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.
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.
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.
Hibernate defines and supports the following object states:
1. Transient - Transient instances will be destroyed by the garbage collector if the application does not hold a reference anymore. 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. 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).
2. Persistent - 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. 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. Developers do not execute manual UPDATE statements, or DELETE statements when an object should be made transient.
3. Detached - A detached instance can be reattached to a new Session at a later point in time, making it (and all the modifications) persistent again. a detached instance is an object that has been persistent, but its Session has been closed. The reference to the object is still valid, of course, and the detached instance might even be modified in this state. 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.