I am looking for a good pattern/methodology to use when coding
services in small Wicket webapps. I want to use the services from
Wicket but it will also be exposed via RMI and SOAP/REST. For example,
a service for Houses and Inhabitants might have methods like this:
// Returns the houses matching the filter (but not yet the
inhabitants)
List<House> findHouses(House filter);
// Returns the inhabitants of the given house
List<Inhabitant> getInhabitants(int houseId);
The class House exposes an id (for use in the getInhabitants() method)
and some data (address, etc.). It does not provide the list of
inhabitants because this list might be huge, might not be needed by
the caller at all, etc. The service returns lists because this comes
in handy with Wicket components.
My question: Should the above service return objects (HouseImpl,
InhabitantImpl) from the persistence layer? Or do you introduce
another set of public (DTO) classes for exposure in the service?
Ideally, I want to keep things simple and manageable, with little code.
For instance, for the above service one could use a Hibernate mapping
like
public class HouseImpl
{
...
@OneToMany(mappedBy = "house", fetch = FetchType.LAZY)
private final Collection<InhabitantImpl> inhabitants;
}
and
public class InhabitantImpl
{
...
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(nullable = false, updatable = false)
private HouesImpl house;
}
and the service can directly return instances of these classes. I have
used this approach in a project and we have seen these problems:
- There's some code used to properly detach the objects and load all
exposed data in order to avoid LazyInitializationException's. For
instance, if House has a list of caretakers and you want them to be
exposed in the service's House, you may need to tell Hibernate to load
them first. This code is cumbersome to maintain.
- I am not sure whether this would work with RMI, for example. I
suppose the Hibernate proxies that exist for some of the members would
get transmitted and would have to exist at the client end, too...
Therefore: How do you design your services? Is anybody using some
method in which the service objects are generated (semi-)automatically
from the persistence layer objects?
Many thanks for your input!
Kaspar
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