https://www.cnblogs.com/JimmyZhang/archive/2007/09/04/880967.html
IHttpModule
HTTPRuntime（运行时）。在一个控制台程序中，程序的入口是Program中的Main方法。那么，一个网站的入口在哪里呢？在最开始的ashx中，有个ProcessRequest方法，后来在WebForm中，在后台是一个不分类，继承自Page类，在Page_Load方法中去写代码。其实Page类型也有一个ProcessRequest的虚方法。
都是这个ProcessRequest方法来处理请求的。在MVC中也是如此。在MVC中，任何一个Http请求，一定有一个IHttpHandler来处理，在这个接口中，定义了一个ProcessRequest方法。HttpApplication继承自IHttpHandler接口。任何一个Http请求就是一个HttpApplication对象来处理的，然后处理过程固定包含权限认证、缓存处理、Session处理、Cookie出库、生成html、输出客户端，与此同时，千千万万的开发者，又有各种各样的扩展诉求，任何一个环节都有可能扩展，如果是我们来设计，该怎么设计？
其实在MVC框架里面，用到的是观察者模式：
在HttpApplication类型，有这些事件：
//
// 摘要:
// Occurs just before ASP.NET sends HTTP headers to the client.
public event EventHandler PreSendRequestHeaders;
//
// 摘要:
// Occurs when the handler is selected to respond to the request.
public event EventHandler MapRequestHandler;
//
// 摘要:
// Occurs when the application is disposed.
public event EventHandler Disposed;
//
// 摘要:
// Occurs as the first event in the HTTP pipeline chain of execution when ASP.NET
// responds to a request.
public event EventHandler BeginRequest;
//
// 摘要:
// Occurs when a security module has established the identity of the user.
public event EventHandler AuthenticateRequest;
//
// 摘要:
// Occurs when a security module has established the identity of the user.
public event EventHandler PostAuthenticateRequest;
//
// 摘要:
// Occurs when a security module has verified user authorization.
public event EventHandler AuthorizeRequest;
//
// 摘要:
// Occurs when the user for the current request has been authorized.
public event EventHandler PostAuthorizeRequest;
//
// 摘要:
// Occurs when ASP.NET finishes an authorization event to let the caching modules
// serve requests from the cache, bypassing execution of the event handler (for
// example, a page or an XML Web service).
public event EventHandler ResolveRequestCache;
//
// 摘要:
// Occurs when ASP.NET bypasses execution of the current event handler and allows
// a caching module to serve a request from the cache.
public event EventHandler PostResolveRequestCache;
//
// 摘要:
// Occurs just before ASP.NET sends content to the client.
public event EventHandler PreSendRequestContent;
//
// 摘要:
// Occurs when ASP.NET has mapped the current request to the appropriate event handler.
public event EventHandler PostMapRequestHandler;
//
// 摘要:
// Occurs when ASP.NET has completed processing all the event handlers for the System.Web.HttpApplication.LogRequest
// event.
public event EventHandler PostLogRequest;
//
// 摘要:
// Occurs when the managed objects that are associated with the request have been
// released.
public event EventHandler RequestCompleted;
//
// 摘要:
// Occurs when the request state (for example, session state) that is associated
// with the current request has been obtained.
public event EventHandler PostAcquireRequestState;
//
// 摘要:
// Occurs just before ASP.NET starts executing an event handler (for example, a
// page or an XML Web service).
public event EventHandler PreRequestHandlerExecute;
//
// 摘要:
// Occurs when the ASP.NET event handler (for example, a page or an XML Web service)
// finishes execution.
public event EventHandler PostRequestHandlerExecute;
//
// 摘要:
// Occurs after ASP.NET finishes executing all request event handlers. This event
// causes state modules to save the current state data.
public event EventHandler ReleaseRequestState;
//
// 摘要:
// Occurs when ASP.NET has completed executing all request event handlers and the
// request state data has been stored.
public event EventHandler PostReleaseRequestState;
//
// 摘要:
// Occurs when ASP.NET finishes executing an event handler in order to let caching
// modules store responses that will be used to serve subsequent requests from the
// cache.
public event EventHandler UpdateRequestCache;
//
// 摘要:
// Occurs when ASP.NET finishes updating caching modules and storing responses that
// are used to serve subsequent requests from the cache.
public event EventHandler PostUpdateRequestCache;
//
// 摘要:
// Occurs just before ASP.NET performs any logging for the current request.
public event EventHandler LogRequest;
//
// 摘要:
// Occurs when ASP.NET acquires the current state (for example, session state) that
// is associated with the current request.
public event EventHandler AcquireRequestState;
//
// 摘要:
// Occurs as the last event in the HTTP pipeline chain of execution when ASP.NET
// responds to a request.
public event EventHandler EndRequest;
//
// 摘要:
// Occurs when an unhandled exception is thrown.
public event EventHandler Error;
这里用的是观察者模式，把固定的步骤直接写在handler里面，在步骤前后分别放一个事件，然后开发者可以对事件注册动作，等着请求进来了，然后就可以按照顺讯执行一下。这种设计是不是很完美？但是仍有不完美的地方，就是每个请求都要执行这些事件，太多管闲事了。在.NET Core中出现了中间件，比这种更加完美。后续再详细介绍。
请见下列代码
public class HttpProcessDemo
{
public class HttpApplicationDemo : IHttpHandler
{
public bool IsReusable => true; public event Action BeginRequest;
public event Action EndRequest;
public event Action PreSomething1Handler;
public event Action PostSomething1Handler;
public event Action PreSomething2Handler;
public event Action PostSomething2Handler;
public event Action PreSomething3Handler;
public event Action PostSomething3Handler;
public event Action PreSomething4Handler;
public event Action PostSomething4Handler;
public event Action PreSomething5Handler;
public event Action PostSomething5Handler;
public event Action PreSomething6Handler;
public event Action PostSomething6Handler;
public void ProcessRequest(HttpContext context)
{
this.BeginRequest?.Invoke(); this.PreSomething1Handler?.Invoke();
Console.WriteLine("Something 1");
this.PostSomething1Handler?.Invoke(); this.PreSomething2Handler?.Invoke();
Console.WriteLine("Something 2");
this.PostSomething2Handler?.Invoke();
this.PreSomething3Handler?.Invoke();
Console.WriteLine("Something 3");
this.PostSomething3Handler?.Invoke();
this.PreSomething4Handler?.Invoke();
Console.WriteLine("Something 4");
this.PostSomething4Handler?.Invoke(); this.PreSomething5Handler?.Invoke();
Console.WriteLine("Something 5");
this.PostSomething5Handler?.Invoke();
this.PreSomething6Handler?.Invoke();
Console.WriteLine("Something 6");
this.PostSomething6Handler?.Invoke(); this.EndRequest?.Invoke();
}
//任何请求进来，只能是 123456
//事件升级后，可以在程序启动时，实例化HttpApplicationDemo后，可以给事件注册动作，请求再进来时，处理不仅是123456了，还有多个事件里面的动作
}
对HttpApplication里面的事件进行动作注册，就叫IHttpModule。
自定义一个HttpModule+配置文件注册，然后任何一个请求都会执行Init里面注册给Application事件的动作。
public class CustomHttpModule : IHttpModule
{
public void Dispose()
{
Console.WriteLine();
} public event EventHandler CustomHttpModuleHandler; /// <summary>
/// 注册动作
/// </summary>
/// <param name="context"></param>
public void Init(HttpApplication application)
{
application.BeginRequest += (s, e) =>
{
this.CustomHttpModuleHandler?.Invoke(application, null);
};
//application.EndRequest += (s, e) =>
//{
// HttpContext.Current.Response.Write("CustomHttpModule.EndRequest");
//};
#region 为每一个事件，都注册了一个动作，向客户端输出信息
application.AcquireRequestState += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "AcquireRequestState "));
application.AuthenticateRequest += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "AuthenticateRequest "));
application.AuthorizeRequest += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "AuthorizeRequest "));
application.BeginRequest += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "BeginRequest "));
application.Disposed += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "Disposed "));
application.EndRequest += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "EndRequest "));
application.Error += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "Error "));
application.LogRequest += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "LogRequest "));
application.MapRequestHandler += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "MapRequestHandler "));
application.PostAcquireRequestState += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PostAcquireRequestState "));
application.PostAuthenticateRequest += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PostAuthenticateRequest "));
application.PostAuthorizeRequest += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PostAuthorizeRequest "));
application.PostLogRequest += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PostLogRequest "));
application.PostMapRequestHandler += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PostMapRequestHandler "));
application.PostReleaseRequestState += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PostReleaseRequestState "));
application.PostRequestHandlerExecute += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PostRequestHandlerExecute "));
application.PostResolveRequestCache += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PostResolveRequestCache "));
application.PostUpdateRequestCache += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PostUpdateRequestCache "));
application.PreRequestHandlerExecute += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PreRequestHandlerExecute "));
application.PreSendRequestContent += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PreSendRequestContent "));
application.PreSendRequestHeaders += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "PreSendRequestHeaders "));
application.ReleaseRequestState += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "ReleaseRequestState "));
application.RequestCompleted += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "RequestCompleted "));
application.ResolveRequestCache += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "ResolveRequestCache "));
application.UpdateRequestCache += (s, e) => application.Response.Write(string.Format("<h1 style='color:#00f'>来自MyCustomModule 的处理，{0}请求到达 {1}</h1><hr>", DateTime.Now.ToString(), "UpdateRequestCache "));
#endregion
}
}
访问下页面，就是这样的结果：
正常流程下，会按照顺序执行19个事件。
学完HttpModule，我们可以做点什么有用的扩展？
任何一个请求都会执行HttpModuleInit里面注册给Application的事件
1、日志-性能监控
2、权限
3、缓存
4、页面加点东西
5、请求过滤
6、MVC就是一个Module的扩展
不适合的：不是针对全部请求的，就不太适合用Module，因为有性能损耗
1、多语言，根据Cookie信息去查询不同的数据做不同的展示，如果是全部一套处理，最后HttpModule拦截+处理，适合httpModule
2、跳转到不同界面，也不合适
3、防盗链，针对一类的后缀来处理的，而不是全部请求---判断----再防盗链
在HttpModule里面发布一个CustomHttpModuleHandler，在Global增加一个动作CustomHttpModuleBingle_CustomHttpModuleHandler（配置文件module名称_module里面事件名称），请求响应时，该事件会执行
protected void CustomHttpModuleBingle_CustomHttpModuleHandler(object sender, EventArgs e)
{
this.logger.Info("this is CustomHttpModuleBingle_CustomHttpModuleHandler");
}
HttpModule是对HttpApplication的事件动作注册动作，Global是对HttpModule里面的事件注册动作。