What are HTTP Service Clients in Spring Boot 4 and how do you define them?

// Define a declarative HTTP client interface:
public interface ProductClient {

    @GetExchange("/api/products/{id}")
    ProductDto getProduct(@PathVariable String id);

    @GetExchange("/api/products")
    List<ProductDto> getAllProducts(
        @RequestParam int page,
        @RequestParam int size
    );

    @PostExchange("/api/products")
    ProductDto createProduct(@RequestBody CreateProductRequest request);

    @DeleteExchange("/api/products/{id}")
    void deleteProduct(@PathVariable String id);
}

// Spring Boot 4: auto-configuration via spring.http.clients.*
# application.properties:
spring.http.clients.product.base-url=https://products-api.example.com
spring.http.clients.product.connect-timeout=5s
spring.http.clients.product.read-timeout=10s

// Use it like any other Spring bean:
@Service
@RequiredArgsConstructor
public class OrderService {

    private final ProductClient productClient;  // injected automatically

    public Order createOrder(String productId, int qty) {
        ProductDto product = productClient.getProduct(productId);
        // ...
        return new Order(product, qty);
    }
}

// Manual configuration (without auto-config):
@Configuration
public class ClientConfig {
    @Bean
    public ProductClient productClient(RestClient.Builder builder) {
        return HttpServiceProxyFactory
            .builderFor(RestClientAdapter.create(
                builder.baseUrl("https://products-api.example.com").build()
            ))
            .build()
            .createClient(ProductClient.class);
    }
}