RestClient in Spring Boot 4: The Modernization of Synchronous Communication

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.client.RestClient;

@Configuration
public class RestClientConfiguration {

    @Bean
    public RestClient githubRestClient(RestClient.Builder builder) {
        // Configure a base URL and default header for all requests
        return builder
            .baseUrl("https://api.github.com")
            .defaultHeader("Accept", "application/json")
            .build();
    }
}

// Assuming 'githubRestClient' is already injected

public String getRepositoryInfo(String owner, String repo, boolean verbose) {
    String uriTemplate = "/repos/{owner}/{repo}";

    return githubRestClient.get()
        .uri(uriTemplate, owner, repo) // Maps {owner} and {repo}
        .queryParam("verbose", verbose) // Adds ?verbose=true or false
        .retrieve()
        .body(String.class);
}

import org.springframework.http.MediaType;

public User createNewUser(NewUserDTO userData) {
    return githubRestClient.post()
        .uri("/users")
        .contentType(MediaType.APPLICATION_JSON) // Sets Content-Type
        .body(userData) // Object that will be serialized to JSON
        .retrieve()
        .body(User.class); // Deserializes response to a User object
}

import org.springframework.http.HttpStatus;

public User safeGetUser(long id) {
    return githubRestClient.get()
        .uri("/users/{id}", id)
        .retrieve()
        .onStatus(HttpStatus.NOT_FOUND, (request, response) -> {
            // Specific handling for 404
            System.out.println("User not found, returning default");
            // Can throw an exception or, in this case, return a default value
            throw new CustomResourceNotFoundException("User ID " + id + " not found");
        })
        .onStatus(HttpStatus::is5xxServerError, (request, response) -> {
            // Handling for 5xx (server errors)
            throw new RuntimeException("Internal service error.");
        })
        .body(User.class);
}

// 1. Client Interface (Contract Definition)
import org.springframework.web.service.annotation.GetExchange;
import org.springframework.web.service.annotation.HttpExchange;

@HttpExchange(url = "https://api.external.com/api/v1") // Base URL
public interface ExternalApi {
    @GetExchange("/items/{itemId}") // Maps to GET https://api.external.com/api/v1/items/{itemId}
    Item getItem(@PathVariable String itemId);
}

// 2. Usage in a Service (Spring Boot 4.0 injects the proxy automatically after configuring @ImportHttpServices)
@Service
public class ItemService {
    private final ExternalApi externalApi;

    // The generated proxy implementation is injected
    public ItemService(ExternalApi externalApi) {
        this.externalApi = externalApi;
    }

    public Item findItem(String id) {
        // Feels like calling a local method
        return externalApi.getItem(id);
    }
}