HTTP Interface Clients - Declarative Replacement for RestTemplate

🌐 HTTP Interface Clients: The Declarative Replacement for RestTemplate

By a Senior Java Backend Engineer tired of boilerplate HTTP clients

RestTemplate is deprecated. Feign is heavy. Spring's @HttpExchange (backed by HttpServiceProxyFactory) is the modern, compile-time safe, AOT-friendly way to call external APIs.

📝 The Interface-Driven Approach

@HttpExchange(url = "/api/v1/orders")
public interface OrderClient {
    @GetExchange("/{id}")
    OrderDto getOrder(@PathVariable String id);

    @PostExchange
    @ResponseStatus(HttpStatus.CREATED)
    OrderDto createOrder(@RequestBody CreateOrderRequest request);
}

@Configuration
public class ClientConfig {
    @Bean
    OrderClient orderClient(WebClient.Builder webClientBuilder) {
        var baseUrl = "https://orders.example.com";
        var client = webClientBuilder.baseUrl(baseUrl).build();
        return HttpServiceProxyFactory.builderFor(WebClientAdapter.create(client))
            .build().createClient(OrderClient.class);
    }
}

🔍 Why It Matters in Production

• Compile-time contract validation (no more typo'd paths)
• Automatic JSON serialization via Jackson
• Seamless error mapping with @ExceptionHandler or ProblemDetail
• Native AOT safe (no runtime reflection tricks)

🚀 Migration Tip

Don't rewrite everything at once. Use HttpServiceProxyFactory alongside RestTemplate during transition. Wire interceptors for auth, retries, and correlation IDs exactly like you would with WebClient. The only difference? You write less glue code.