从detached entity passed to persist到JPA EntityManager: Why use persist() over merge()?

今天在测试添加通知功能的时候，报了detached entity passed to persist错误，原因是在保存通知到数据库的时候，通知里需要设置一个user对象，而传递过来的user对象是一个游离态的对象，在我的通知类中

@ManyToOne(cascade = {CascadeType.MERGE,CascadeType.REFRESH，CascadeType.PERSIST},optional = false)
private User source; //发起通知的人

后改成

@ManyToOne(cascade = {CascadeType.MERGE,CascadeType.REFRESH},optional = false)
private User source; //发起通知的人

即去掉了级联保存，问题解决了

后来又尝试了在不去掉级联保存的情况下，从数据库中查找出持久化对象填充notification对象，也能成功

notification.setSource(userRepository.findOne(source.getId()));

notificationRepository.save(notification);

jpa的persist()方法和merge()方法有什么区别呢？找到了一个很好的解释：这里

摘录下来，便于以后查看

persist:

• Insert a new register to the database
• Attach the object to the entity manager.

merge:

• Find an attached object with the same id and update it.
• If exists update and return the already attached object.
• If doesn't exist insert the new register to the database.

persist() efficiency:

• It could be more efficient for inserting a new register to a database than merge().
• It doesn't duplicates the original object.

persist() semantics:

• It makes sure that you are inserting and not updating by mistake.

实体的状态

• New (Transient)

  A newly created object that hasn’t ever been associated with a Hibernate Session (a.k.a Persistence Context) and is not mapped to any database table row is considered to be in the New (Transient) state.

  To become persisted we need to either explicitly call the EntityManager#persist method or make use of the transitive persistence mechanism.

• Persistent (Managed)

  A persistent entity has been associated with a database table row and it’s being managed by the current running Persistence Context. Any change made to such entity is going to be detected and propagated to the database (during the Session flush-time). With Hibernate, we no longer have to execute INSERT/UPDATE/DELETE statements. Hibernate employs a transactional write-behind working style and changes are synchronized at the very last responsible moment, during the current Session flush-time.

• Detached

  Once the current running Persistence Context is closed all the previously managed entities become detached. Successive changes will no longer be tracked and no automatic database synchronization is going to happen.

  To associate a detached entity to an active Hibernate Session, you can choose one of the following options:

  • Reattaching

    Hibernate (but not JPA 2.1) supports reattaching through the Session#update method. A Hibernate Session can only associate one Entity object for a given database row. This is because the Persistence Context acts as an in-memory cache (first level cache) and only one value (entity) is associated to a given key (entity type and database identifier). An entity can be reattached only if there is no other JVM object (matching the same database row) already associated to the current Hibernate Session.

  • Merging

  The merge is going to copy the detached entity state (source) to a managed entity instance (destination). If the merging entity has no equivalent in the current Session, one will be fetched from the database. The detached object instance will continue to remain detached even after the merge operation.

• Removed

  Although JPA demands that managed entities only are allowed to be removed, Hibernate can also delete detached entities (but only through a Session#delete method call). A removed entity is only scheduled for deletion and the actual database DELETE statement will be executed during Session flush-time.

To understand the JPA state transitions better, you can visualize the following diagram:

Or if you use the Hibernate specific API:

回答者还给出了自己写的书，https://vladmihalcea.com/tutorials/hibernate/