Mastering Spring 5 Functional Endpoints: A Hands‑On Guide

1. Introduction

Typical Spring MVC controller code uses @RestController and @RequestMapping . This article demonstrates a different style by using the functional endpoints introduced in Spring 5, which also work with Spring WebFlux.

@RestController
@RequestMapping("/v1/userinfo")
public class UserInfoController {
    @GetMapping("/foo")
    public String foo() {
        return "felord.cn";
    }
}

Instead of the annotation‑based approach, we use Spring 5’s functional routing API.

Note: The functional endpoint feature requires Spring 5.2 or later.

2. Dependency

For the demo we only add the Spring MVC starter:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

3. RouterFunction

In functional endpoints the traditional @RequestMapping is replaced by a RouterFunction. The equivalent code is:

@Bean
public RouterFunction<ServerResponse> fooFunction() {
    return RouterFunctions.route()
        .GET("/v1/userinfo/foo", request -> ServerResponse.ok()
            .body("felord.cn"))
        .build();
}

4. ServerRequest / ServerResponse

ServerRequest

abstracts the incoming HTTP request, while ServerResponse abstracts the outgoing response. Both are processed by a HandlerFunction that maps request to response.

5. HandlerFunction

HandlerFunction

is a functional interface that receives a ServerRequest and returns a ServerResponse. Example:

HandlerFunction<ServerResponse> handlerFunction = request ->
    ServerResponse.ok().body("felord.cn");

6. RequestPredicate

RequestPredicate

allows routing decisions based on request method, headers, parameters, etc. The following example routes only when the query parameter plan is present:

@Bean
public RouterFunction<ServerResponse> predicateFunction() {
    return RouterFunctions.route()
        .GET("/v1/userinfo/predicate",
            request -> request.param("plan").isPresent(),
            request -> ServerResponse.ok().body("felord.cn"))
        .build();
}

Calling GET http://localhost:8080/v1/userinfo/predicate?plan=... returns felord.cn with status 200, while a request without the plan parameter results in a 404 response.

7. Summary

Functional endpoints are a new programming model introduced in Spring 5 (supported from 5.2) that aligns with functional programming practices. This article covered the key concepts; a follow‑up will explore more advanced usage.