为什么要关注对象及其状态？而不是sql语句？
Hibernate is a full object/relational mapping solution that not only shields the developer from
the details of the underlying database management system, but also offers state management
of objects. This is, contrary to the management of SQL statements in common JDBC/SQL
persistence layers, a natural object-oriented view of persistence in Java applications.
In other words, Hibernate application developers should always think about the state of their
objects, and not necessarily about the execution of SQL statements. This part is taken care of
by Hibernate and is only relevant for the application developer when tuning the performance of
the system.
Hibernate object states
Hibernate defines and supports the following object states:
• Transient - an object is transient if it has just been instantiated using the new operator, and it
is not associated with a Hibernate Session. It has no persistent representation in the database
and no identifier value has been assigned. Transient instances will be destroyed by the garbage
collector if the application does not hold a reference anymore. Use the Hibernate Session to
make an object persistent (and let Hibernate take care of the SQL statements that need to be
executed for this transition).
1 数据库中不存在与瞬时对象相对应的记录 （根据配置文件unsaved-value来判断）
2 没有hibernate根据配置文件设置主键生成生成策略分配的identifier值
• Persistent - a persistent instance has a representation in the database and an identifier value.
It might just have been saved or loaded, however, it is by definition in the scope of a Session.
Hibernate will detect any changes made to an object in persistent state and synchronize the
state with the database when the unit of work completes. Developers do not execute manual
UPDATE statements, or DELETE statements when an object should be made transient.
1 数据库中存在与持久对象相对应的记录
2 有hibernate根据配置文件设置主键生成生成策略分配的identifier值
3 通过执行saved （从对象到session）or loaded（从db到session）
4 因为持久对象在session的管理范围之内 ，hibernate 可以将 持久对象的任何改变在事务提交时同步到数据库中（对持久对象调用update方法没有意义）
• Detached - a detached instance is an object that has been persistent, but its Session has been closed. （注意这句话,如果这个对象在session被删除了，那么这个对象是什么状态？）The reference to the object is still valid, of course, and the detached instance might even be modified in this state. A detached instance can be reattached to a new Session at alater point in time, making it (and all the modifications) persistent again. This feature enables a programming model for long running units of work that require user think-time. We call them application transactions, i.e., a unit of work from the point of view of the user.
1 脱管对象 是曾经容纳过这个持久对象的session，现在已经关闭了。（脱管对象有identifier值,）