http-interface-clients

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.