I'm new in Hibernate i've got source like this when i compile the error occure


Session sesja = new Configuration().configure("/hibernate.cfg.xml").buildSessionFactory().openSession();
Transaction tx = sesja.beginTransaction();
Kategoria koc = (Kategoria) sesja.get(Kategoria.class, new Long(1));
tx.commit();
sesja.close();

Kategoria kat = new Kategoria();
kat.setKategoriaNazwa("Koza");
// here the error start
koc.getSetOfKategoria().add(kat);
kat.setKategoria(koc);

the error what i made wrong ?

11527 [main] ERROR org.hibernate.LazyInitializationException - failed to lazily initialize a collection (com.manabit.model.Kategoria.SetOfKategoria) - no session or session was closed
org.hibernate.LazyInitializationException: failed to lazily initialize a collection (com.manabit.model.Kategoria.SetOfKategoria) - no session or session was closed
at org.hibernate.collection.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:180)
at org.hibernate.collection.AbstractPersistentCollection.write(AbstractPersistentCollection.java:61)
at org.hibernate.collection.PersistentSet.add(PersistentSet.java:158)
at com.manabit.cwiczenia.Kategorie.main(Kategorie.java:43)
Exception in thread "main" org.hibernate.LazyInitializationException: failed to lazily initialize a collection (com.manabit.model.Kategoria.SetOfKategoria) - no session or session was closed
at org.hibernate.collection.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:180)
at org.hibernate.collection.AbstractPersistentCollection.write(AbstractPersistentCollection.java:61)
at org.hibernate.collection.PersistentSet.add(PersistentSet.java:158)
at com.manabit.cwiczenia.Kategorie.main(Kategorie.java:43)

Well, it told you exactly what the problem is - your session is closed. Most likely cuases are: a) you closed the sesion earlier, b) the object is detached. Most likely, it's b.

Objects become detached when the session that loaded them closes. In order to use the object with a new session, you must call session.update() or session.lock() to bind the object to the session.

A great way to prevent this is to use the Open Session In View (search the site and the forums). If you're a GUI, then the Thread Local pattern might do, which the Open Session in View pattern is based on.

Also, buy Hibernate in Action. It has chapters on lazy objects, detached persistent objects, and the open session in view pattern.

Also try to identify if you are using the anti-patter: Session per operation.[/list]

I just looked more closely at your code.

You are closing your session and then trying to access lazy associations. Keep the session open for the whole operation.

My earlier suggestions still hold true, however. Session management should be off-loaded to a process outside your data access code. I still recommend the Thread Local or Open Session in View patterns to manage your Hibernate Sessions. Manually opening and closing them is not only tedius, but it's asking for errors.

thanks for replay i now that i close the session
i read now Hibernate in Action

Session = getSession();
Tr ...
Category computer = Session.get(Category.class, new Long(1));
...
Session.close();


// here i've got the same problem // what i understood that
computer.getChildrenCategories().add(laptops); // this section must be independent we work ony on object which is independet from session
laptops.setParentCategory(computer); //


Session = getSession();
Tr ...

session.save(laptops);
...
Session.close();


maybe i do somethink wrong ??

Hi
You have to close the session only when you finsih your entire transaction. You cant say session.close and still try to set the value for the child and save.
The process should be done with in one entrire session for Lazy loading ie if you have enable lazy=true

thanks

thnx