创建UserController、BookController、OrderController、CartController类，分别对应四张表的请求处理。在Controller层中调用Service层，完成对应的业务逻辑操作，并返回结果。

好的，下面是示例代码：

UserController：

@RestController
@RequestMapping("/users")
public class UserController {
    
    @Autowired
    private UserService userService;
    
    @GetMapping("/{id}")
    public ResponseEntity<User> getUserById(@PathVariable Long id) {
        User user = userService.getUserById(id);
        return ResponseEntity.ok(user);
    }
    
    @PostMapping("/")
    public ResponseEntity<User> createUser(@RequestBody User userRequestBody) {
        User user = userService.createUser(userRequestBody);
        return ResponseEntity.status(HttpStatus.CREATED).body(user);
    }
    
    @PutMapping("/{id}")
    public ResponseEntity<User> updateUser(@PathVariable Long id, @RequestBody User userRequestBody) {
        User user = userService.updateUser(id, userRequestBody);
        return ResponseEntity.ok(user);
    }
    
    @DeleteMapping("/{id}")
    public ResponseEntity<Void> deleteUser(@PathVariable Long id) {
        userService.deleteUser(id);
        return ResponseEntity.noContent().build();
    }
}

BookController：

@RestController
@RequestMapping("/books")
public class BookController {
    
    @Autowired
    private BookService bookService;
    
    @GetMapping("/{id}")
    public ResponseEntity<Book> getBookById(@PathVariable Long id) {
        Book book = bookService.getBookById(id);
        return ResponseEntity.ok(book);
    }
    
    @PostMapping("/")
    public ResponseEntity<Book> createBook(@RequestBody Book bookRequestBody) {
        Book book = bookService.createBook(bookRequestBody);
        return ResponseEntity.status(HttpStatus.CREATED).body(book);
    }
    
    @PutMapping("/{id}")
    public ResponseEntity<Book> updateBook(@PathVariable Long id, @RequestBody Book bookRequestBody) {
        Book book = bookService.updateBook(id, bookRequestBody);
        return ResponseEntity.ok(book);
    }
    
    @DeleteMapping("/{id}")
    public ResponseEntity<Void> deleteBook(@PathVariable Long id) {
        bookService.deleteBook(id);
        return ResponseEntity.noContent().build();
    }
}

OrderController：

@RestController
@RequestMapping("/orders")
public class OrderController {
    
    @Autowired
    private OrderService orderService;
    
    @GetMapping("/{id}")
    public ResponseEntity<Order> getOrderById(@PathVariable Long id) {
        Order order = orderService.getOrderById(id);
        return ResponseEntity.ok(order);
    }
    
    @PostMapping("/")
    public ResponseEntity<Order> createOrder(@RequestBody Order orderRequestBody) {
        Order order = orderService.createOrder(orderRequestBody);
        return ResponseEntity.status(HttpStatus.CREATED).body(order);
    }
    
    @PutMapping("/{id}")
    public ResponseEntity<Order> updateOrder(@PathVariable Long id, @RequestBody Order orderRequestBody) {
        Order order = orderService.updateOrder(id, orderRequestBody);
        return ResponseEntity.ok(order);
    }
    
    @DeleteMapping("/{id}")
    public ResponseEntity<Void> deleteOrder(@PathVariable Long id) {
        orderService.deleteOrder(id);
        return ResponseEntity.noContent().build();
    }
}

CartController：

@RestController
@RequestMapping("/carts")
public class CartController {
    
    @Autowired
    private CartService cartService;
    
    @GetMapping("/{id}")
    public ResponseEntity<Cart> getCartById(@PathVariable Long id) {
        Cart cart = cartService.getCartById(id);
        return ResponseEntity.ok(cart);
    }
    
    @PostMapping("/")
    public ResponseEntity<Cart> createCart(@RequestBody Cart cartRequestBody) {
        Cart cart = cartService.createCart(cartRequestBody);
        return ResponseEntity.status(HttpStatus.CREATED).body(cart);
    }
    
    @PutMapping("/{id}")
    public ResponseEntity<Cart> updateCart(@PathVariable Long id, @RequestBody Cart cartRequestBody) {
        Cart cart = cartService.updateCart(id, cartRequestBody);
        return ResponseEntity.ok(cart);
    }
    
    @DeleteMapping("/{id}")
    public ResponseEntity<Void> deleteCart(@PathVariable Long id) {
        cartService.deleteCart(id);
        return ResponseEntity.noContent().build();
    }
}

这里假设每个Controller都有对应的Service层实现了相应的逻辑，示例代码就不再赘述。