Why Doesn't Reactive Spring Support HttpServletRequest in REST Endpoints?

Why is HttpServletRequest not supported as a parameter in Reactive Spring REST endpoints?

Reactive Spring, designed for non-blocking, event-driven applications, does not support `HttpServletRequest` as a parameter in its REST endpoints due to the fundamental differences between the reactive and servlet programming models. The reasons and solutions to this issue are outlined below.

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.http.server.reactive.ServerHttpRequest;
import reactor.core.publisher.Mono;

@RestController
public class ReactiveController {
    @GetMapping("/example")
    public Mono<String> exampleEndpoint(ServerHttpRequest request) {
        // Use ServerHttpRequest for non-blocking access
        String clientIp = request.getRemoteAddress().getHostString();
        return Mono.just("Client IP: " + clientIp);
    }
}

Causes

• Reactive programming is inherently non-blocking, while `HttpServletRequest` is tightly coupled with the traditional, blocking servlet model.
• Using `HttpServletRequest` would introduce blocking behavior, which contradicts the core principles of a reactive architecture.
• Reactive Spring emphasizes the use of reactive types like `Mono` and `Flux` to handle asynchronous data streams, a paradigm incompatible with the servlet-based request handling.

Solutions

• Use the `ServerHttpRequest` class provided by Spring WebFlux instead of `HttpServletRequest`, which is designed for reactive applications.
• Refactor your code to utilize reactive types (`Mono`, `Flux`) for handling the request and response, ensuring non-blocking operations throughout your application.

Mistake: Attempting to directly use `HttpServletRequest` in a Reactive controller.

Solution: Switch to using `ServerHttpRequest` in your Spring WebFlux controllers.

Mistake: Forgetting to handle data reactively when dealing with request or response processing.

Solution: Ensure to return `Mono` or `Flux` types from your controller methods.