Chapter 10. Working with objects
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 very natural object-oriented view of persistence in Java applications.
10.1. 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 doesn't 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).
-
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 don't execute manual UPDATE
statements, or DELETE
statements when an object should be made transient.
-
Detached
- 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. A detached instance can be reattached to a new Session
at a later 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.
We'll now discuss the states and state transitions (and the Hibernate methods that trigger a transition) in more detail.