场景X:

表:spititter(1)，表:Spittles(许多)(Spittles是与FK:spitter_id关系的所有者)

这个场景的结果是节省:喷壶和两个喷壶就像被同一个喷壶拥有一样。

        Spitter spitter=new Spitter();  
    Spittle spittle3=new Spittle();     
    spitter.setUsername("George");
    spitter.setPassword("test1234");
    spittle3.setSpittle("I love java 2");       
    spittle3.setSpitter(spitter);               
    dao.addSpittle(spittle3); // <--persist     
    Spittle spittle=new Spittle();
    spittle.setSpittle("I love java");
    spittle.setSpitter(spitter);        
    dao.saveSpittle(spittle); //<-- merge!!

场景Y:

这将拯救喷壶，将拯救2喷壶，但他们不会引用同一喷壶!

        Spitter spitter=new Spitter();  
    Spittle spittle3=new Spittle();     
    spitter.setUsername("George");
    spitter.setPassword("test1234");
    spittle3.setSpittle("I love java 2");       
    spittle3.setSpitter(spitter);               
    dao.save(spittle3); // <--merge!!       
    Spittle spittle=new Spittle();
    spittle.setSpittle("I love java");
    spittle.setSpitter(spitter);        
    dao.saveSpittle(spittle); //<-- merge!!

关于merge的更多细节将帮助你使用merge over persist:

Returning a managed instance other than the original entity is a critical part of the merge process. If an entity instance with the same identifier already exists in the persistence context, the provider will overwrite its state with the state of the entity that is being merged, but the managed version that existed already must be returned to the client so that it can be used. If the provider did not update the Employee instance in the persistence context, any references to that instance will become inconsistent with the new state being merged in. When merge() is invoked on a new entity, it behaves similarly to the persist() operation. It adds the entity to the persistence context, but instead of adding the original entity instance, it creates a new copy and manages that instance instead. The copy that is created by the merge() operation is persisted as if the persist() method were invoked on it. In the presence of relationships, the merge() operation will attempt to update the managed entity to point to managed versions of the entities referenced by the detached entity. If the entity has a relationship to an object that has no persistent identity, the outcome of the merge operation is undefined. Some providers might allow the managed copy to point to the non-persistent object, whereas others might throw an exception immediately. The merge() operation can be optionally cascaded in these cases to prevent an exception from occurring. We will cover cascading of the merge() operation later in this section. If an entity being merged points to a removed entity, an IllegalArgumentException exception will be thrown. Lazy-loading relationships are a special case in the merge operation. If a lazy-loading relationship was not triggered on an entity before it became detached, that relationship will be ignored when the entity is merged. If the relationship was triggered while managed and then set to null while the entity was detached, the managed version of the entity will likewise have the relationship cleared during the merge."

以上所有信息都摘自Mike Keith和Merrick Schnicariol的“Pro JPA 2掌握Java™Persistence API”。第六章。分段分离与合并。这本书实际上是作者专门介绍JPA的第二本书。这本新书比前一本有许多新信息。我非常推荐那些将认真参与JPA的人阅读这本书。我很抱歉匿名发布了我的第一个答案。

我注意到，当我使用em.merge时，每个INSERT都有一个SELECT语句，即使JPA没有为我生成字段——主键字段是我自己设置的UUID。我切换到em.persist(myEntityObject)，然后只得到INSERT语句。

persist(entity)应该与全新的实体一起使用，将它们添加到DB(如果实体已经存在于DB中，则会抛出EntityExistsException)。

应该使用Merge(实体)，如果实体被分离并被更改，则将实体放回持久性上下文中。

可能持久是生成INSERT sql语句和合并UPDATE sql语句(但我不确定)。

JPA规范说明了关于persist()的以下内容。

如果X是一个分离对象，则在持久化时可能会抛出EntityExistsException 操作被调用，或者EntityExistsException或另一个PersistenceException可能在刷新或提交时抛出。

因此，当对象不应该是分离对象时，使用persist()是合适的。您可能更喜欢让代码抛出PersistenceException，以便快速失败。

尽管规范不清楚，persist()可能会为对象设置@GeneratedValue @Id。但是merge()必须有一个已经生成的@Id对象。

merge和persist之间还有一些区别(我将再次列举已经在这里发布的区别):

D1。Merge不会使传递的实体受管理，而是返回另一个受管理的实例。在另一边持久化将使传递的实体被管理:

//MERGE: passedEntity remains unmanaged, but newEntity will be managed
Entity newEntity = em.merge(passedEntity);

//PERSIST: passedEntity will be managed after this
em.persist(passedEntity);

D2。如果你删除了一个实体，然后决定持久化该实体，你只能使用persist()来做，因为merge会抛出一个IllegalArgumentException。

D3。如果您决定手动处理您的id(例如通过使用uuid)，则合并 operation将触发后续的SELECT查询，以查找具有该ID的存在实体，而persist可能不需要这些查询。

D4。有些情况下，您只是不相信调用您的代码的代码，为了确保没有数据被更新，而是被插入，您必须使用持久化。