Responsibilities and use of Service and DAO Layers

It is a good idea to have those two layers when your business logic is more complex than your data logic. The service layer implements the business logic. In most cases, this layer has to perform more operations than just calling a method from a DAO object. And if you’re thinking of making your application bigger, this is probably the best solution.

Imagine you want to include a City entity and create a relationship between People and City. Here is an example:

@Transactional
public class PeopleService {

    ....
    private PeopleDAO pDAO;
    private CityDAO cDAO;

    ...

    public void createPerson(String name, String city)
     throws PeopleServiceException {
        Person p = new Person();
        p.setName(name);

        City c = cDAO.getCityByName(city);
        if (c == null) throw new ServiceException(city + " doesn't exist!");
        if (c.isFull()) throw new ServiceException(city + " is full!");
        c.addPeople(p);

        sess().save(p);
        sess().save(c);
    }

    ...
}

In this example, you can implement more complex validations, like checking the consistency of the data. And PersonDAO has not been modified.

Another example:

DAO and Service layers with Spring

Definition of Service layer pattern