3.8 External API Communication (RestTemplate, WebClient, RestClient)

Learn about the various HTTP client libraries available in Spring Boot for calling external services, including both synchronous and asynchronous processing methods.

1. RestTemplate (Traditional Approach)

RestTemplate has been the most popular Synchronous, Blocking HTTP client since Spring 3.0.

Characteristics

• The thread waits (blocks) until it receives a response for each call.
• Suitable for simple designs following RESTful principles.
• Currently in maintenance mode; it is recommended to use WebClient or RestClient for new projects.

Example Usage

@Service
@RequiredArgsConstructor
public class ExternalApiService {
    private final RestTemplate restTemplate;

    public UserDto getUser(Long id) {
        String url = "https://api.example.com/users/" + id;
        return restTemplate.getForObject(url, UserDto.class);
    }
}

2. WebClient (Asynchronous Processing)

WebClient is a modern Non-blocking, Asynchronous client introduced with Spring 5.0 (WebFlux).

Characteristics

• High resource efficiency as threads can perform other tasks while waiting for a response.
• Can also be used synchronously via .block().
• Essential for high-traffic or reactive programming environments.

Asynchronous Example

public Mono<UserDto> getUserAsync(Long id) {
    return webClient.get()
            .uri("/users/{id}", id)
            .retrieve()
            .bodyToMono(UserDto.class); // Returns a Mono containing data asynchronously
}

3. RestClient (New in Spring 6.1)

Introduced in Spring 6.1, RestClient is a new synchronous HTTP client that combines the blocking nature of RestTemplate with the modern fluent API of WebClient.

Characteristics

• The successor to RestTemplate, featuring a contemporary API structure similar to WebClient.
• Allows writing elegant code without requiring the full WebFlux dependency.

Example Usage

UserDto user = restClient.get()
        .uri("/users/{id}", id)
        .retrieve()
        .body(UserDto.class);

4. Java Standard HttpClient (Java 11+)

The built-in client provided by pure Java (JDK 11 and above) without external library requirements.

Example Usage

HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
        .uri(URI.create("https://api.example.com"))
        .build();

// Asynchronous processing
client.sendAsync(request, HttpResponse.BodyHandlers.ofString())
      .thenApply(HttpResponse::body)
      .thenAccept(System.out::println);

5. Summary Comparison

Client	Mode	Recommendation
RestTemplate	Synchronous	Maintenance of legacy projects
RestClient	Synchronous	** Recommended for new Spring 6.1+ sync calls**
WebClient	Sync/Async	** Essential for high traffic or async needs**
HttpClient	Sync/Async	Environments avoiding extra local libraries

tip

If you are using Spring Boot 3.2 or above, we strongly recommend using the more intuitive and modern RestClient over the deprecated RestTemplate.