Mastering Spring @Autowired: 9 Powerful Injection Techniques

Environment: spring5.2.15

The @Autowired annotation first attempts injection by type; if multiple candidates exist it falls back to by name, and if no matching name is found the container throws an error. You can narrow the target with @Qualifier.

As of Spring Framework 4.3, an @Autowired annotation on a constructor is no longer necessary if the target bean defines only one constructor. However, if several constructors are available and there is no primary/default constructor, at least one constructor must be annotated with @Autowired to indicate which one the container should use.

Function 1

Inject a dependency via a setter method.

public class SimpleMovieLister {
  private MovieFinder movieFinder;
  @Autowired
  public void setMovieFinder(MovieFinder movieFinder) {
    this.movieFinder = movieFinder;
  }
}

Function 2

@Autowired can be applied to a method with any name and multiple parameters.

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

Function 3

@Autowired can be used on fields and can be mixed with constructor injection.

public class MovieRecommender {
  private final CustomerPreferenceDao customerPreferenceDao;
  @Autowired
  private MovieCatalog movieCatalog;
  @Autowired
  public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) {
    this.customerPreferenceDao = customerPreferenceDao;
  }
}

Function 4

Inject an array of beans by annotating a field or method.

public class MovieRecommender {
  @Autowired
  private MovieCatalog[] movieCatalogs;
}

Function 5

Inject a collection (Set) of beans via a field or method.

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

Note: To preserve order when injecting a List or array, implement the Ordered interface, use @Order, or the standard @Priority annotation. A Set does not guarantee order.

Function 6

Inject a Map of beans, where the key is the bean name.

public class MovieRecommender {
  private Map<String, MovieCatalog> movieCatalogs;
  @Autowired
  public void setMovieCatalogs(Map<String, MovieCatalog> movieCatalogs) {
    this.movieCatalogs = movieCatalogs;
  }
}

Note: For arrays, collections, or maps, at least one matching bean must exist; otherwise an error occurs.

By default, @Autowired marks a dependency as required, causing an error if the bean is missing. You can change this behavior:

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

Note: If no suitable MovieFinder bean exists, the setter will not be called. Constructor injection will still fail even with required = false.

Function 7

Use Java 8's java.util.Optional to represent an optional dependency.

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

Function 8

Mark a dependency as optional with the @Nullable annotation.

@Autowired
public void setMovieFinder(@Nullable MovieFinder movieFinder) {
  System.out.println("-----movie");
  this.movieFinder = movieFinder;
}

Function 9

@Autowired can inject built‑in Spring components such as BeanFactory, ApplicationContext, Environment, ResourceLoader, ApplicationEventPublisher, and MessageSource.

@Autowired
private ApplicationContext context;

The @Autowired, @Inject, @Value, and @Resource annotations are processed by Spring BeanPostProcessor implementations. They cannot be used inside custom BeanPostProcessor or BeanFactoryPostProcessor classes; those must obtain beans via XML configuration or @Bean methods.

End of guide.