I've got a set of classes, namely, a data transfer object, a service implementation object, and a data access object. I currently have business logic in the service implementation object; it uses the dao to get data to populate the dto that is shipped back to the client/gui code.

The issue is that I can't create a lightweight junit test of the service implementtion object(it's a servlet); I think the business logic should be elsewhere, but the only thing I can think of is putting business logic in the dao or in yet another layer that goes between the dao and the service implementation.

Are there other options, or am I thinking about this the wrong way?

It's a GWT/App Engine project.