Declarative HTTP Interface Clients (Boot 4)

Define a remote API as an interface with @HttpExchange methods. Boot 4 registers the proxy with @ImportHttpServices, so do not create a manual factory for every client.

Define and register the interface

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

    @PostExchange
    OrderDto create(@RequestBody CreateOrderRequest request);
}

@SpringBootApplication
@ImportHttpServices(group = "orders", basePackages = "com.example.client.orders")
public class Application { }

Inject OrderApiClient like any other bean. The interface needs no @Component and no implementation class.

Configure groups and transport

spring:
  http:
    clients:
      connect-timeout: 2s
      read-timeout: 5s
    serviceclient:
      orders:
        base-url: https://orders.internal.example.com
        read-timeout: 10s

spring.http.clients.* contains global transport defaults. spring.http.serviceclient.<group>.* contains per-group settings, and the group name must match @ImportHttpServices.

The default client type is blocking RestClient. Select HttpServiceGroup.ClientType.WEB_CLIENT for interfaces returning Mono or Flux, and include the WebClient starter. Group configurers apply to a group, not one interface.

Manual HttpServiceProxyFactory wiring remains valid for one-off clients, but it is the Boot 3 pattern and should not be the default in Boot 4.

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 4.1.0 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 creates a HttpServiceProxyFactory bean for every client - use @ImportHttpServices and groups.
• Agent adds @Component or an implementation to the interface - generated proxies need neither.
• Agent uses spring.http.client.* - global defaults are under spring.http.clients.*; group settings use serviceclient.
• Agent hard-codes a host in @HttpExchange and sets a group base URL - keep the host in configuration.
• Agent returns Mono or Flux with the default client - select WEB_CLIENT explicitly.
• Agent assumes a group configurer customizes one interface - configure the group or separate the group.