BookRentalsService

package bookrental.service.book.rentals;

import bookrental.model.account.User;
import bookrental.model.book.Book;
import bookrental.model.book.BookRentals;
import bookrental.repository.account.UserRepository;
import bookrental.repository.book.BookRepository;
import bookrental.repository.book.BookRentalsRepository;
import flexjson.JSONSerializer;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.stereotype.Service;

import java.util.ArrayList;
import java.util.List;

@Service
public class BookRentalService {

    private final UserRepository userRepository;
    private final BookRepository bookRepository;
    private final BookRentalsRepository bookRentalsRepository;

    @Autowired
    public BookRentalService(BookRepository bookRepository, BookRentalsRepository bookRentalsRepository, UserRepository userRepository) {
        this.bookRepository = bookRepository;
        this.bookRentalsRepository = bookRentalsRepository;
        this.userRepository = userRepository;
    }
....

    public ResponseEntity<String> findAllRentals() {
        List<BookRentals> rentedBooks = new ArrayList<>();
        bookRentalsRepository.findAll().forEach(rentedBooks::add);
        HttpHeaders headers = new HttpHeaders();
        headers.add("Content-Type", "application/json; charset=utf-8");
        return new ResponseEntity<>(new JSONSerializer().exclude("class").exclude("available").serialize(rentedBooks), headers,HttpStatus.OK);
    }
}

controller

package bookrental.controller.book.rentals;

import bookrental.service.book.rentals.BookRentalService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class BookRentalController {

    private final BookRentalService bookRentalService;

    @Autowired
    public BookRentalController(BookRentalService bookRentalService) {
        this.bookRentalService = bookRentalService;
    }
...

    @GetMapping(value = "/books/rentals", produces ="application/json")
    public ResponseEntity<String> findAllRentals() {
        return bookRentalService.findAllRentals();
    }
}

output in json

[
    {
        "book": {
            "author": "Henryk Sienkiewicz",
            "available": false,
            "category": "powieść historyczna",
            "class": "bookrental.model.book.Book",
            "id": 1,
            "title": "Krzyżacy"
        },
        "id": 1,
        "user": {
            "class": "bookrental.model.account.User",
            "id": 2,
            "name": "piotri",
            "password": "123"
        }
    }
]