublisher, and MessageSource.

public class MovieRecommender {

    @Autowired
    private ApplicationContext context;

    public MovieRecommender() {
    }
    // ...
}

@Autowired Annotation Optional Elements

• boolean required - Declares whether the annotated dependency is required. Example:

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Autowired(required = false)
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
    // ...
}