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本文介紹了休眠：無法將數(shù)據(jù)回取到Map&lt；&gt；的處理方法，對(duì)大家解決問題具有一定的參考價(jià)值，需要的朋友們下面隨著小編來一起學(xué)習(xí)吧！

問題描述

擁有這些實(shí)體：

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);
}

和最終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 |
+-------+-----------+----------+-----------------+

如您所見，所有朋友都已保存。然而，這一聲明：

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

返回null。即使fetchedUser已獲取朋友圖：

        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，當(dāng)Mapfriendships被完全獲取(從上面的語句中可以看到，所有的朋友都被獲取了)時(shí)，為什么無法獲取朋友f1(更準(zhǔn)確地說是null)？

PS：

我已經(jīng)刪除了@DataLombok批注(剛剛添加了@Getter、@Setter和@NoArgsConstructor`)，并親自重寫了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);
    }

或者換句話說，equals()方法使用User類的所有字段。

推薦答案

如您所見，所有朋友都已保存。然而，這一聲明：

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

即使fetchedUser擁有好友地圖
已提取：

    System.out.println(fetchedUser);

打?。?/p>

 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=[])

問題是，當(dāng)f1User添加到friendshipsHashMap時(shí)，主鍵id不存在。它稍后會(huì)在某個(gè)時(shí)候通過Hibernate進(jìn)行更新。這會(huì)更改HashCode值！

hashcode鍵的值在添加到Map后不應(yīng)更改。這就是導(dǎo)致問題的原因。模擬問題的簡單測(cè)試代碼-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;
}

解決方案

將用戶添加到映射后，不應(yīng)更改hashcode值。我認(rèn)為有幾個(gè)選擇可以嘗試解決這個(gè)問題-

在將數(shù)據(jù)庫中的好友放入friendship地圖之前將其持久化。因此該ID已分配。
根本不要覆蓋equals和hashcode。使用默認(rèn)設(shè)置?；趯?duì)象標(biāo)識(shí)。
使用固定的哈希碼。例如，如果username分配后沒有變化，則該字段可用于生成hashcode值。

這篇關(guān)于休眠：無法將數(shù)據(jù)回取到Map&lt；&gt；的文章就介紹到這了，希望我們推薦的答案對(duì)大家有所幫助，