I've been doing some profiling of late and noticed a small issue with one of my application. In this application I end up loading a heirarchy of objects in a parent/child relationship. Any node in teh tree might have children and the children are stored as a List.

The issue comes in that the majority of the nodes are in fact leaf nodes which have no children. On these nodes Hibernate sets a empty list which well enough until you take a look at memory consumption. The empty list consumes over 300 bytes. So in my application were I might have 10,000+ nodes in memory at a time and most are leaf nodes I'm using up 3MB of memory to hold these empty lists.

So anyway I've been tinkering for several days now trying to figure out a way to change things so that a null value is used to represent these list. I tried checking in the setter and when Hibernate provided a empty list I threw it away and just kept the reference null. Unfortunately this triggered Hibernate to do spurious updates which is pretty much what I expected.

I've considered switching to arrays but that would really complicate the code handling updates. I'm now considering a user type but a user type to hande lists seems a little more complicated then the ones I've done so far. I was wondering if anybody had any better ideas?

Write a UserCollectionType and implement UserType.instantiate() like so:

We're currently migrating from 2.1.8 to 3.0 and it looks like your suggestion was for the 3.x series. I'll try this ouot during our current conversion.

I was wondering though. I assumed I'd have to at least check and see if the list would be empty and then either return null for a empty list or return the normal Hibernate persistent list. Will what you showed result in the attribute being set null or in the attribute being set with a empty standard Java ArrayList?