Why hide DAO behind the service?

Question

Having looked at a bunch of MVC-style web applications, I noticed that it is very important for them to have a very large service interface located in front of the business layer. The implementation of this interface usually includes a DAO group. So you have something like:

public class FoodServiceImpl implements FoodService {

  private FruitDAO fruitDAO;
  private VegetableDAO vegetableDAO;
  private MeatDAO meatDAO;

  // ... DAO injection stuff

  public List<Meat> getMeat() {
    return meatDAO.getMeat();
  }

  public void addMeat(Meat meat) {
    meatDAO.add(meat);  
  }

  public List<Fruit> getFruit() {
    return fruitDAO.getFruit();
  }

  // ... tons of methods that mostly just delegate to DAOs

}

Assuming your DAOs are not specific, first of all, why not just expose the DAO to the next level?

So, instead of:

// assume foodService is injected at runtime
public void controllerMethod() {
  foodService.getFruit();
  foodService.getMeat();
}

You just

// assume DAOs are injected at runtime
public void controllerMethod() {
  fruitDAO.getFruit();
  meatDAO.getMeat();
}

On the one hand, it would be nice to have whole guts of the application wrapped in one interface ... on the other hand, you can find a gigantic interface whose implementation does nothing more than delegate the DAO.

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