(TUTORIAL) Hibernate Tutorial by Murali

Hibernate is an Object Relational Mapping (ORM) tool. It manages the persistence of java objects in a relational database. The idea is that a programmer should be able to design his business objects as standard Java objects with very little interference from the problems of making these objects persist in a database. Together with a little help from the programmer, Hibernate saves the objects into the database, retrieves them when needed and supports queries on the database written in a form similar to SQL but which refers to objects and object properties instead of tables and column names. The end result is that the code that needs to be written to interact with the database is considerably shorter and simpler.

The Hibernate Object Life Cycle

Transient objects do not (yet) have any association with the database. they act like any normal Java object and are not saved to the database. When the last reference to a transient object is lost, the object itself is lost and is (eventually) garbage collected. There is no connection between transactions and such objects: commits and rollbacks have no effects on them. They can be turned into persistent objects via one of the save method calls if the Session object or by adding a reference from a persistent object to this object.

Persistent objects do have an association with the database. They are always associated with a persistence manager, i.e., a Session object and they always participate in a transaction. Actual updates of a database from the persistent object may occur at any time between when the object is updated to the end of the transaction: it does not necessarily happen immediately. However, this feature, which allows important optimizations in database interactions, is essentially invisible to the programmer. For example, one place where one might expect to notice the difference between the in-memory persistent object and the database version is at the point of executing a query. In such a case, Hibernate will, if necessary, synchronise any dirty objects with the database (i.e., save them) in order to ensure that the query returns the correct results.

A persistent object has a primary key value set, whether or not it has been actually saved to the database yet.

Calling the delete method of the Session object on a persistent object will cause its removal from the database and will make it transient.

Detached objects are objects that were persistent but no longer have a connection to a Session object (usually because you have closed the session). Such an object contains data that was synchronised with the database at the time that the session was closed, but, since then, the database may have changed; with the result that this object is now stale.

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