Using `@Autowired`

JSR 330’s @Inject annotation can be used in place of Infra @Autowired annotation in the examples included in this section. See here for more details.

You can apply the @Autowired annotation to constructors, as the following example shows:

Java

public class MovieRecommender {

  private final CustomerPreferenceDao customerPreferenceDao;

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

  // ...
}

As of TODAY Framework 4.3, an @Autowired annotation on such a constructor is no longer necessary if the target bean defines only one constructor to begin with. However, if several constructors are available and there is no primary/default constructor, at least one of the constructors must be annotated with @Autowired in order to instruct the container which one to use. See the discussion on constructor resolution for details.

You can also apply the @Autowired annotation to traditional setter methods, as the following example shows:

Java

public class SimpleMovieLister {

  private MovieFinder movieFinder;

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

  // ...
}

You can also apply the annotation to methods with arbitrary names and multiple arguments, as the following example shows:

Java

public class MovieRecommender {

  private MovieCatalog movieCatalog;

  private CustomerPreferenceDao customerPreferenceDao;

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

  // ...
}

You can apply @Autowired to fields as well and even mix it with constructors, as the following example shows:

Java

public class MovieRecommender {

  private final CustomerPreferenceDao customerPreferenceDao;

  @Autowired
  private MovieCatalog movieCatalog;

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

  // ...
}

Make sure that your target components (for example, MovieCatalog or CustomerPreferenceDao) are consistently declared by the type that you use for your @Autowired-annotated injection points. Otherwise, injection may fail due to a "no type match found" error at runtime.

For XML-defined beans or component classes found via classpath scanning, the container usually knows the concrete type up front. However, for @Bean factory methods, you need to make sure that the declared return type is sufficiently expressive. For components that implement several interfaces or for components potentially referred to by their implementation type, consider declaring the most specific return type on your factory method (at least as specific as required by the injection points referring to your bean).

You can also instruct Infra to provide all beans of a particular type from the ApplicationContext by adding the @Autowired annotation to a field or method that expects an array of that type, as the following example shows:

Java

public class MovieRecommender {

  @Autowired
  private MovieCatalog[] movieCatalogs;

  // ...
}

The same applies for typed collections, as the following example shows:

Java

public class MovieRecommender {

  private Set<MovieCatalog> movieCatalogs;

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

  // ...
}

Your target beans can implement the cn.taketoday.core.Ordered interface or use the @Order or standard @Priority annotation if you want items in the array or list to be sorted in a specific order. Otherwise, their order follows the registration order of the corresponding target bean definitions in the container.

You can declare the @Order annotation at the target class level and on @Bean methods, potentially for individual bean definitions (in case of multiple definitions that use the same bean class). @Order values may influence priorities at injection points, but be aware that they do not influence singleton startup order, which is an orthogonal concern determined by dependency relationships and @DependsOn declarations.

Note that the standard jakarta.annotation.Priority annotation is not available at the @Bean level, since it cannot be declared on methods. Its semantics can be modeled through @Order values in combination with @Primary on a single bean for each type.

Even typed Map instances can be autowired as long as the expected key type is String. The map values contain all beans of the expected type, and the keys contain the corresponding bean names, as the following example shows:

Java

public class MovieRecommender {

  private Map<String, MovieCatalog> movieCatalogs;

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

  // ...
}

By default, autowiring fails when no matching candidate beans are available for a given injection point. In the case of a declared array, collection, or map, at least one matching element is expected.

The default behavior is to treat annotated methods and fields as indicating required dependencies. You can change this behavior as demonstrated in the following example, enabling the framework to skip a non-satisfiable injection point through marking it as non-required (i.e., by setting the required attribute in @Autowired to false):

Java

public class SimpleMovieLister {

  private MovieFinder movieFinder;

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

  // ...
}

A non-required method will not be called at all if its dependency (or one of its dependencies, in case of multiple arguments) is not available. A non-required field will not get populated at all in such cases, leaving its default value in place.

In other words, setting the required attribute to false indicates that the corresponding property is optional for autowiring purposes, and the property will be ignored if it cannot be autowired. This allows properties to be assigned default values that can be optionally overridden via dependency injection.

Injected constructor and factory method arguments are a special case since the required attribute in @Autowired has a somewhat different meaning due to Infra constructor resolution algorithm that may potentially deal with multiple constructors. Constructor and factory method arguments are effectively required by default but with a few special rules in a single-constructor scenario, such as multi-element injection points (arrays, collections, maps) resolving to empty instances if no matching beans are available. This allows for a common implementation pattern where all dependencies can be declared in a unique multi-argument constructor — for example, declared as a single public constructor without an @Autowired annotation.

Only one constructor of any given bean class may declare @Autowired with the required attribute set to true, indicating the constructor to autowire when used as a Infra bean. As a consequence, if the required attribute is left at its default value true, only a single constructor may be annotated with @Autowired. If multiple constructors declare the annotation, they will all have to declare required=false in order to be considered as candidates for autowiring (analogous to autowire=constructor in XML). The constructor with the greatest number of dependencies that can be satisfied by matching beans in the Infra container will be chosen. If none of the candidates can be satisfied, then a primary/default constructor (if present) will be used. Similarly, if a class declares multiple constructors but none of them is annotated with @Autowired, then a primary/default constructor (if present) will be used. If a class only declares a single constructor to begin with, it will always be used, even if not annotated. Note that an annotated constructor does not have to be public.

Alternatively, you can express the non-required nature of a particular dependency through Java 8’s java.util.Optional, as the following example shows:

public class SimpleMovieLister {

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

As of TODAY Framework 5.0, you can also use a @Nullable annotation (of any kind in any package — for example, javax.annotation.Nullable from JSR-305) or just leverage Kotlin built-in null-safety support:

Java

public class SimpleMovieLister {

  @Autowired
  public void setMovieFinder(@Nullable MovieFinder movieFinder) {
    ...
  }
}

You can also use @Autowired for interfaces that are well-known resolvable dependencies: BeanFactory, ApplicationContext, Environment, ResourceLoader, ApplicationEventPublisher, and MessageSource. These interfaces and their extended interfaces, such as ConfigurableApplicationContext or PatternResourceLoader, are automatically resolved, with no special setup necessary. The following example autowires an ApplicationContext object:

Java

public class MovieRecommender {

  @Autowired
  private ApplicationContext context;

  public MovieRecommender() {
  }

  // ...
}

The @Autowired, @Inject, @Value, and @Resource annotations are handled by Infra BeanPostProcessor implementations. This means that you cannot apply these annotations within your own BeanPostProcessor or BeanFactoryPostProcessor types (if any). These types must be 'wired up' explicitly by using XML or a Infra @Bean method.

Using @Autowired

Using `@Autowired`