Declarative HTTP Interface Clients (Boot 3)

Spring Framework 6 supports @HttpExchange interfaces, but Boot 3 does not provide Boot 4's @ImportHttpServices group auto-registration. Build the client adapter explicitly.

@HttpExchange("/orders")
interface OrderApiClient {
    @GetExchange("/{id}")
    OrderDto get(@PathVariable UUID id);
}

@Configuration
class ClientConfig {
    @Bean
    OrderApiClient orderApiClient(RestClient.Builder builder,
                                  @Value("${clients.orders.base-url}") String baseUrl) {
        RestClient client = builder.baseUrl(baseUrl).build();
        HttpServiceProxyFactory factory =
            HttpServiceProxyFactory.builderFor(RestClientAdapter.create(client)).build();
        return factory.createClient(OrderApiClient.class);
    }
}

Use WebClientAdapter for reactive interfaces returning Mono or Flux. Configure base URLs, timeouts, authentication, and error translation in the client adapter layer rather than in controllers or domain services. Prefer one factory/configurer per external service.

Gotchas

• Agent uses @ImportHttpServices - that Boot 4 registration API is not available in Boot 3.
• Agent adds @Component or an implementation to the interface - register the generated proxy as a bean.
• Agent uses RestClient for Mono or Flux - use WebClientAdapter for reactive return types.
• Agent hard-codes remote hosts in annotations - keep URLs in configuration.
• Agent lets transport errors leak through the domain - translate them in the client adapter.
• Agent creates a new client per request - configure and reuse a singleton proxy.