HttpModule是如何工作的

当一个HTTP请求到达HttpModule时，整个ASP.NET Framework系统还并没有对这个HTTP请求做任何处理，也就是说此时对于HTTP请求来讲，HttpModule是一个HTTP请求的“必经之路”，所以可以在这个HTTP请求传递到真正的请求处理中心（HttpHandler）之前附加一些需要的信息在这个HTTP请求信息之上，或者针对截获的这个HTTP请求信息作一些额外的工作，或者在某些情况下干脆终止满足一些条件的HTTP请求，从而可以起到一个Filter过滤器的作用。

示例1：

using System;

using System.Collections.Generic;

using System.Text;

using System.Web;

namespace MyHttpModule

{

     /// <summary>

     /// 说明：用来实现自己的HttpModule类。

     /// 作者：文野

     /// 联系：[email protected]

     /// </summary>

     public class MyFirstHttpModule : IHttpModule

     {

         private void Application_BeginRequest(object sender, EventArgs e)

         {

              HttpApplication application = (HttpApplication)sender;

              HttpContext context = application.Context;

              HttpRequest request = application.Request;

              HttpResponse response = application.Response;

              response.Write("我来自自定义HttpModule中的BeginRequest<br />");

         }

         private void Application_EndRequest(object sender, EventArgs e)

         {

              HttpApplication application = (HttpApplication)sender;

              HttpContext context = application.Context;

              HttpRequest request = application.Request;

              HttpResponse response = application.Response;

              response.Write("我来自自定义HttpModule中的EndRequest<br />");

         }

         #region IHttpModule 成员

         public void Dispose()

         {}

         public void Init(HttpApplication application)

         {

              application.BeginRequest += new EventHandler(Application_BeginRequest);

              application.EndRequest += new EventHandler(Application_EndRequest);

         }

         #endregion

     }

}

在Web.config进行如下配置

<add name="MyFirstHttpModule" type="MyHttpModule.MyFirstHttpModule,MyHttpModule"/>

深入了解HttpModule

一个HTTP请求在HttpModule容器的传递过程中，会在某一时刻（ResolveRequestCache事件）将这个HTTP请求传递给HttpHandler容器。在这个事件之后，HttpModule容器会建立一个HttpHandler的入口实例，但是此时并没有将HTTP请求控制权交出，而是继续触发AcquireRequestState事件以及PreRequestHandlerExcute事件。在PreRequestHandlerExcute事件之后，HttpModule窗口就会将控制权暂时交给HttpHandler容器，以便进行真正的HTTP请求处理工作。

而在HttpHandler容器内部会执行ProcessRequest方法来处理HTTP请求。在容器HttpHandler处理完毕整个HTTP请求之后，会将控制权交还给HttpModule，HttpModule则会继续对处理完毕的HTTP请求信息流进行层层的转交动作，直到返回到客户端为止。

图1：HttpModule生命周期示意图

示例2：验证HttpModule生命周期

using System;

using System.Collections.Generic;

using System.Text;

using System.Web;

namespace MyHttpModule

{

    public class ValidaterHttpModule : IHttpModule

    {

        #region IHttpModule 成员

        public void Dispose()

        {}

        public void Init(HttpApplication application)

        {

            application.BeginRequest += new EventHandler(application_BeginRequest);

            application.EndRequest += new EventHandler(application_EndRequest);

            application.PreRequestHandlerExecute += new EventHandler(application_PreRequestHandlerExecute);

            application.PostRequestHandlerExecute += new EventHandler(application_PostRequestHandlerExecute);

            application.ReleaseRequestState += new EventHandler(application_ReleaseRequestState);

            application.AcquireRequestState += new EventHandler(application_AcquireRequestState);

            application.AuthenticateRequest += new EventHandler(application_AuthenticateRequest);

            application.AuthorizeRequest += new EventHandler(application_AuthorizeRequest);

            application.ResolveRequestCache += new EventHandler(application_ResolveRequestCache);

            application.PreSendRequestHeaders += new EventHandler(application_PreSendRequestHeaders);

            application.PreSendRequestContent += new EventHandler(application_PreSendRequestContent);

        }

        void application_PreSendRequestContent(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_PreSendRequestContent<br/>");

        }

        void application_PreSendRequestHeaders(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_PreSendRequestHeaders<br/>");

        }

        void application_ResolveRequestCache(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_ResolveRequestCache<br/>");

        }

        void application_AuthorizeRequest(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_AuthorizeRequest<br/>");

        }

        void application_AuthenticateRequest(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_AuthenticateRequest<br/>");

        }

        void application_AcquireRequestState(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_AcquireRequestState<br/>");

        }

        void application_ReleaseRequestState(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_ReleaseRequestState<br/>");

        }

        void application_PostRequestHandlerExecute(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_PostRequestHandlerExecute<br/>");

        }

        void application_PreRequestHandlerExecute(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_PreRequestHandlerExecute<br/>");

        }

        void application_EndRequest(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_EndRequest<br/>");

        }

        void application_BeginRequest(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.Context.Response.Write("application_BeginRequest<br/>");

        }

        #endregion

    }

}

多个自定义的Http Module的运作

从运行结果可以看到，在web.config文件中引入自定义HttpModule的顺序就决定了多个自定义HttpModule在处理一个HTTP请求的接管顺序。注：系统默认那几个HttpModule是最先衩ASP.NET Framework所加载上去的。

示例3：（代码类同示例2）

在HttpModule中终止此次的HTTP请求

可以利用HttpModule通过调用HttpApplication.CompleteRequest()方法实现当满足某一个条件时终止此次的HTTP请求。

需要注意的是，即使调用了HttpApplication.CompleteRequest()方法终止了一个HTTP请求，ASP.NET Framework仍然会触发HttpApplication后面的这3个事件：EndRequest事件、PreSendRequestHeaders事件、PreSendRequestContent事件。

如果存在多个自定义的HttpModule的话，当Module1终止了一个HTTP请求，这个HTTP请求将不会再触发Module2中相应的事件了，但Module2的最后三个事件仍会被触发。

示例4：

using System;

using System.Collections.Generic;

using System.Text;

using System.Web;

namespace MyHttpModule

{

    public class CompleteRequestHttpModule : IHttpModule

    {

        #region IHttpModule 成员

        public void Dispose()

        {}

        public void Init(HttpApplication application)

        {

            application.BeginRequest += new EventHandler(Application_BeginRequest);

        }

        void Application_BeginRequest(object sender, EventArgs e)

        {

            HttpApplication application = (HttpApplication)sender;

            application.CompleteRequest();

            application.Context.Response.Write("请求被终止。");

        }

        #endregion

    }

}

参考资料

《ASP.NET深入解析》

《ASP.NET实用全书》

 
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