Functional style error fallback support in RestClient.retrieve()

[vinsguru]

The RestClient is great. However, its error handling design is currently limited to side effect style callbacks through .onStatus(...), where the typical behavior is to throw exceptions. 😬

In many cases, especially when writing resilient or fault-tolerant clients, developers would prefer to use functional style fallbacks, returning a default or cached value when the response status indicates an error, instead of throwing and catching exceptions manually. It helps us to write clean code.

Current Behavior

Book book;
try {
    book = restClient.get()
        .uri("/books/{id}", id)
        .retrieve()
        .body(Book.class);
} catch (RestClientException ex) {
    book = defaultBook;
}

Proposed Enhancement Introduce a functional-style error fallback mechanism (similar to WebClient’s reactive operators).

Book book = restClient.get()
                 .uri("/books/{id}", id)
                 .retrieve()
                 .onStatus(HttpStatusCode::is5xxServerError, (req, res) -> defaultBook)
                 .body(Book.class);

Motivation

• Keeps the API consistent with the functional style of WebClient.
• Reduces boilerplate and improves readability for simple fallback cases.
• Encourages exception-free control flow for predictable errors like 404 or 500.