hibernate笔记，结束第九章

conversation implementation with detached object state有时候需要考虑重写equals(), 这时候要考虑implementing equality with a business key

The save() operation also returns the database identifier of the persistent instance.

 

get和load的区别：

If no row with the given identifier value exists in the
database,  get()  returns  null.  The  load()  method  throws  an  ObjectNotFoundException.

The  get() method on the other hand never returns a proxy, it always hits the database.

 

you  don’t  have  to  reattach  (with  update()  or  lock())  a  detached
instance to delete it from the database.

 

item.getId(); // The database identity is "1234"
item.setDescription(...);
Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
Item item2 = (Item) session.get(Item.class, new Long(1234));
session.update(item); // Throws exception!
tx.commit();
session.close();

 

A persistent instance with the same database identifier is already associated with the Session!

 

merge的时候，如果persistence context有当前的item, update它，如果没有从db里找，要是找到了，update它，要是找不到，create一条记录。