Is it possible to implement a component acting as a controller without using the @Controller annotation?

Yes, it is possible to implement a component that acts as a controller without using the @Controller annotation in Spring. The @Controller annotation is simply a stereotype that helps Spring detect and register the class as a controller in the MVC context.

Alternatives:

• Use the @Component or @Service annotation and manually configure routing through configuration.
• In Spring WebFlux, you can use functional routes (RouterFunction) and handlers without the @Controller annotation.
• You can create a controller bean programmatically and register it in the context.

However, without @Controller or similar annotations, Spring cannot automatically associate HTTP requests with class methods. Therefore, @Controller or @RestController are usually used for convenience and declarativity.

Example without @Controller, using functional style in Spring WebFlux:

@Bean
public RouterFunction<ServerResponse> route() {
    return RouterFunctions.route()
        .GET("/hello", request -> ServerResponse.ok().bodyValue("Hello World"))
        .build();
}

Thus, a controller can be implemented without the annotation, but it requires a different approach to routing and request handling.