这两种方法都会将实体添加到PersistenceContext中，区别在于之后如何处理实体。

Persist获取一个实体实例，将其添加到上下文中，并使该实例被管理(即将跟踪实体的未来更新)。

Merge返回与状态合并的托管实例。它确实返回存在于PersistenceContext中的内容或创建实体的新实例。在任何情况下，它都会从提供的实体复制状态，并返回一个托管副本。传入的实例不会被管理(你所做的任何更改都不会成为事务的一部分——除非你再次调用merge)。不过您可以使用返回的实例(托管实例)。

也许一个代码示例会有所帮助。

MyEntity e = new MyEntity();

// scenario 1
// tran starts
em.persist(e); 
e.setSomeField(someValue); 
// tran ends, and the row for someField is updated in the database

// scenario 2
// tran starts
e = new MyEntity();
em.merge(e);
e.setSomeField(anotherValue); 
// tran ends but the row for someField is not updated in the database
// (you made the changes *after* merging)
      
// scenario 3
// tran starts
e = new MyEntity();
MyEntity e2 = em.merge(e);
e2.setSomeField(anotherValue); 
// tran ends and the row for someField is updated
// (the changes were made to e2, not e)

场景1和3大致相同，但在某些情况下，您可能希望使用场景2。

Merge不会更新传递的实体，除非该实体被管理。即使实体ID被设置为一个现有的DB记录，一个新的记录将在数据库中创建。

我发现Hibernate文档中的解释很有启发性，因为它们包含了一个用例:

The usage and semantics of merge() seems to be confusing for new users. Firstly, as long as you are not trying to use object state loaded in one entity manager in another new entity manager, you should not need to use merge() at all. Some whole applications will never use this method. Usually merge() is used in the following scenario: The application loads an object in the first entity manager the object is passed up to the presentation layer some modifications are made to the object the object is passed back down to the business logic layer the application persists these modifications by calling merge() in a second entity manager Here is the exact semantic of merge(): if there is a managed instance with the same identifier currently associated with the persistence context, copy the state of the given object onto the managed instance if there is no managed instance currently associated with the persistence context, try to load it from the database, or create a new managed instance the managed instance is returned the given instance does not become associated with the persistence context, it remains detached and is usually discarded

来自:http://docs.jboss.org/hibernate/entitymanager/3.6/reference/en/html/objectstate.html

我在我的实体上得到了lazyLoading异常，因为我试图访问一个会话中的惰性加载集合。

我要做的是在一个单独的请求中，从会话检索实体，然后尝试访问我的jsp页面中的一个集合，这是有问题的。

为了缓解这个问题，我在控制器中更新了相同的实体，并将其传递给我的jsp，尽管我想象当我在会话中重新保存时，它也可以通过SessionScope访问，而不会抛出一个LazyLoadingException，这是示例2的修改:

以下是我行之有效的方法:

// scenario 2 MY WAY
// tran starts
e = new MyEntity();
e = em.merge(e); // re-assign to the same entity "e"

//access e from jsp and it will work dandy!!

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。

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!!