The HTTP Client Showdown Every Spring Developer Needs to Understand

If you've been writing Spring applications for a while, you've probably had this conversation: "Should I use RestTemplate? Someone said it's deprecated. What about WebClient? And wait, what on earth is RestClient?"

Let's settle this once and for all.

The Short Version (For the Impatient)

If you're starting a new project today and your code is synchronous — use RestClient. If you're doing reactive or truly async I/O — use WebClient. If you have an existing RestTemplate codebase — you don't have to migrate tomorrow, but plan for it.

Now let's dig into why.

A Quick History Lesson (Because Context Matters)

To understand these three clients, you need to understand the problems each was born to solve.

2009 — RestTemplate arrives. Back then, calling REST APIs in Java was painful. You were stitching together HttpURLConnection, Apache HttpClient, and custom serializers. RestTemplate wrapped all that ceremony behind a clean, synchronous API. It was revolutionary — and for a decade, it was the way.

2017 — WebClient arrives. The reactive revolution (RxJava, Project Reactor) changed how we think about I/O. Microservices meant one request often fanned out into ten. Blocking threads became expensive. Spring introduced WebFlux and, with it, WebClient — a non-blocking HTTP client built on Reactor's Mono and Flux.

The problem? WebClient became the only modern choice. If you wanted modern features (builder pattern, better error handling, interceptors that don't fight you), you had to adopt reactive types — even if your app had zero reactive code. Teams were writing .block() everywhere and pretending they weren't.

2023 — RestClient arrives. The Spring team finally admitted the obvious: most applications are synchronous, and they deserve a modern, fluent, blocking API that doesn't force reactive types down their throat. RestClient is what RestTemplate would look like if we designed it today.

RestTemplate: The Veteran

@Service
public class UserService {
    private final RestTemplate restTemplate;

    public UserService(RestTemplateBuilder builder) {
        this.restTemplate = builder
            .rootUri("https://api.example.com")
            .build();
    }

    public User getUser(Long id) {
        return restTemplate.getForObject("/users/{id}", User.class, id);
    }

    public User createUser(User user) {
        return restTemplate.postForObject("/users", user, User.class);
    }
}

What's good:

• Everyone knows it. Zero learning curve.
• Massive ecosystem, battle-tested, boring in the best way.
• Works synchronously — matches how most business logic thinks.

What hurts:

• The API is overloaded beyond recognition. Look at the Javadoc — exchange, execute, getForObject, getForEntity, postForObject, postForEntity, postForLocation... twenty ways to do one thing.
• No fluent builder. Headers, query params, and body-building are awkward.
• Error handling defaults are surprising. A 4xx throws; a 2xx with no body returns null.
• Not evolving. New features go to RestClient and WebClient.

Official status: Not deprecated. This is important. The Spring team has explicitly said RestTemplate will not be removed. But it's in maintenance mode — no new features, just bug fixes.

WebClient: The Reactive Powerhouse

@Service
public class UserService {
    private final WebClient webClient;

    public UserService(WebClient.Builder builder) {
        this.webClient = builder
            .baseUrl("https://api.example.com")
            .build();
    }

    public Mono<User> getUser(Long id) {
        return webClient.get()
            .uri("/users/{id}", id)
            .retrieve()
            .bodyToMono(User.class);
    }

    public Flux<User> getAllUsers() {
        return webClient.get()
            .uri("/users")
            .retrieve()
            .bodyToFlux(User.class);
    }
}

What's good:

• Truly non-blocking. Perfect for high-concurrency fan-out (a single Netty event-loop thread can handle thousands of in-flight requests).
• Fluent, modern, discoverable API.
• First-class streaming support — Flux<User> lets you process a 10GB response without loading it into memory.
• Backpressure. If you're consuming a fast producer from a slow consumer, WebClient handles it natively.

What hurts:

• You're now in Reactor-land. Mono, Flux, flatMap, zip, onErrorResume — there's a real learning curve.
• Stack traces are ugly. Debugging reactive code is genuinely harder.
• If you .block() everything, you get WebClient's complexity with RestTemplate's performance characteristics — the worst of both worlds.
• Needs spring-webflux on the classpath even in a servlet app.

When it shines: WebFlux apps, streaming responses, SSE/WebSockets, fan-out aggregation where latency matters, any place you're genuinely doing async I/O.

RestClient: The Goldilocks Client

@Service
public class UserService {
    private final RestClient restClient;

    public UserService(RestClient.Builder builder) {
        this.restClient = builder
            .baseUrl("https://api.example.com")
            .build();
    }

    public User getUser(Long id) {
        return restClient.get()
            .uri("/users/{id}", id)
            .retrieve()
            .body(User.class);
    }

    public User createUser(User user) {
        return restClient.post()
            .uri("/users")
            .contentType(MediaType.APPLICATION_JSON)
            .body(user)
            .retrieve()
            .body(User.class);
    }

    public User getUserWithCustomErrorHandling(Long id) {
        return restClient.get()
            .uri("/users/{id}", id)
            .retrieve()
            .onStatus(HttpStatusCode::is4xxClientError, (req, res) -> {
                throw new UserNotFoundException(id);
            })
            .body(User.class);
    }
}

Look at that code. Notice something? It reads exactly like WebClient — but there's not a Mono or Flux in sight. It's synchronous. It blocks. It returns the value directly.

What's good:

• Same fluent API as WebClient, zero reactive types.
• Modern error handling with .onStatus().
• Can use any underlying HTTP client — JDK's HttpClient, Apache, Jetty, Reactor Netty.
• Integrates cleanly with Spring's @HttpExchange declarative clients (think Feign, but built in).
• Familiar mental model for anyone coming from RestTemplate or WebClient.

What hurts:

• Newer. Smaller community base of examples and Stack Overflow answers (though that's changing fast).
• Not reactive. If you need non-blocking, it's the wrong tool.

Pro tip: You can build a RestClient from an existing RestTemplate — a huge win for gradual migration:

RestClient restClient = RestClient.create(existingRestTemplate);

All your existing interceptors, message converters, and error handlers come along for the ride.

The Deep Comparison

1. Syntax and Ergonomics

RestClient and WebClient share near-identical fluent APIs. RestTemplate is the outlier with its overloaded method zoo.

2. Threading Model

• RestTemplate — one thread per request, blocked until response arrives.
• RestClient — same model (blocking), but the underlying transport is pluggable.
• WebClient — event-loop based, one thread handles many in-flight requests.

For a typical CRUD service handling a few hundred req/sec on a well-sized thread pool, the difference is invisible. For a gateway aggregating 50 downstream calls per request at 10K RPS, WebClient wins by orders of magnitude.

3. Error Handling

RestTemplate throws HttpClientErrorException / HttpServerErrorException by default. You override a ResponseErrorHandler globally to change it. Not fine-grained.

RestClient and WebClient offer per-request .onStatus() handling — you decide what a 404 means in the context of this specific call. Huge ergonomic win.

4. Testing

• RestTemplate — MockRestServiceServer is mature and easy.
• WebClient — needs either ExchangeFunction mocking or an MockWebServer setup.
• RestClient — works with MockRestServiceServer (shares the infrastructure). Big win for teams migrating.

5. Declarative Clients (the underrated feature)

Both RestClient and WebClient support Spring's @HttpExchange:

@HttpExchange(url = "/users")
public interface UserApi {
    @GetExchange("/{id}")
    User getUser(@PathVariable Long id);

    @PostExchange
    User createUser(@RequestBody User user);
}

Wire it up once and you have a type-safe client — no implementation code to write. This is effectively what Feign gave us, now built into Spring. RestTemplate doesn't support this.

The Decision Framework

Ask yourself three questions, in order:

1. Is your codebase reactive (WebFlux, R2DBC, Kafka reactive)?
→ Use WebClient. Don't fight the paradigm.

2. Do you need genuine non-blocking I/O for performance (high fan-out, streaming, SSE)?
→ Use WebClient. The complexity pays for itself.

3. Everything else?
→ Use RestClient. It's the modern synchronous default.

And for legacy code on RestTemplate — there's no emergency. Plan a gradual migration as you touch code. RestClient.create(restTemplate) makes incremental migration almost free.

A Word on Performance

I'll save you the benchmark drama: for synchronous workloads, RestTemplate and RestClient are effectively identical. They share transport layers. The real performance story is WebClient vs. everything else in high-concurrency async scenarios — and even there, it only matters if you're actually constrained on threads.

Don't pick WebClient for "performance" if your app does 50 RPS. You'll pay the complexity tax for benefits you'll never measure.

Common Mistakes I See on Code Reviews

1. Using WebClient with .block() everywhere. You're burning all of WebClient's benefits and eating its complexity. Switch to RestClient.
2. Creating a new RestTemplate/RestClient per request. These are designed to be singletons. Inject the builder, build once, reuse.
3. Ignoring timeouts. Default timeouts are "forever." Always set connect and read timeouts — production will thank you.
4. Catching RestClientException broadly. Use .onStatus() for semantic error handling. Catch specific exceptions at the boundary.
5. Not using @HttpExchange. If you're writing repetitive CRUD wrappers, you're writing code that Spring will generate for you.

Closing Thoughts

Spring's HTTP client story went from "just use RestTemplate" to "use WebClient and suffer" to — finally — a clean, ergonomic split based on what your code actually needs. RestClient in particular is one of the best Spring improvements of the last five years. It quietly fixes the biggest complaint developers had without breaking anyone's existing code.

If I had to sum it up in one sentence: use RestClient by default, reach for WebClient when you're genuinely reactive, and migrate off RestTemplate at your own pace.

That's it. No dogma, no hype. Pick the tool that matches your workload.

If this helped, a clap goes a long way. And if you disagree — especially on the "don't migrate tomorrow" take for RestTemplate — I'd love to hear your war stories in the comments.