Working with persistent objects(Chapter 4 of Hibernate In Action)

　　In Hibernate, objects instantiated using the new operator aren’t immediately persistent. Their state is transient, which means they aren’t associated with any database table row, and so their state is lost as soon as they’re dereferenced (no longer referenced by any other object) by the application.

　　A persistent instance is any instance with a database identity, as defined in chapter 3, section 3.4, “Understanding object identity.” That means a persistent instance has a primary key value set as its database identifier.