Handle 404 from external data source

Problem

You want to serve under /books/{isbn} a dynamically generated page for every book retrieved from an external API .

When sending GET requests to external API to retrieve book details for some ISBNs the API returns 404 Not Found status.

How to make Spring return 404 as well?

Solution

To handle special cases in an uniform way Spring @ResponseStatus can be applied to a custom exception class.

BookNotFoundException.java

@ResponseStatus(value=HttpStatus.NOT_FOUND)
public class BookNotFoundException extends RuntimeException {
  // ...
}

When controller encounters such exception Spring will set the response type according to the value defined in the annotation.

Exceptions should be thrown right away so the best place to so is the service class responsible for invoking the external API.

BookApiClient.java

@ResponseStatus(value=HttpStatus.NOT_FOUND)
@Component
public class BookApiClient {

  private final WebClient webClient = WebClient.builder().build();

  public Mono<Book> getBook(String isbn) {
    webClient.get()
      .uri("https://book.api/books/" + isbn)
      .retrieve()
      .onStatus(status -> status.equals(HttpStatus.NOT_FOUND),
          response -> Mono.error(new BookNotFoundException()))
      .bodyToMono(Book.class);
  }

}

If getBook() method is invoked outside Markuply context (in controller) then BookNotFoundException will reach Spring exception handling mechanism.

However if that same method is invoked within Markuply component then it will be swallowed by the default ComponentErrorHandler.

To mitigate that we have to customize the error handler by providing it with exception types which should be passed downstream as they should affect the generation of the whole page rather than just a fragment.

SpringConfiguration.java

@Bean
ComponentErrorHandler handler() {
  return ComponentErrorHandlers.logAllExcept(BookNotFoundException.class);
}