http://java.sun.com/blueprints/corej2eepatterns/Patterns/index.html describes all the patterns you mentioned.
Essentially they break down as follows:
[Business Delegate]
This can mean different things. I usually (maybe not correctly) use it to refer to a client side "facade" that is responsible for talking to a server side object.
[Session Facade]
This is usually a fine grained stateless session bean which talks about use cases. The implementation of this facade *may* delegate to one or more finer grained POJOs/EJBs. Another common use is to use a session facade to wrap all external access to entity beans.
[DAO]
Data Access Object is a layer behind which lives all your data access code. For example, you SF (Session Facade) might choose to use a DAO to load stuff from persistent storage.
Examples:
public final class ClientSideBusinessDelegate {
public UserVO getUser(final String username) throws UserNotFoundException {
try {
return userStateLessSessionBean.getUser(username);
} catch (RemoteException e) {
throw new MySpecificRuntimeException(e);
}
}
}
public final class ServerSideStatelessSessionBean {
public UserVO getUser(final String username) throws UserNotFoundException, RemoteException {
UserVO user = dao.loadUser(username);
}
}
public interface UserDAO {
UserVO loadUser(username) throws UserNotFoundException;
}
public class EntityBeanBackedUserDAO implements UserDAO {
public UserVO loadUser(username) throws UserNotFoundException {
try {
UserVO user = new UserVO();
UserEntityBean userBean = userHome.findByPrimaryKey(username);
user.setUsername(userBean.getUsername());
} catch (ObjectNotFoundException e) {
throw new UserNotFoundException(username);
}
}
}
Note: The DAO will typically be an interface with one or more implementations. Note also that the exceptions coming from a DAO should not be implementation specific :)
Hope this helps.
Comments + feedback welcome.
Col
P.S. Ask 10 people, get 12 different answers ;)
