休眠：无法将数据回取到Map&lt；&gt； [英] Hibernate: Cannot fetch data back to Map<>

问题描述

拥有这些实体：

User.java：

@Entity
@Data
@NoArgsConstructor
public class User {
    @Id @GeneratedValue
    private int id;
    private String username;
    private String about;
    @OneToMany(mappedBy = "owner", cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.EAGER)
    private Map<User, Friendship> friendships = new HashMap<>();
    @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.EAGER)
    private Collection<Post> posts = new ArrayList<>();

    public User(String username) {
        this.username = username;
    }

    public void addFriend(User friend){
        Friendship friendship = new Friendship();
        friendship.setOwner(this);
        friendship.setFriend(friend);
        this.friendships.put(friend, friendship);
    }

    public void addPost(Post post){
        post.setAuthor(this);
        this.posts.add(post);
    }
}

Friendship.java：

@Entity
@Getter
@Setter
@NoArgsConstructor
public class Friendship {
    @EmbeddedId
    private FriendshipId key = new FriendshipId();
    private String level;
    @ManyToOne
    @MapsId("ownerId")
    private User owner;
    @ManyToOne
    @MapsId("friendId")
    private User friend;
}

Friendship Id.Java：

@Embeddable
public class FriendshipId implements Serializable {
    private int ownerId;
    private int friendId;
}

UserRepository.Java：

public interface UserRepository extends JpaRepository<User, Integer> {
    public User findByUsername(String username);
}

和最终DemoApplication.java：

@Bean
    public CommandLineRunner dataLoader(UserRepository userRepo, FriendshipRepository friendshipRepo){
        return new CommandLineRunner() {
            @Override
            public void run(String... args) throws Exception {
                User f1 = new User("friend1");
                User f2 = new User("friend2");
                User u1 = new User("user1");

                u1.addFriend(f1);
                u1.addFriend(f2);
                userRepo.save(u1);

                User fetchedUser = userRepo.findByUsername("user1");
            System.out.println(fetchedUser);
            System.out.println(fetchedUser.getFriendships().get(f1));

            }
        };
    }

userRepo.save(u1)操作后，各表如下：

mysql> select * from user;
+----+-------+----------+
| id | about | username |
+----+-------+----------+
|  1 | NULL  | user1    |
|  2 | NULL  | friend1  |
|  3 | NULL  | friend2  |
+----+-------+----------+

select * from friendship;
+-------+-----------+----------+-----------------+
| level | friend_id | owner_id | friendships_key |
+-------+-----------+----------+-----------------+
| NULL  |         2 |        1 |               2 |
| NULL  |         3 |        1 |               3 |
+-------+-----------+----------+-----------------+

如您所见，所有朋友都已保存。然而，这一声明：

        System.out.println(fetchedUser.getFriendships().get(f1));

返回null。即使fetchedUser已获取朋友图：

        System.out.println(fetchedUser);

打印：

User(id=1, username=user1, about=null, friendships={User(id=2, username=friend1, about=null, friendships={}, posts=[])=com.example.demo.model.Friendship@152581e8, User(id=3, username=friend2, about=null, friendships={}, posts=[])=com.example.demo.model.Friendship@58a5d38}, posts=[])

那么，当Mapfriendships被完全获取(从上面的语句中可以看到，所有的朋友都被获取了)时，为什么无法获取朋友f1(更准确地说是null)？

PS：

我已经删除了@DataLombok批注(刚刚添加了@Getter、@Setter和@NoArgsConstructor`)，并亲自重写了equalsAndHashCode：

@Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof User)) return false;
        User user = (User) o;
        return id == user.id && Objects.equals(username, user.username) && Objects.equals(about, user.about) && Objects.equals(friendships, user.friendships) && Objects.equals(posts, user.posts);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, username, about, friendships, posts);
    }

或者换句话说，equals()方法使用User类的所有字段。

推荐答案

如您所见，所有朋友都已保存。然而，这一声明：

    System.out.println(fetchedUser.getFriendships().get(f1)); returns null. 

即使fetchedUser拥有好友地图 已提取：

    System.out.println(fetchedUser);

打印：

 User(id=1, username=user1, about=null, friendships={User(id=2, username=friend1, about=null, friendships={}, posts=[])=com.example.demo.model.Friendship@152581e8, User(id=3, username=friend2, about=null, friendships={}, posts=[])=com.example.demo.model.Friendship@58a5d38}, posts=[])

问题是，当f1User添加到friendshipsHashMap时，主键id不存在。它稍后会在某个时候通过Hibernate进行更新。这会更改HashCode值！

hashcode键的值在添加到Map后不应更改。这就是导致问题的原因。模拟问题的简单测试代码-https://www.jdoodle.com/a/3Bg3

import lombok.*;
import java.util.*;

public class MyClass {
    public static void main(String args[]) {
      Map<User, String> friendships = new HashMap<>();
        User f1 = new User();
        f1.setUsername("friend1");
        
        User f2 = new User();
        f2.setUsername("friend2");
        friendships.put(f1, "I am changed. Can't find me");
        friendships.put(f2, "Nothing changed. So, you found me");
        
        System.out.println(f1.hashCode()); // -600090900
        f1.setId(1); // Some id gets assigned by hibernate. Breaking the hashcode
        System.out.println(f1.hashCode()); // -600090841 (this changed !!!)

        System.out.println(friendships); // prints f1, f2 both
        System.out.println(friendships.get(f1)); // prints null
        System.out.println(friendships.get(f2));
    }
}

// @Data
@Getter
@Setter
@EqualsAndHashCode
@ToString
class User
{
    private int id;
    private String username;
}

解决方案

将用户添加到映射后，不应更改hashcode值。我认为有几个选择可以尝试解决这个问题-

1. 在将数据库中的好友放入friendship地图之前将其持久化。因此该ID已分配。
2. 根本不要覆盖equals和hashcode。使用默认设置。基于对象标识。
3. 使用固定的哈希码。例如，如果username分配后没有变化，则该字段可用于生成hashcode值。

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