I'm trying out hibernate for the first time and I've run into a problem that I can't figure out. I'm trying to load a portion of a collection through a collection filter and I keep getting this exception:



The weird thing is that I don't always get this exception. I'll explain more after my code. I've trimmed it down to just the applicable sections.



The function getWritingsByTag() is loading the Tag object's collection of Writings via pagination. When I initially run my web app and getWritingsByTag() is called with values of firstResult=0 and numResults=15, the call is successful. The page can then be refreshed, revisited, etc. without error. But when I go to the next "page" of results and getWritingsByTag() is called with firstResult=15, the above exception is thrown. This behavior occurs every time.

Another interesting behavior is that if the user visiting my site visits page 2 first (firstResult=15), the function completes successfully (and the page can be refreshed and revisited without problem). If the same user then tries to go back to page one (firstResult=0), the exception gets thrown.

I've found other posts with people who are getting the same exception, but I don't think it's for the same reason. I haven't found any posts where the collection filter works on one "page" but not on the next.

Any ideas?

BTW, I have seen the section in the Hibernate FAQ regarding this exception:



I don't see how that answer helps as I am using a persistent collection.

Well, I figured out the cause of the problem, but I'm sure how to fix it. The root of the problem lies in caching.



The above function caches the tag object (I'm using ehcache) that is being used to populate the loaded page (via getWritingsByTag function). So here is what I think is happening:

-The user loads page 1 for the tag "someTagName".
-getTag(someTagName) is called which fetches the tag from the database, loads it, caches it, and returns it as someTagObject.
-getWritingsByTag(0, 10, someTagObject)
-The page is populated with the results of the getWritingsByTag() and everything works fine.
-The user loads page 2 for the tag "someTagName".
-The collection someTagObject.getTaggedByThis() goes out of scope (something to do with the way hibernate handles persistent collections by wrapping them in a PersistentCollection object)
-getTag(someTagName) is called which gets intercepted and redirected to cache which returns someTagObject
-getWritingsByTag(10, 10, someTagObject) is called
-Query query = sessionFactory.getCurrentSession().createFilter(tag.getTaggedByThis(), "order by votes desc"); creates an exception because tag.getTaggedByThis() is unreferenced.

I'm still not sure why all of the above is happening the way it is. It seems like hibernate would be smart enough to try to load the collection from the database if it ever becomes "unreferenced". The entire problem goes away if I disable caching on the getTag() function. This is obviously undesirable because it would incur a performance hit.

Has anyone else run into this problem?