Having read the free chapter 2 of the Hibernate book, I understand how they open a session, perform a unit of work, then close the session. gurther more this ties in nicely with the default save/delete methods in the facades that work in the same way.

What I'm not sure about is when you create a findByxxxxx method in the facade, you open the session, perform your query, receiving a List of Object back and return that from your findBy method.

Initially I wasn't closing the session at the end of the findBy method, everything worked fine in the jsp/java, but later in the running of the program I would get errors regarding exitsing references or similar, often these could be solved by adding a session.clear.

However I eventually got a "Found two representations of same collection when flush called" error in the save method of a facade, that only occurred once the method had been used once. I found that a session close as the first command solved the problem.

So I thought maybe I should be closing the session at the end of the findBy methods too. However when I tried this, I got errors from the JSP when it was trying to work with the list returned from findBy, the error stated the session was closed, so obviously I shouldn't closed the session.

Anyone able to give me some pointers here?

Tom

Hi Tom.

This is one of the most popular controversy available on the Hibernate usage: where to open and where to close your sessions. AND, if you're working with a web-based MVC, there's even more to consider.

The fact is that you should close your session after using it in a way or another, sessions holds connections, transactions, cached objects, and lots of features and those are managed when you open and close it.

I'm working heavily with Hibernate on a project right now. We have adopted an approach where my DAO factory has no idea of there it's session came from, it's constructor injected by it's caller. This solves part of the problem.

Also, we're using an MVC framework called VRaptor (http://www.vraptor.org/) that comes with some solutions that allows you to delegate the session's opening and closing to interceptors. It gives you freedom to transparently handle lazy lists and attributes on your jsp's...

Is that any help? Hope so!

Regards

All of this stuff is explained in detail on http://hibernate.org/42.html