I've been reading through forum posts regarding granularity of DAOs and I've decided that for my application, I'll have granular DAOs for complex queries, but I also want a generic DAO for general-purpose loading and saving of object subgraphs.

Now to my question: I want one of my generic DAO methods to load an object (by ID) and all of its children (one level deep). Is there an easy way to write this without having to interrogate the configuration metadata for the entity class? Seems like this should be very straightforward, but the reference manual, Hibernate in Action, etc. don't seem to talk about strategies for writing generic DAOs.

Why do you necessarily want "one level deep"? In my code, I have a fairly generic DAO that can load, save, saveOrUpdate, saveOrUpdateCopy, or delete a single object or a collection of objects. Works pretty well expect where it doesn't, then I write something specific :-)

Actually, I want arbitrary depth, but I figured I'd start simple.

For example, in our GUI, we have a tree structure that the user needs to be able to navigate. The entire tree can be quite large, so we only want to bring in one level of the hierarchy at a time (or maybe a couple).

So I want the UI to be able to pass the DAO the ID and class of a parent object, and the DAO should retrieve all of the children, which may be in several different collections.

Well that doesn't sound very generic. It sounds like a pretty specific requirement.

Your GUI then could call your service layer (do you have one, or does your GUI communicate directly with your DAO?) which would in turn contact the DAO to get it to ask Hibernate to load the object. If your class' mapping document defines lazy collections then you'd need to Hibernate.initialize() them before transporting them back to your GUI.

Is this making sense? I'm not really seeing your problem...

The problem is that there are many different types of objects in the tree, and I don't want to write a method for each different object type that does basically the same thing.

The logic can be describe generically like this: For a given persistent class and ID, retrieve all of its children. I shouldn't need to couple the DAO to the particular collections found in each entity type.

So, when you talk about a persistent class, what do you mean by "all of its children"?

What do you need to do differently from just asking Hibernate to load an instance of a persistent class?

It's a tree. So I mean all of the related entities contained in collections owned by any given parent node, which is also an entity.

So, you have a persistent class that has a lazily loaded collection of children instances of the same type?

I would just ask Hibernate to load the instance, then initialize the collection of children. Eg:

Session s = // get the Hibernate Session
PersistentClass instance = (PersistentClass) s.load(PersistentClass.class, id);
Hibernate.initialize(instance.getChildren());


Then, when your user clicks on a child, you again ask Hibernate to load it. Will this work?

No, because each persistent class could have different sets of child collections. There is no single "getChildren" method on each persistent class.

I think I've found a way to do this using the Hibernate ClassMetadata. I was just wondering if there was an easier way. I basically want eager fetching for all of the collections owned by an entity, rather than just one.

Well, yes, maybe you could do that, or maybe each persistent class could implement an interface that would help you out. For example, why not make each persistent class implement an interface (say) TreeEntry that defines a method:



so that each persistent class would need to return these. Then your "generic" DAO could do something like:




and that should sort it out!

I didnt really read all of this post. I skimmed....


However, it is my understanding that Hibernate only provides eager fetching for ONE association of an entity.

Well true, but if you're forcing it to instantiate the other associations by calling Hibernate.initialize() on them. That's the whole point!

Thanks for the input, but modifying the entity model is even more intrusive than creating separate DAO methods for each type.

What I really want is some kind of Hibernate.initializeAll method that initializes an object and its associations or maybe just the associated collections. I can write a method to do it using the class metadata. I was just hoping for something built-in.

Well OK, I'm reminded of the expression "There ain't no such thing as a free lunch"! There are obviously consequences of anything you do...

Why don't you use reflection then to find the collections on the persistent class and call initialize on them.