Spring Boot 4 Microservices & REST APIs · Lesson

Reactive Controllers

Build non-blocking endpoints with WebFlux.

Reactive Controllers is a free Spring Boot 4 Microservices & REST APIs lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Spring Boot 4 Microservices & REST APIs learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Reactive Web Endpoints

With WebFlux you write @RestController classes just like in Spring MVC, but methods return reactive types instead of plain objects.

Return Mono<T> for a single resource
Return Flux<T> for a collection or stream

Returning a Mono

A handler returning Mono resolves to a single JSON object. The framework subscribes for you.

@RestController
@RequestMapping("/users")
public class UserController {
    @GetMapping("/{id}")
    public Mono<User> getUser(@PathVariable String id) {
        return userService.findById(id);
    }
}

Returning a Flux

A handler returning Flux serializes to a JSON array by default.

@GetMapping
public Flux<User> getAllUsers() {
    return userService.findAll();
}

No Blocking Allowed

Inside reactive handlers you must not call blocking code. Return the publisher and let the chain stay reactive.

// BAD: blocks the event loop
@GetMapping("/bad")
public User bad() {
    return userService.findById("1").block();
}

// GOOD: stays reactive
@GetMapping("/good")
public Mono<User> good() {
    return userService.findById("1");
}

Path and Query Params

Binding annotations work the same as in MVC: @PathVariable, @RequestParam, and @RequestBody.

@GetMapping("/search")
public Flux<User> search(@RequestParam String name) {
    return userService.findByName(name);
}

Accepting a Reactive Body

A POST handler can accept a Mono request body, keeping the whole flow non-blocking.

@PostMapping
public Mono<User> create(@RequestBody Mono<User> body) {
    return body.flatMap(userService::save);
}

Setting Status Codes

Use @ResponseStatus or wrap results in ResponseEntity within a Mono.

@PostMapping
@ResponseStatus(HttpStatus.CREATED)
public Mono<User> create(@RequestBody Mono<User> body) {
    return body.flatMap(userService::save);
}

ResponseEntity in a Mono

To control headers and status dynamically, return Mono<ResponseEntity<T>>.

@GetMapping("/{id}")
public Mono<ResponseEntity<User>> getUser(@PathVariable String id) {
    return userService.findById(id)
        .map(ResponseEntity::ok)
        .defaultIfEmpty(ResponseEntity.notFound().build());
}

Streaming with Server-Sent Events

Set the content type to text/event-stream to stream a Flux to the client over time.

@GetMapping(value = "/stream",
    produces = MediaType.TEXT_EVENT_STREAM_VALUE)
public Flux<Long> stream() {
    return Flux.interval(Duration.ofSeconds(1));
}

Error Handling

Use onErrorResume to map errors to a fallback publisher, or a @ControllerAdvice for global handling.

@GetMapping("/{id}")
public Mono<User> getUser(@PathVariable String id) {
    return userService.findById(id)
        .onErrorResume(e -> Mono.error(
            new ResponseStatusException(HttpStatus.NOT_FOUND)));
}

The Netty Server

WebFlux runs on Netty by default — an event-loop server with a small fixed thread pool. This is why blocking calls are so harmful: blocking one thread stalls many requests.

Quick Check

Test your understanding of reactive controllers.

Recap

You learned how to build reactive endpoints:

@RestController methods return Mono or Flux
Never call block() inside a handler
Use ResponseEntity in a Mono for dynamic status
Stream with text/event-stream

Frequently asked questions

Is the “Reactive Controllers” lesson free?

Yes — the full text of “Reactive Controllers” is free to read here on the web, and the Spring Boot 4 Microservices & REST APIs course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Spring Boot 4 Microservices & REST APIs course, upgrade to CoddyKit PRO.

What will I learn in “Reactive Controllers”?

Build non-blocking endpoints with WebFlux. You practise Spring Boot 4 Microservices & REST APIs with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Spring Boot 4 Microservices & REST APIs?

No prior experience is required. Spring Boot 4 Microservices & REST APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Reactive Controllers” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Spring Boot 4 Microservices & REST APIs lesson?

Yes. Every Spring Boot 4 Microservices & REST APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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