Andere APIs aufrufen

So far BookVault serves HTTP. Real applications also consume it - calling other services for data. BookVault might enrich a book with cover images or ratings from an external catalog API. Spring's modern tool for this is the RestClient.

RestClient: a fluent, synchronous client

RestClient (introduced in Spring Framework 6.1) offers a clean, chainable API for making HTTP calls and mapping responses straight to Java objects:

@Service
class CoverService {

    private final RestClient client;

    CoverService(RestClient.Builder builder) {          // Boot provides the builder
        this.client = builder.baseUrl("https://covers.example.com").build();
    }

    CoverArt fetchCover(String isbn) {
        return client.get()
            .uri("/covers/{isbn}", isbn)
            .retrieve()                                 // send the request
            .body(CoverArt.class);                      // deserialize JSON -> record
    }
}

The response JSON is deserialized by the same Jackson you already know. Notice you inject a RestClient.Builder - Boot auto-configures it (with sensible timeouts and converters), and you customize per client.

Handling failures

Remote calls fail - timeouts, 404s, 500s. RestClient lets you handle status codes explicitly instead of letting exceptions surprise you:

CoverArt cover = client.get()
    .uri("/covers/{isbn}", isbn)
    .retrieve()
    .onStatus(status -> status.value() == 404, (req, res) -> {
        throw new CoverNotFoundException(isbn);        // map remote 404 to your domain
    })
    .body(CoverArt.class);

When to reach for WebClient

You'll meet WebClient and the reactive model properly in the Reactive module. For BookVault's blocking, request-per-thread style, RestClient is the natural choice.