Why is a bean declared in @SpringBootApplication class registered even though it is not a stereotyped class?

Question

I have this main class in my project

@SpringBootApplication
@EnableOAuth2Sso
public class App 
{
    public static void main(String[] args) throws Exception {
        SpringApplication.run(App.class, args);    
    }

    @Bean public RequestContextListener requestContextListener(){
        return new RequestContextListener();
    }

}

As far as I know, component scan scans beans in stereotyped classes which are one of @Component, @Service, @Repository, @Controller if I am not wrong.

From spring docs

By default, classes annotated with @Component, @Repository, @Service, @Controller, or a custom annotation that itself is annotated with @Component are the only detected candidate components.

I cannot understand how the bean in this class is registered. As it is not a stereotyped class and no annotation is annotated with @Component it shouldn't be scanned in the first place but this code works perfectly. In fact for my use case having the bean in this class was the only way my problem was solved, but that is a different thing. Can anyone please explain this. Thanks !!

Answer 1

@SpringBootApplication is a meta annotation which looks like:

// Some details omitted
@SpringBootConfiguration
@EnableAutoConfiguration
public @interface SpringBootApplication { ... }

@SpringBootConfiguration is also a meta annotation:

// Other annotations
@Configuration
public @interface SpringBootConfiguration { ... }

And @Configuration is:

// Other annotations
@Component
public @interface Configuration { ... }

It works Since:

By default, classes annotated with @Component, @Repository, @Service, @Controller, or a custom annotation that itself is annotated with @Component are the only detected candidate components.

Answer 2

This is because @SpringBootApplication acts also as a @Configuration annotation.

@Configuration is used to create define beans as in xml spring configurarion files.

You can have a bean configuration class.

@Configuration
class MyConfiguration{
@bean MyBean myBean(){...};
}

o you can have a Spring configuration file

<beans>
   <bean id="myBean" class="MyBean" />
</beans>

In your case you're unsing Spring configuration class as you're using @SpringBootApplication

You can see more here

http://www.tutorialspoint.com/spring/spring_java_based_configuration.htm

http://docs.spring.io/spring-boot/docs/current/api/org/springframework/boot/autoconfigure/SpringBootApplication.html

http://docs.spring.io/spring/docs/4.2.1.RELEASE/javadoc-api/org/springframework/context/annotation/Configuration.html