下面只是列出主要流程，如果喜欢钻研的同学可以用Reflector去查看
一：ISAPIRuntime
       bool uSEOOP = iWRType == 1;
        wr = ISAPIWorkerRequest.CreateWorkerRequest(ecb, uSEOOP);
        wr.Initialize();
        string appPathTranslated = wr.GetAppPathTranslated();
        string appDomainAppPathInternal = HttpRuntime.AppDomainAppPathInternal;
        if ((appDomainAppPathInternal == null) || StringUtil.EqualsIgnoreCase(appPathTranslated, appDomainAppPathInternal))
        {
            HttpRuntime.ProcessRequestNoDemand(wr);
            return 0;
        }
        HttpRuntime.ShutdownAppDomain(ApplicationShutdownReason.PhysicalApplicationPathChanged, SR.GetString("Hosting_Phys_Path_Changed", new object[] { appDomainAppPathInternal, appPathTranslated }));
        return 1;
它的主要作用是调用一些非托管代码生成HttpWorkerRequest对象，该对象包含当前请求的所有信息，然后传递给HttpRuntime，这里生成的HttpWorkerRequest对象可以直接在我们的页面中调用的，通过它取得原始的请求信息：
            IServiceProvider provider = (IServiceProvider)HttpContext.Current;
            HttpWorkerRequest wr = (HttpWorkerRequest)provider.GetService(typeof(HttpWorkerRequest));

二：HttpRuntime
最主要的就是private void ProcessRequestInternal(HttpWorkerRequest wr)方法：         context = new HttpContext(wr, false);

        IHttpHandler applicationInstance = HttpApplicationFactory.GetApplicationInstance(context);

  IHttpAsyncHandler handler2 = (IHttpAsyncHandler) applicationInstance;
  context.AsyncAppHandler = handler2;
  handler2.BeginProcessRequest(context, this._handlerCompletionCallback, context);
        1、根据HttpWorkerRequest对象生成HttpContext，HttpContext应该大家都很熟悉的，它包含request、response等属性，在页面中经常会用到的；
2、调用HttpApplicationFactory来生成IHttpHandler(这里生成的是一个默认的HttpApplication对象，HttpApplication也是IHttpHandler接口的一个实现)
3、调用HttpApplication对象执行请求
三：HttpApplicationFactory
正如2.2中所提到的，这里主要是生成一个HttpApplication对象：

 internal static string GetApplicationFile()
 {
     return Path.Combine(HttpRuntime.AppDomainAppPathInternal, "global.asax");
 }
首先会查看是否存在global.asax文件，如果有的话就用它来生成HttpApplication对象，从这里我们可以看到global.asax的文件名是在asp.net的框架中写死的，不能修改的。如果这个文件不存在就使用默认的对象。
创建好HttpApplication之后对它进行初始化：
    application = (HttpApplication) HttpRuntime.CreateNonPublicInstance(this._theApplicationType);
 using (ApplicationImpersonationContext context2 = new ApplicationImpersonationContext())
 {
     application.InitInternal(context, this._state, this._eventHandlerMethods);
 }
 四、HttpApplication
这个是比较复杂也比较重要的一个对象
首先是执行初始化操作，比较重要的一步就是进行HttpModule的初始化：         private void InitModules()
        {
            this._moduleCollection = RuntimeConfig.GetAppConfig().HttpModules.CreateModules();
            this.InitModulesCommon();
        }它会读取web.config中所有HttpModule的配置
在HookupEventHandlersForApplicationAndModules方法中绑定Module的事件处理程序接着进行事件实际绑定：
 if (HttpRuntime.UseIntegratedPipeline)
 {
     this._stepManager = new PipelineStepManager(this);
 }
 else
 {
     this._stepManager = new ApplicationStepManager(this);
 }
 this._stepManager.BuildSteps(this._resumeStepsWaitCallback);
在ApplicationStepManager的BuildSteps方法中可以看到事件的绑定执行顺序：

app.CreateEventExecutionSteps(HttpApplication.EventBeginRequest, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventAuthenticateRequest, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventDefaultAuthentication, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventPostAuthenticateRequest, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventAuthorizeRequest, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventPostAuthorizeRequest, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventResolveRequestCache, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventPostResolveRequestCache, steps);
 steps.Add(new HttpApplication.MapHandlerExecutionStep(app));
 app.CreateEventExecutionSteps(HttpApplication.EventPostMapRequestHandler, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventAcquireRequestState, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventPostAcquireRequestState, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventPreRequestHandlerExecute, steps);
 steps.Add(new HttpApplication.CallHandlerExecutionStep(app));
 app.CreateEventExecutionSteps(HttpApplication.EventPostRequestHandlerExecute, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventReleaseRequestState, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventPostReleaseRequestState, steps);
 steps.Add(new HttpApplication.CallFilterExecutionStep(app));
 app.CreateEventExecutionSteps(HttpApplication.EventUpdateRequestCache, steps);
 app.CreateEventExecutionSteps(HttpApplication.EventPostUpdateRequestCache, steps);
 this._endRequestStepIndex = steps.Count;
 app.CreateEventExecutionSteps(HttpApplication.EventEndRequest, steps);
 steps.Add(new HttpApplication.NoopExecutionStep());
注意上面红色标注的MapHandlerExecutionStep(读取所有的HttpHandler配置)、CallHandlerExecutionStep就是对Handle程序进行处理的，也就是说在web.config中配置的HttpHandler都是在这里进行处理的，执行顺序如上所示

然后就是调用2.3中的方法执行请求：
Code
        IAsyncResult IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
        {
            this._context = context;
            this._context.ApplicationInstance = this;
            this._stepManager.InitRequest();
            this._context.Root();
            HttpAsyncResult result = new HttpAsyncResult(cb, extraData);
            this.AsyncResult = result;
            if (this._context.TraceIsEnabled)
            {
                HttpRuntime.Profile.StartRequest(this._context);
            }
            this.ResumeSteps(null);
            return result;
        }
在ResumeSteps中就是执行事件处理程序。

五、HttpModule
在系统web.config中默认的配置有：
Code
    <httpModules>
      <add name="OutputCache" type="System.Web.Caching.OutputCacheModule"/>
      <add name="Session" type="System.Web.SessionState.SessionStateModule"/>
      <add name="WindowsAuthentication" type="System.Web.Security.WindowsAuthenticationModule"/>
      <add name="FormsAuthentication" type="System.Web.Security.FormsAuthenticationModule"/>
      <add name="PassportAuthentication" type="System.Web.Security.PassportAuthenticationModule"/>
      <add name="RoleManager" type="System.Web.Security.RoleManagerModule"/>
      <add name="UrlAuthorization" type="System.Web.Security.UrlAuthorizationModule"/>
      <add name="FileAuthorization" type="System.Web.Security.FileAuthorizationModule"/>
      <add name="AnonymousIdentification" type="System.Web.Security.AnonymousIdentificationModule"/>
      <add name="Profile" type="System.Web.Profile.ProfileModule"/>
      <add name="ErrorHandlerModule" type="System.Web.Mobile.ErrorHandlerModule, System.Web.Mobile, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
      <add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
    </httpModules>
基本使用方法可以参见我的上一篇文章

六、HttpHandlerFactory、HttpHandler
这两个对象在web.config中的配置方法是相同的，默认配置有：
Code
     <add path="*.rules" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.xoml" verb="*" type="System.ServiceModel.Activation.HttpHandler, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" validate="false"/>
      <add path="trace.axd" verb="*" type="System.Web.Handlers.TraceHandler" validate="true"/>
      <add path="WebResource.axd" verb="GET" type="System.Web.Handlers.AssemblyResourceLoader" validate="true"/>
      <add path="*.axd" verb="*" type="System.Web.HttpNotFoundHandler" validate="true"/>
      <add path="*.aspx" verb="*" type="System.Web.UI.PageHandlerFactory" validate="true"/>
      <add path="*.ashx" verb="*" type="System.Web.UI.SimpleHandlerFactory" validate="true"/>
      <add path="*.asmx" verb="*" type="System.Web.Services.Protocols.WebServiceHandlerFactory, System.Web.Services, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" validate="false"/>
      <add path="*.rem" verb="*" type="System.Runtime.Remoting.Channels.Http.HttpRemotingHandlerFactory, System.Runtime.Remoting, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" validate="false"/>
      <add path="*.soap" verb="*" type="System.Runtime.Remoting.Channels.Http.HttpRemotingHandlerFactory, System.Runtime.Remoting, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" validate="false"/>
      <add path="*.asax" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.ascx" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.master" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.skin" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.browser" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.sitemap" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.dll.config" verb="GET,HEAD" type="System.Web.StaticFileHandler" validate="true"/>
      <add path="*.exe.config" verb="GET,HEAD" type="System.Web.StaticFileHandler" validate="true"/>
      <add path="*.config" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.cs" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.csproj" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.vb" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.vbproj" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.webinfo" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.licx" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.resx" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.resources" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.mdb" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.vjsproj" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.java" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.jsl" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.ldb" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.ad" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.dd" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.ldd" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.sd" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.cd" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.adprototype" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.lddprototype" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.sdm" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.sdmDocument" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.mdf" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.ldf" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.exclude" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.refresh" verb="*" type="System.Web.HttpForbiddenHandler" validate="true"/>
      <add path="*.svc" verb="*" type="System.ServiceModel.Activation.HttpHandler, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" validate="false"/>
      <add path="*" verb="GET,HEAD,POST" type="System.Web.DefaultHttpHandler" validate="true"/>
      <add path="*" verb="*" type="System.Web.HttpMethodNotAllowedHandler" validate="true"/>
    </httpHandlers>
要注意的是相同的后缀名配置多次的话，后面的配置会把前面的覆盖。

这里我们重点看一下aspx的配置：System.Web.UI.PageHandlerFactory
这是一个HttpHandlerFactory对象，根据不同的Page生成不同的HttpHandler对象(我们自己的Page页面都是一个IHttpHandler)：
    Page page = BuildManager.CreateInstanceFromVirtualPath(virtualPath, typeof(Page), context, true, true) as Page;
    if (page == null)
    {
        return null;
    }
    page.TemplateControlVirtualPath = virtualPath;
    return page;
这里会调用web.config中的buildProviders配置编译页面：
Code
<buildProviders>
        <add extension=".aspx" type="System.Web.Compilation.PageBuildProvider"/>
        <add extension=".ascx" type="System.Web.Compilation.UserControlBuildProvider"/>
        <add extension=".master" type="System.Web.Compilation.MasterPageBuildProvider"/>
        <add extension=".asmx" type="System.Web.Compilation.WebServiceBuildProvider"/>
        <add extension=".ashx" type="System.Web.Compilation.WebHandlerBuildProvider"/>
        <add extension=".soap" type="System.Web.Compilation.WebServiceBuildProvider"/>
        <add extension=".resx" type="System.Web.Compilation.ResXBuildProvider"/>
        <add extension=".resources" type="System.Web.Compilation.ResourcesBuildProvider"/>
        <add extension=".wsdl" type="System.Web.Compilation.WsdlBuildProvider"/>
        <add extension=".xsd" type="System.Web.Compilation.XsdBuildProvider"/>
        <add extension=".js" type="System.Web.Compilation.ForceCopyBuildProvider"/>
        <add extension=".lic" type="System.Web.Compilation.IgnoreFileBuildProvider"/>
        <add extension=".licx" type="System.Web.Compilation.IgnoreFileBuildProvider"/>
        <add extension=".exclude" type="System.Web.Compilation.IgnoreFileBuildProvider"/>
        <add extension=".refresh" type="System.Web.Compilation.IgnoreFileBuildProvider"/>
        <add extension=".xoml" type="System.ServiceModel.Activation.WorkflowServiceBuildProvider, System.WorkflowServices, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"/>
        <add extension=".svc" type="System.ServiceModel.Activation.ServiceBuildProvider, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
      </buildProviders>

这样就可以进入我们的Page执行了，大的执行顺序可以参见第四部分的描述，它也就是一个HttpHandler