Please be careful the cake is a lie

How lazy is Hibernate's lazy initialization?

Originally I was going to give a more catchy title, but then I figured there is no need to pick on Hibernate. It actually does its job well when you manage to tame it.

Part of our application is using AngularJS which talks to a REST API, and the data travels as JSON. So we needed a way to serialize our entities into JSON strings, so we can send it back to the client. We are using JAX RS for the REST API, which uses Jackson by default. It seemed like a straightforward decision to use Jackson as our JSON serializer library.

Our domain model is a bit problematic and has lots of bidirectional associations, which is not best kind to have when you want to serialize it.

I knew upfront that Hibernate proxies will cause some trouble, but I would have never imagined how much. When the serializer tried to access a lazily initialized property without a hibernate session, it threw an exception. There is no surprise there. Luckily others have encountered this before, so there is a handy library. Unfortunately there is a known issue about this as well.

This is the point where I made a mistake. I assumed that Hibernate4Module can handle Hibernate proxies correctly, so I reattached the entity to a Hibernate session and hoped for the best. It was like raining blood. Everything blew up. The whole thing got stuck in an infinite loop because of the circular nature of the associations in our domain model.

I went down the debugging rabbit hole to see where all the Hibernate proxies get handled and it seemed fine. It seemed to return null for the uninitialized proxies just as it was advertised in the docs. But somehow those proxies got initialized during serialization despite all of my efforts.

It seems that Hibernate’s lazy initialization is a bit more greedier than I thought. I had the impression that it only loaded an entity or a collection when you accessed one of its properties. So for an Employee ---> Workspace relation calling a employee.getWorkspace() would not fire a query to the database. When working with Jackson this was not the case. It loaded everything and caused me some serious headache. It might have been a bug in the Hibernate4Module that I did not notice or my setup was somehow broken.

I ended up using mixins and ignored all the properties that caused the trouble. I was lucky because I did not need those properties on the client.