barcode vb.net 2008 Session management using the current session API in Visual C#

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Listing 10.4 Session management using the current session API
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using NHibernate.Context; public void PlaceBidForItem(long itemId, long userId, double bidAmount) { try { using(ISession session = NHibernateHelper.OpenSession()) using(session.BeginTransaction()) { CurrentSessionContext.Bind(session); Item item = ItemDAO.FindByIdAndLock(itemId); double maxBidAmount = ItemDAO.GetMaxBidAmount(itemId); User bidder = UserDAO.FindById(userId); item.PlaceBid(bidder, bidAmount, maxBidAmount); session.Transaction.Commit(); } } catch (HibernateException ex) { throw new InfrastructureException( "Error while accessing the database", ex ); } finally { CurrentSessionContext.Unbind(NHibernateHelper.SessionFactory); } }
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Designing the persistence layer
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Once the session is opened, you attach it to the current context so it s available to any object executed in this context B. After that, you can execute the logic as before C. Note that a pessimistic lock can now be used. At the end of the operation, you must detach the (closed) session from the current context D. With this implementation, DAOs can access the same session in a neat and transparent way. It s time to address the other issue in the implementation of a persistence layer: minimizing redundancy.
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DESIGNING DAOS USING GENERICS
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Whenever you find yourself repeating a similar code repeatedly, it s time to think composition, inheritance, and generics. Note that this section assumes you have a good understanding of .NET 2.0 generics. If you re still using .NET 1.1, it s possible to adapt the following idea, but the result won t be as clean. As you saw when implementing the method FindById() for the ItemDAO and UserDAO classes, only the name of the entity changes for basic CRUD operations. Therefore, it s possible to use generics to abstract this operation. The new DAOs may look like this:
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public abstract class GenericNHibernateDAO<T, ID> { public T FindById(ID id) { try { return NHibernateHelper.GetCurrentSession().Load<T>(id); } catch(HibernateException ex) { throw new Exceptions.InfrastructureException(ex); } } //... } public class UserDAO : GenericNHibernateDAO<User, long> { }
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The GenericNHibernateDAO class only needs the types of the entity T and its identifier ID to implement the FindById() method. After that, implementing the UserDAO class means inheriting from GenericNHibernateDAO and providing these types. Many other methods can be implemented like that. Before you fill the GenericNHibernateDAO class with them, let s take a step back and think about the final design. As far as the business layer is concerned, the persistence layer should provide a set of interfaces to perform all the operations that are needed. This means the underlying implementation doesn t matter and can be changed as long as the interfaces don t change. In your design, you ll have a GenericDAO interface with operations common to all entities and DAO interfaces, inheriting from the GenericDAO interface for each entity. These interfaces will all have implementations using NHibernate. Figure 10.1 illustrates this design. Our experience tells us that even though some interfaces may not have any methods, it s still important to create them because they re likely candidates for future extension.
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GenericDAO<T,ID>
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FindById(ID id) : T FindByIdAndLock(ID id) : T FindAll() : IList<T> MakePersistent(T entity) : T MakeTransient(T entity)
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Architectural patterns for persistence
GenericNHibernateDAO<T,ID>
ItemDAO<Item,long>
Bid GetMaxBid(long itemId) Bid GetMinBid(long itemId)
ItemDAOImpl
UserDAO<User,long>
UserDAOImpl
Figure 10.1 Generic DAO interfaces with a separated NHibernate implementation
You ll use the Abstract Factory pattern as a fa ade to provide the implementations of the interfaces. This means the NHibernate classes will be completely hidden from the other layers. It will even be possible to switch the persistence mechanism at runtime. Enough theory. Let s look at the GenericDAO interface:
public interface GenericDAO<T, ID> { T FindById(ID id); T FindByIdAndLock(ID id); IList<T> FindAll(); T MakePersistent(T entity); void MakeTransient(T entity); }
This interface defines methods to load (the Find...() methods), save (using MakePersistent()), and delete (using MakeTransient()) entities.
Why MakePersistent() and MakeTransient() instead of Save() and Delete()
It s simpler to explain persistence operations using verbs like save and delete. But NHibernate is a state-oriented framework. This notion was introduced in section 5.1. For example, when you delete an entity, that entity becomes transient. Its row in the database is eventually deleted, but the entity doesn t cease to exist (and it can even be persisted again). Because these methods are created for the business layer, their names should reflect what happens at that level.
The interfaces inheriting from the GenericDAO interface look like this:
public interface ItemDAO : GenericDAO<Item, long> { Bid GetMaxBid(long itemId); Bid GetMinBid(long itemId); }
Here, you avoid defining a too-specific method like GetMaxBidAmount(). Now, let s implement these interfaces. First, GenericNHibernateDAO:
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