Why are shared references to collections bad?

stewart.cambridge · **Joined:** Sat Nov 19, 2005 2:46 pm **Posts:** 69

This conversation thread explains the cause of the exception "Found shared references to a collection":

http://forums.hibernate.org/viewtopic.php?t=944335

However, I'm curious as to why shared references to collections are a problem? Is there something inherent in DB relations that make it impossible, or is it a design decision by the Hibernate team to encourage best practice?
In short, why does Hibernate throw an exception in this circumstance?

I can think of several examples of object model where you might want different entities to reference collections that end up the same. An example might be product SKUs in stock (on the warehouse object) and product SKUs manufactured by ACME (on the manufacturer object) where all that's left in stock are ACME products.

How does hibernate cope with this - or have I missed something in understanding mapped associations?

Thanks,

axismundi · **Joined:** Mon Aug 22, 2005 5:45 am **Posts:** 146

There are numerous issues involved.
According to my experience you are free to reference instances in different collections at the same time as long as they are attached (via proxy) to the same hibernate session.
You gonna run into severe trouble once you need to re-attach a persistent collection to a new session and an instance contained in this collection is referenced elsewhere in another session.

hth.

stewart.cambridge · **Joined:** Sat Nov 19, 2005 2:46 pm **Posts:** 69

Is that so? If this is true, then it's news to me. This would explain why I've sometimes been getting session errors, and sometimes shared reference errors.

Presumably I can share as many references as I like if I can sort out the session situation?

It's not what seems obvious to me, based upon what I've experienced though. I would expect that the statement:

Code:

session.createCriteria(clazz).list();

would all occurr within the same session, but I was getting a "collection is not associated with any session" because that class contains a list of references to all instances of itself - meaning each object refers to the same list - a perfectly feasible thing in "unpersisted" java. (If I add a restriction to the above criteria, the exception becomes "Found shared references to a collection".