java - JPA/Hibernate preUpdate 不更新父对象

Question

在我的应用程序中，我定义了以下类:

@Entity
@Table(name = "forums")
public class Forum {
    @Id
    @GeneratedValue(generator = "system-uuid")
    @GenericGenerator(name = "system-uuid", strategy = "uuid")
    private String id;

    private String name;
    private Date lastActivity;

    @OneToMany(mappedBy = "forum", cascade = { CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE })
    private List<Post> posts;

---

@Entity
@Table(name = "posts")
public class Post {
    @Id
    @GeneratedValue(generator = "system-uuid")
    @GenericGenerator(name = "system-uuid", strategy = "uuid")
    private String id;

    private String username;
    private String content;
    private Date creationDate;

    @ManyToOne(optional = false, cascade = { CascadeType.MERGE, CascadeType.PERSIST })
    private Forum forum;

    public Post() {
        creationDate = new Date();
    }

    @PrePersist
    private void onPersist() {
        System.out.println(getClass().getName() + ": onPersist");
        if (creationDate == null) {
            creationDate = new Date();
        }

        forum.setLastActivity(creationDate);
    }

    @PreUpdate
    private void onUpdate() {
        forum.setLastActivity(new Date());
    }

如果我尝试向论坛实体添加新帖子，lastActivity 字段会通过 @PrePersist 回调在数据库中正确更新。 但是，如果我尝试使用以下代码更新帖子实体:

  entityManager.getTransaction().begin();
  Post post = entityManager.find(Post.class, "postId");
  post.setContent("New post text");
  entityManager.merge(post);
  entityManager.getTransaction().commit();

只更新发布数据，lastActivity 字段值不变。在我看来，@PreUpdate 方法应该可以解决问题并更新论坛实体。 这是错误还是我遗漏了什么？

Answer 1

这不是错误，即使通过快速尝试，这对我来说也符合您的预期。负面消息是它不能保证有效，因为:

从 JPA 2.0 规范的第 93 页开始:

In general, the lifecycle method of a portable application should not invoke EntityManager or Query operations, access other entity instances, or modify relationships within the same persistence context.[43] A lifecycle callback method may modify the non-relationship state of the entity on which it is invoked.

第 95 页:

It is implementation-dependent as to whether callback methods are invoked before or after the cascading of the lifecycle events to related entities. Applications should not depend on this ordering.