我是 Jpa 的新手,我在我的 spring boot 中添加了 spring boot jpa 用于一对一映射。所以,
package com.jpa.onetoone.onetoonejpa.model;
import org.hibernate.annotations.GeneratorType;
import javax.persistence.*;
@Entity
public class User {
@Id
@GeneratedValue
private Long id;
private String username;
private String password;
@OneToOne(cascade=CascadeType.ALL,mappedBy = "user",fetch = FetchType.LAZY)
private Address address;
public User(String username, String password) {
this.username = username;
this.password = password;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public Address getAddress() {
return address;
}
public void setAddress(Address address) {
this.address = address;
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", username='" + username + '\'' +
", password='" + password + '\'' +
'}';
}
public User() {
}
}
Address.java文件是
package com.jpa.onetoone.onetoonejpa.model;
import com.fasterxml.jackson.annotation.JsonIgnore;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToOne;
@Entity
public class Address {
@Id
@GeneratedValue
private Long id;
private String homeAddress;
private int homeNumber;
@OneToOne
@JsonIgnore
private User user;
public Address(String homeAddress, int homeNumber) {
this.homeAddress = homeAddress;
this.homeNumber = homeNumber;
}
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getHomeAddress() {
return homeAddress;
}
public void setHomeAddress(String homeAddress) {
this.homeAddress = homeAddress;
}
public int getHomeNumber() {
return homeNumber;
}
public void setHomeNumber(int homeNumber) {
this.homeNumber = homeNumber;
}
public Address() {
}
}
我在 User 和 Address 之间添加了一对一的关系。我已经将数据插入数据库并希望通过 RestController 获取数据。所以,我只是在 User 类中使用 FetchType.Lazy 和 FetchType.Eager 测试我的应用程序但是我得到相同的 Json。使用 FetchType 的 JSOn 没有变化。
当我点击 URL 时:http://localhost:8080/users在 fetchtype.lazy 和 fetchtype.eager 中,我得到了相同的 JSON,因为我在地址字段中使用 fetchtype.lazy 时需要删除地址字段。
@OneToOne(cascade=CascadeType.ALL,mappedBy = "user",fetch = FetchType.LAZY)
private Address address;
我在这两种情况下得到的 JSON 是:
[
{
"id":3,
"username":"ashwin",
"password":"karki",
"address":{
"id":4,
"homeAddress":"kapan",
"homeNumber":71444
}
}
]
我想问的另一个问题是,当我尝试访问 http://localhost:8080/address 时,我还想要与地址关联的用户对象。循环进入无限期,我添加了 JsonIgnore,然后 Json 打印如下。有什么办法,我们可以在点击这两个 URL 时从地址访问用户和从用户访问地址?
[
{
"id":4,
"homeAddress":"kapan",
"homeNumber":71444
}
]
默认 Controller .java
package com.jpa.onetoone.onetoonejpa.controller;
import com.jpa.onetoone.onetoonejpa.model.Address;
import com.jpa.onetoone.onetoonejpa.model.User;
import com.jpa.onetoone.onetoonejpa.repository.AddressRepository;
import com.jpa.onetoone.onetoonejpa.repository.UserRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import java.util.List;
@RestController
public class DefaultController {
@Autowired
private UserRepository userRepository;
@Autowired
private AddressRepository addressRepository;
@GetMapping(value = "/users")
public List<User> getUsers() {
List<User> users = userRepository.findAll();
return users;
}
@GetMapping(value = "/address")
public List<Address> getAddress() {
List<Address> address = addressRepository.findAll();
return address;
}
}
最佳答案
使用 FetchType 的 JSON 没有变化——也不应该。 fetch 类型是一种从数据库加载数据的策略。
通过一个 SQL 调用同时加载 EAGER 用户和地址:
@GetMapping(value = "/users")
public List<User> getUsers() {
List<User> users = userRepository.findAll(); // users and addresses was loaded
return users;
}
使用 LAZY,当您不阅读地址时,不会加载地址。但是,当您从 Controller 返回用户时,JSON 映射器读取地址属性,因此它将通过每个用户一个 SQL 调用加载:
@GetMapping(value = "/users")
public List<User> getUsers() {
List<User> users = userRepository.findAll(); // users was loaded
return users; // addresses was loaded for each user one by one
}
第二个问题。有很多方法。参见 JsonView ,例如。
关于java - 在 Fetchtype Lazy 和 Eager 上获得相同的值,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/57351892/