Fetching arbitrary object graphs in JPA 2
In Java EE, JPA (Java Persistence API) is used to store and retrieve graphs of objects. This works by specifying relations between objects via annotations (or optionally XML). Hand over the root of an object graph to the entity manager and it will persist it. Ask the entity manager for an object with a given Id and you'll get the graph back. This is all fine and well, but how in this model do we control which branches of the graph are retrieved and to which depth branches should be followed? The primary mechanism to control this with is the eager/lazy mechanism. Mark a relation as eager and JPA will fetch it upfront, mark it as lazy and it will dynamically fetch it when the relation is traversed. In practice, both approaches have their cons and pros. Mark everything eager and you'll risk pulling in the entire DB for every little bit of data that you need. Mark everything lazy, and you'll not only have to keep the persistence context around (which by itself can be trou