PO即Persistence Object
VO即Value Object
一个vo就是一个普通的对象，对应到对象状态当中是transient, detached两种状态。
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 applicatio[......]
