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.

Transport failure contract

Define an adapter policy before adding retry annotations. The companion CatalogHttpClient implements the CatalogRetryService.CatalogTransport port from the resilience-retry skill and uses explicit JDK connect/read timeouts. It is a small imperative adapter example, not a reason to replace an existing HTTP interface or Boot-managed client.

For POST/payment calls, preserve the same idempotency key and request bytes across retries; a timeout does not prove the server did nothing. Never forward inbound Authorization to an arbitrary destination: configure credentials per trusted service. Do not allow an end user to choose the base URL. Avoid logging tokens or raw upstream error bodies.

Test the real client adapter against a stub HTTP server, including permanent status codes, transient-then-success responses, malformed bodies, delayed responses and cancellation. Keep connect, read, pool-acquisition and total deadlines distinct. When using WebClient, apply a total timeout around the whole retry sequence and avoid blocking the event loop.

See Spring REST clients.

Verification scope

The repository fixture targets Boot 3.5.16 with Java 17/21 and checks the imperative catalog adapter against a local HTTP server; not every declarative group-registration option. Check the coverage matrix before treating another minor version or an illustrative snippet as verified.

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.