Working with lazy associations

Persistent Object: An object with an id existing in the database that we pulled to the session.


Transient Object: An object with no id that has not yet been persisted.

Detached Object: It is the object that is formed as a result of closing the Session to which the persistent object is connected. The object's id field is full and probably corresponds to a record in the database.


! After hibernate3, the default value lazy is set to "true".


"By default, Hibernate3 uses lazy select fetching for collections and lazy proxy fetching for single-valued associations."


Lazy initialization exception

Fires if there is access to the detached object after commit. For example:

{

   s = sessions.openSession();
   Transaction tx = s.beginTransaction();

   Hibernate.initialize();

   User u = (User) s.createQuery("from User u where u.name=:userName").setString("userName", userName).uniqueResult();

   Map permissions = u.getPermissions();

   tx.commit();
   s.close();

   Integer accessLevel = (Integer) permissions.get("accounts"); // Error! Attempting to access detached object.

}

"Hibernate does not support lazy initialization for detached objects"


If we use non-lazy links (lazy="false") too much, we draw too many objects into memory. Instead, it may make more sense to use join fetch (non-lazy fetch) in the tansactions we want.

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