Because of its simplicity, the most common pattern used in web applications for managing NHibernate sessions is session-per-request. In this recipe, I'll show you how to set up the session-per-request pattern using NHibernate's contextual sessions feature.
- Create a new ASP.NET Web Forms or ASP.NET MVC application.
- Add references to
NHibernate.dll,NHibernate.ByteCode.Castle.dll,log4net.dll, and ourEg.Coremodel and mappings project from Chapter 1. - In the
web.configfile, set up the NHibernate and log4net configuration sections. Refer to the Configuring NHibernate with App.config recipes in Chapter 2.
- In the
hibernate-configurationsection ofweb.config, add thecurrent_session_context_classproperty with a value ofweb. - If it doesn't exist already, add a new Global application class (
Global.asax). - In
Global.asax, add these using statements.using NHibernate; using NHibernate.Cfg; using NHibernate.Context;
- Create a static property named
SessionFactory.public static ISessionFactory SessionFactory { get; private set; } - In the
Application_Startmethod, add the following code.protected void Application_Start(object sender, EventArgs e) { log4net.Config.XmlConfigurator.Configure(); var nhConfig = new Configuration().Configure(); SessionFactory = nhConfig.BuildSessionFactory(); } - In the
Application_BeginRequestmethod, add the following code.protected void Application_BeginRequest(object sender, EventArgs e) { var session = SessionFactory.OpenSession(); CurrentSessionContext.Bind(session); } - In the
Application_EndRequestmethod, add the following code:protected void Application_EndRequest(object sender, EventArgs e) { var session = CurrentSessionContext.Unbind(SessionFactory); session.Dispose(); }
In web applications, it's common to use a session for each web request. We open the session when the request begins and close it when the request ends.
NHibernate's contextual session feature allows a session to be associated with some application-specific scope that approximates a single unit of work. This context is configured with the
current_session_context_class property, which specifies an implementation of NHibernate.Context.ICurrentSessionContext. In this case, we'll associate it with the web request. web is the short name for NHibernate.Context.WebSessionContext.
Even with contextual sessions, NHibernate does not open, close or dispose the session for us. We associate and dissociate a session with the current web request using the CurrentSessionContext.Bind and Unbind methods.
To get the NHibernate session for the current web request, we use
SessionFactory.GetCurrentSession(). In our example web application, it might look something like this:
Guid productId = new Guid(Request["id"]);
Eg.Core.Product product;
var session = Global.SessionFactory.GetCurrentSession();
using (var tran = session.BeginTransaction())
{
product = session.Get<Eg.Core.Product>(productId);
tran.Commit();
}
Page.Title = product.Name;
Label1.Text = product.Name;
Label2.Text = product.Description;This naive example fetches a product from the database and displays the name and description to the user. In a production-worthy application, we would use dependency injection rather than directly access the singleton. The free TekPub Concepts screencast provides a thorough introduction to dependency injection, and can be found at http://tekpub.com/view/concepts/1.
Note
NHibernate sessions are extremely lightweight and cheap to make. Simply opening a session doesn't open a database connection. NHibernate makes every effort to avoid any delay opening a connection. On the other hand, NHibernate goes through great effort to create the session factory. You should only create one session factory for the entire lifecycle of the application.
There are many implementations of session per request using Inversion of Control containers and even HTTP modules. Some use contextual sessions. Others manage the session without NHibernate's help. A complete session per request implementation has four characteristics:
- Create the one and only session factory when the application starts
- Open a session when the web request begins
- Close the session when the request ends
- Provide a standard way to access the current session throughout the data access layer