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Hibernate version: 2.1.7



Hi

In our application on startup we load via hibernate a lot of read only objects which stay around in memory during the life time of the jvm. There are many other objects that are created as needed via HQL and have more standard lifespans. These objects reference the read only objects.

Our problem is when hibernate loads these more standard objects we need hibernate to make them reference to the already created read only objects. However by default hibernate will create new instances of these read only objects instead of reusig the ones we have already created.

Any ideas?

I thank you in advance

maybe there is another way to do this, but you could re attach (.lock(obj, LockMode.NONE), everytime you get persistant instance referencing your read only objects.

It might be easy to it automatically on the app level

i'm idiot, what about using second level cache?

The 2nd level cache will always return a new instance though won't they?

What is the real issue with the new instances? The fact that they are new instances or that you end up going back to the DB to load up the data?

1- first time, read only instances (ROI) are loaded from an hibernate session and put in the second level cache
2- when detached, they still are in second level cache
3- querying persistant instance (PI) that refers to our ROI will only hit the db for what is needed, when fetching associated ROI, db won't be hit and there won't be new instances too

eh - gettings objects from 2nd level cache always create a new fresh instance ... it is build up from a "tuple" which is only read from the database once.

I'm with max :D

sorry for the mistake ;)

Hi guys

Thanks for your replies.

The issue with re-attaching using the lock method is that there are a large amount of read only objects. And the application has no way of knowing which ones to re-attach. To re-attach them all on each session would be a performance issue.

So I am still left clueless as to how to do this.

Any ideas?

But do you actually need to lock them? You're not updating them (they're read only) so as long as you aren't cascading to them you don't need to lock them in your session.

The objects your're saving can still reference them via many-to-ones, etc. Am I missing something?

I want the objects that I query out of the database to reference the read only objects and not new copies of the read only objects.

If I do not lock the read only objects to the session and I query via hibernate for some other object that is supposed to reference the read only object Hibernate will create a new instance or copy of the read only object.

The problem with this is that the read only object references a whole graph of other read only objects. So hibernate will start recreating the whole graph of read only objects

I have no idea why your would want this. What is wrong with a new object out of the second-level cache.

Well, you can do it with a UserType, if you absolutely insist.

But requirements like this ring alarm bells that you are doing something else wrong.

Gavin,

Thanks for your reply.

Our system is very similar to the system you describe in "Metadata driven applications"
http://blog.hibernate.org/cgi-bin/blosxom.cgi/2004/06/24#metadata-driven

The dynamic model is a dense graph. The Items reference the dynamic model. When Hibernate loads an Item we do not want Hibernate to rebuild the dense dynamic model in each session.

This is a Swing application that holds the dynamic model (an ontology) in memory across Hibernate sessions.

So use custom types.

Have you actually tested the performance of rebuilding the model each time?

In that case, UserTypes would be used for entities (Java classes that are mapped into the database under a <class> mapping tag). Is that possible? From what I've seen, UserTypes are used for value objects only.