This application is a proof of concept for how to support functional bean registration in Spring Boot. See spring-projects/spring-boot#8115 for background.

The approach here is to provide a modified version of Spring Boot's (internal) BeanDefinitionLoader. Example app:

@SpringBootApplication
public class InitializerApplication
		implements ApplicationContextInitializer<GenericApplicationContext> {

	public static void main(String[] args) {
		SpringApplication.run(InitializerApplication.class, args);
	}

	@Override
	public void initialize(GenericApplicationContext context) {
		context.registerBean(RouterFunction.class, this::userEndpoints);
	}

	private RouterFunction<?> userEndpoints() {
		return route(GET("/"), request -> ok().body(Mono.just("Hello"), String.class));
	}

}

This works as a main application class and in integration tests because the same BeanDefinitionLoader is used.

There is also a custom AutoConfigurationImportSelectorAutoConfigurationImportSelector which looks at autoconfig classes to see if they are ApplicationContextInitializer. If so the initializer is applied to the current context and the autoimport is cancelled.