Hibernate / JPA：未初始化的代理传递给persist（）

Question

我有两张表友谊和用户。我想要做的是与现有用户建立友谊关系。

@Entity(name = "friendship")
public class Friendship {

    private static final long serialVersionUID = -8950073132358183472L;

    @ManyToOne(fetch = FetchType.EAGER, cascade = { CascadeType.ALL})
    @JoinColumn(name = "requester_id", referencedColumnName = "id")
    private User requester;

    @ManyToOne(fetch = FetchType.EAGER, cascade = { CascadeType.ALL})
    @JoinColumn(name = "receiver_id", referencedColumnName = "id")
    private User receiver; 

}

@Entity(name = "user")
public class User {

    @OneToMany(mappedBy = "requester")
    private Set<Friendship> requesterFriendships;

    @OneToMany(mappedBy = "receiver")
    private Set<Friendship> requestedFriendships;

}

public class FriendshipServiceImpl implements FriendshipService {

    @Autowired
    FriendshipRepository friendshipRepository;

    public Friendship save(Friendship friendship) {

        return friendshipRepository.save(friendship);
    }


    public Friendship requestFriendship(User requester, User receiver) {
        Friendship friendship = new Friendship(requester, receiver, LocalDateTime.now(), FriendshipStatus.REQUESTED);
        return save(friendship);
    }
}

调用控制器方法：

@RequestMapping(value = "/add/{id}", method = RequestMethod.GET)
public String addFriend(@PathVariable("id") long id, Principal principal) {
    User requester = userService.getUserByUsername(principal.getName());
    User receiver = userService.getUser(id);

    if (requester != null && receiver != null) {
        friendshipService.requestFriendship(requester, receiver);
    }

    return "friendsManagement";
}

FriendshipRepository扩展了Spring数据CrudRepository。现在，当我尝试保存时，我收到以下错误。请求者和接收者User对象已经是现有对象。帮我理解这个错误。

SEVERE: Servlet.service() for servlet [dispatcherServlet] in context with path [/sample-app] threw exception [Request processing failed; nested exception is org.springframework.dao.InvalidDataAccessApiUsageException: uninitialized proxy passed to persist(); nested exception is org.hibernate.PersistentObjectException: uninitialized proxy passed to persist()] with root cause
org.hibernate.PersistentObjectException: uninitialized proxy passed to persist()
    at org.hibernate.event.internal.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:80)
    at org.hibernate.internal.SessionImpl.firePersist(SessionImpl.java:764)
    at org.hibernate.internal.SessionImpl.persist(SessionImpl.java:757)
    at org.hibernate.jpa.event.internal.core.JpaPersistEventListener$1.cascade(JpaPersistEventListener.java:80)
    at org.hibernate.engine.internal.Cascade.cascadeToOne(Cascade.java:379)
    at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:304)
    at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:146)
    at org.hibernate.engine.internal.Cascade.cascade(Cascade.java:95)