原文地址：http://www.cnblogs.com/stwyhm/archive/2006/08/09/471729.html
HttpModule是如何工作的
當一個HTTP請求到達HttpModule時，整個ASP.NET Framework系統還並沒有對這個HTTP請求做任何處理，也就是說此時對於HTTP請求來講，HttpModule是一個HTTP請求的“必經之路”，所以可以在這個HTTP請求傳遞到真正的請求處理中心（HttpHandler）之前附加一些需要的信息在這個HTTP請求信息之上，或者針對截獲的這個HTTP請求信息作一些額外的工作，或者在某些情況下乾脆終止滿足一些條件的HTTP請求，從而可以起到一個Filter過濾器的作用。
示例1：
using System;
using System.Collections.Generic;
using System.Text;
using System.Web;
namespace MyHttpModule
{
/// <summary>
/// 說明：用來實現自己的HttpModule類。
/// 作者：文野
/// 聯繫：[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("我來自自定義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("我來自自定義HttpModule中的EndRequest<br />");
}
#region IHttpModule 成員
public void Dispose()
{}
public void Init(HttpApplication application)
{
application.BeginRequest += new EventHandler(Application_BeginRequest);
application.EndRequest += new EventHandler(Application_EndRequest);
}
#endregion
}
}
在Web.config進行如下配置
<add name="MyFirstHttpModule" type="MyHttpModule.MyFirstHttpModule,MyHttpModule"/>
深入瞭解HttpModule
一個HTTP請求在HttpModule容器的傳遞過程中，會在某一時刻（ResolveRequestCache事件）將這個HTTP請求傳遞給HttpHandler容器。在這個事件之後，HttpModule容器會建立一個HttpHandler的入口實例，但是此時並沒有將HTTP請求控制權交出，而是繼續觸發AcquireRequestState事件以及PreRequestHandlerExcute事件。在PreRequestHandlerExcute事件之後，HttpModule窗口就會將控制權暫時交給HttpHandler容器，以便進行真正的HTTP請求處理工作。
而在HttpHandler容器內部會執行ProcessRequest方法來處理HTTP請求。在容器HttpHandler處理完畢整個HTTP請求之後，會將控制權交還給HttpModule，HttpModule則會繼續對處理完畢的HTTP請求信息流進行層層的轉交動作，直到返回到客戶端爲止。
圖1：HttpModule生命週期示意圖
示例2：驗證HttpModule生命週期
using System;
using System.Collections.Generic;
using System.Text;
using System.Web;
namespace MyHttpModule
{
public class ValidaterHttpModule : IHttpModule
{
#region IHttpModule 成員
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
}
}
多個自定義的Http Module的運作
從運行結果可以看到，在web.config文件中引入自定義HttpModule的順序就決定了多個自定義HttpModule在處理一個HTTP請求的接管順序。注：系統默認那幾個HttpModule是最先衩ASP.NET Framework所加載上去的。
示例3：（代碼類同示例2）
在HttpModule中終止此次的HTTP請求
可以利用HttpModule通過調用HttpApplication.CompleteRequest()方法實現當滿足某一個條件時終止此次的HTTP請求。
需要注意的是，即使調用了HttpApplication.CompleteRequest()方法終止了一個HTTP請求，ASP.NET Framework仍然會觸發HttpApplication後面的這3個事件：EndRequest事件、PreSendRequestHeaders事件、PreSendRequestContent事件。
如果存在多個自定義的HttpModule的話，當Module1終止了一個HTTP請求，這個HTTP請求將不會再觸發Module2中相應的事件了，但Module2的最後三個事件仍會被觸發。
示例4：
using System;
using System.Collections.Generic;
using System.Text;
using System.Web;
namespace MyHttpModule
{
public class CompleteRequestHttpModule : IHttpModule
{
#region IHttpModule 成員
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("請求被終止。");
}
#endregion
}
}
參考資料
《ASP.NET深入解析》
《ASP.NET實用全書》
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