Hibernate: A Developer's Notebook

5.1. Using Lazy Associations

First rich, then lazy? I suppose that could be a plausible story about someone, as long as it happened in that order. But this really is an object relational mapping topic of some importance. As your data model grows, adding associations between objects and tables, your program gains power, which is great. But you often end up with a large fraction of your objects somehow linked to each other. So what happens when you load one of the objects that is part of a huge interrelated cluster? Since, as you've seen, you can move from one object to its associated objects just by traversing properties, it seems you'd have to load all the associated objects when you load any of them. For small databases this is fine, but in general your database can't hold a lot more than the memory available to your program. Uh oh! And even if it does all fit, rarely will you actually access most of those objects, so it's a waste to load them all.

Luckily, this problem was anticipated by the designers of object/relational mapping software, including Hibernate. The trick is to configure some associations to be "lazy," so that associated objects aren't loaded until they're actually referenced. Hibernate will instead make a note of the linked object's identity and put off loading it until you actually try to access it. This is often done for collections like those we've been using.

5.1.1. How do I do that?

With collections, all you need to do is set the lazy attribute in the mapping ...