如何使用jpa在json中检索我的外键字段值？

@Entity
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;

    // getters and setters
}

@Entity
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "user_id")
    private User user;

    // getters and setters
}

import com.fasterxml.jackson.databind.ObjectMapper;

public class Main {
    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();

        User user = new User();
        user.setId(1L);
        user.setName("John Doe");

        Order order = new Order();
        order.setId(1L);
        order.setUser(user);

        String json = mapper.writeValueAsString(order);
        System.out.println(json);
    }
}

{
  "id": 1,
  "user": {
    "id": 1,
    "name": "John Doe"
  }
}

import com.fasterxml.jackson.annotation.JsonManagedReference;

@Entity
public class Order {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "user_id")
    @JsonManagedReference
    private User user;

    // getters and setters
}