This is my scenario:

My server has a load of persistent objects. However, I need the client to access the domain model, but I don't want to send persistent objects to the client. In fact, I cannot do this since I use WebServices (which lacks supports for stuff like Sets)

So I have a "shadow" class hierarchy that looks just like my persistent classes, except that Sets are java arrays, etc.

Is there an easy way to copy between the persistent and non-persistent object graph?

I guess I could look into the reflection / hibernate metadata API, but if there is a standard solution...

I guess I would also need to specify how much of the graph to copy.

Difficult problem?

I am not you will like it, but you can add some noise to model itself :

Set collection;

void setCollection(Set collection){

this.collection = collection;

}

MyObject[] getArray(){

return this.collection.toArray( new MyObject[]{} );

}

....

Map collections to DB and arrays to web methods.

"I am not you will like it, but you can add some noise to model itself"

You're right: I don't like it ;-)

Besides, I (re)generate the persistent java classes using the hibernate tools, so that's out of the question anyway.

Do you think it is better to have two domain models and copy data between models ?
I am not WebServices expert but I think it must be some way to provide custom type mappings and data transformations to return collection as array of objects.

No, there is no way to map sets and maps. .NET certainly doesn't support it, given a Java webservices server.

Besides, exposing persistent classes on any client is not a good design.