Register a Spring Bean in the Spring IoC Container

Using Java Annotations

Using @Component

• Among annotations, which are a type of metadata that can be added to Java source code, the @Component annotation can be used to register a Spring Bean in the IoC Container

  • Use the @Component annotation when you want to register a class that you developed directly as a Bean!

• When the @Component annotation is registered, Spring recognizes the annotation and automatically registers it as a Bean

  • ex)

    • In the @Controller annotation used to register a Controller, you can see that the @Component annotation is present as shown below

      // Controller.java
      @Target({ElementType.TYPE})
      @Retention(RetentionPolicy.RUNTIME)
      @Documented
      @Component
      public @interface Controller {
      
        /**
        * The value may indicate a suggestion for a logical component name,
        * to be turned into a Spring bean in case of an autodetected component.
        * @return the suggested component name, if any (or empty String otherwise)
        */
        @AliasFor(annotation = Component.class)
        String value() default "";
      
      }

• When using @Component, you must specify the scanning range for components with @ComponentScan in the Main or App class

  • However, if using SpringBoot, it is included by default under @SpringBootConfiguration, so no separate configuration is needed

Registering Beans Directly in a Bean Configuration File and Retrieving Beans Using ApplicationContext

Using @Bean & @Configuration

• Beans are registered using the @Configuration and @Bean annotations

  • As shown below, using @Configuration creates a Class that serves as Configuration for the Spring Project, where Beans can be configured

    import org.springframework.context.annotation.Configuration;
    
    // Annotation indicating that one or more Beans are being registered
    @Configuration
    public class ApplicationConf {
        @Bean
        public Person person(){  // The method name becomes the bean's id
            return new Person();
        }
    
        @Bean
        public Body body(){
            return new Body();
        }
    }

  • In a class using @Bean, you must use the @Configuration annotation to indicate that the class intends to register Beans

    • Using only the @Bean annotation without the @Configuration annotation will still register as a Spring Bean, but singleton behavior is not guaranteed when creating objects through method calls

  • Note that the @Configuration annotation, which is responsible for Bean configuration, also internally has a @Component annotation, so the class with @Configuration is also registered as a Spring Bean

• Using Beans created with @Bean & @Configuration

  • Create an ApplicationContext and use the .getBean() method to retrieve and use Beans

    public class Application {
        public static void main(String[] args){
            AnnotationConfigApplicationContext context =
                new AnnotationConfigApplicationContext(ApplicationConf.class);
    
            Person person = (Person)context.getBean(Person.class);
            person.say(); // print : "hi"
        }
    }