BookService.java

@AllArgsConstructor
@Service
public class BookService {

    @Autowired
    private BookRepository bookRepository;

    public List<Book> findAll() {
        Iterable<Book> book = bookRepository.findAll();
        List<Book> books = new ArrayList<>();
        book.iterator().forEachRemaining(books::add);
        return books;
    }

    public Book save(Book book) {
        return bookRepository.save(book);
    }

    public void deleteById(String id) {
        bookRepository.deleteById(id);
    }
}