EduSpring Part 5: Spring.Net in an ASP.NET environment (including MVC)

Tuesday, June 14, 2011

This post is part of a series on Spring.NET. I recommend starting at the beginning if you haven't already.  Also, I am walking through code in the accompanying GitHub project.

By now, you should have the basics of DI, IoC, and the benefits and drawbacks of the approach.  Now, I'll introduce you to the architecture of Spring.NET in an ASP.NET environment.  I'm sure a lot of other IoC frameworks operate in a similar manner.  If not, you can add code to make them work that way. ;-)

If you look at the IocWithoutSpring project, you'll see this Main function:

static void Main(string[] args)
            //These two lines are handled by the Spring.NET HttpModule
            var container = new IoCContainer();

            // These two lines are also handled by the HttpModule by a special 
            // syntax in the spring configuration
            var service = new DoSomeWork();
            service.Worker = container.GetObject("myObject");

            // This is what the ASP.NET Framework would do
            Console.WriteLine("The output is: " + service.DoTheWork());

Most of the time, creation of the IoC container itself is a single dependency that's particularly hard to get rid of without writing some reflection-style glue code.  Ideally, we want our objects to be completely ignorant of this container, though.  In ASP.NET, the web.config provides us with the concept of an HttpModule, which will look at every request coming from into the web server and have an opportunity to do something with it.  Taking advantage of this feature, the Spring.Net team wrote an ASP.NET HttpModule that will do just that, so the first two lines of main (instantiation and initialization of the container) are handled by the ASP.NET framework.  Awesome!

Now our Spring.Net dictionary is populated, assuming we have a valid configuration.  I'll address more about the pain points of Spring configuration later, but this very early creation of lots of objects is one of the main frustrations of people who want to use Spring.Net.  The next question is, what about setting up dependencies in ASPX pages?

Technically speaking, the ASP.NET framework parses an ASPX file (or MVC view) and code-generates a class.  On each request, it creates an instance of this class and allows it to process the request, before destroying the object.  Very stateless, but this creation and destruction of classes rubs against IoC's Dictionary<string,object> heart.

If you read this contract from the other side (a.k.a. how would I solve this problem if I were writing Spring.Net), you can imagine yourself writing a PageHandlerFactory that can deliver the aspx page class with dependencies already injected.  There are two problems you have to solve, however:

  1. What name do you use to look up the object?
  2. How do you deal with request-specific data?
The answer to #1 is relatively obvious if you look at the problem from the perspective of Spring.  The object doesn't really have a name, so we leave that blank, and the virtual path (~/page.aspx) is used as the type of the object since someone configuring the object will not know the type ASP.NET generates.  The actual type name needs to be figured out by Spring.  The answer to the second question runs a bit contrary to what I've been discussing so far.

In the examples so far, the initialization method of the IoC container has created all the objects in configuration and put them in its internal dictionary.  However, there's no reason that the objects need to be created right away, nor do they have to be held in the dictionary.  In the case of this PageHandlerFactory, objects are created and configured at the time of the request, not at initialization.  However, dependencies follow the normal rules.  

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