本文介紹了休眠：無法將數據回取到Map<；>；的處理方法，對大家解決問題具有一定的參考價值，需要的朋友們下面隨著小編來一起學習吧！
問題描述
擁有這些實體：
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?。?/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=[])
那么，當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，為什么無法獲取朋友f1(更準確地說是null)？
PS：
我已經刪除了@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f1User添加到friendshipsHashMap時，主鍵id不存在。它稍后會在某個時候通過Hibernate進行更新。這會更改HashCode值！
hashcode鍵的值在添加到Map后不應更改。這就是導致問題的原因。模擬問題的簡單測試代碼-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，不應更改hashcode值。我認為有幾個選擇可以嘗試解決這個問題-
-
在將數據庫中的好友放入
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。使用默認設置?；趯ο髽俗R。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。
這篇關于休眠：無法將數據回取到Map<；>；的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，
