HTTP Handlers and HTTP Modules Overview

 

HTTP Handlers and HTTP Modules Overview
.NET Framework 4
An ASP.NET HTTP handler is the process (frequently referred to as the "endpoint") that runs in response to a request made to an ASP.NET Web application. The most common handler is an ASP.NET page handler that processes .aspx files. When users request an .aspx file, the request is processed by the page through the page handler. You can create your own HTTP handlers that render custom output to the browser.
An HTTP module is an assembly that is called on every request that is made to your application. HTTP modules are called as part of the ASP.NET request pipeline and have access to life-cycle events throughout the request. HTTP modules let you examine incoming and outgoing requests and take action based on the request.
 Scenarios
Typical uses for custom HTTP handlers include the following:
RSS feeds   To create an RSS feed for a Web site, you can create a handler that emits RSS-formatted XML. You can then bind a file name extension such as .rss to the custom handler. When users send a request to your site that ends in .rss, ASP.NET calls your handler to process the request.
Image server   If you want a Web application to serve images in a variety of sizes, you can write a custom handler to resize images and then send them to the user as the handler's response.
Typical uses for HTTP modules include the following:
Security   Because you can examine incoming requests, an HTTP module can perform custom authentication or other security checks before the requested page, XML Web service, or handler is called. In Internet Information Services (IIS) 7.0 running in Integrated mode, you can extend forms authentication to all content types in an application.
Statistics and logging   Because HTTP modules are called on every request, you can gather request statistics and log information in a centralized module, instead of in individual pages.
Custom headers or footers   Because you can modify the outgoing response, you can insert content such as custom header information into every page or XML Web service response.
Features
HTTP handler and module features include the following:
The IHttpHandler and IHttpModule interfaces are the starting point for developing handlers and modules.
The IHttpAsyncHandler interface is the starting point for developing asynchronous handlers.
Custom handler and module source code can be put in the App_Code folder of an application, or it can be compiled and put in the Bin folder of an application.
Handlers and modules developed for use in IIS 6.0 can be used in IIS 7.0 with little or no change. For more information, see Moving an ASP.NET Application from IIS 6.0 to IIS 7.0.
Modules can subscribe to a variety of request-pipeline notifications. Modules can receive notification of events of the HttpApplication object.
In IIS 7.0, the request pipeline is integrated with the Web server request pipeline. HTTP modules can be used for any request to the Web server, not just ASP.NET requests.
Background
HTTP Handlers
An ASP.NET HTTP handler is the process that runs in response to a request that is made to an ASP.NET Web application. The most common handler is an ASP.NET page handler that processes .aspx files. When users request an .aspx file, the request is processed by the page handler.
The ASP.NET page handler is only one type of handler. ASP.NET includes several other built-in handlers such as the Web service handler for .asmx files.
Built-in HTTP Handlers in ASP.NET
ASP.NET maps HTTP requests to HTTP handlers based on a file name extension. Each HTTP handler can process individual HTTP URLs or groups of URL extensions in an application. ASP.NET includes several built-in HTTP handlers, as listed in the following table.
Handler  Description
ASP.NET page handler (*.aspx)  The default HTTP handler for all ASP.NET pages.
Web service handler (*.asmx)  The default HTTP handler for Web service pages created as .asmx files in ASP.NET.
Generic Web handler (*.ashx)  The default HTTP handler for all Web handlers that do not have a UI and that include the @ WebHandler directive.

Trace handler (trace.axd)  A handler that displays current page trace information. For details, see How to: View ASP.NET Trace Information with the Trace Viewer.

Creating a Custom HTTP Handler
To create a custom HTTP handler, you create a class that implements the IHttpHandler interface to create a synchronous handler. Alternatively, you can implement IHttpAsyncHandler to create an asynchronous handler. Both handler interfaces require that you implement the IsReusable property and the ProcessRequest method. The IsReusable property specifies whether the IHttpHandlerFactory object (the object that actually calls the appropriate handler) can put the handler in a pool and reuse it to increase performance. If the handler cannot be pooled, the factory must create a new instance of the handler every time that the handler is needed.
The ProcessRequest method is responsible for processing individual HTTP requests. In this method, you write the code that produces the output for the handler.
HTTP handlers have access to the application context. This includes the requesting user's identity (if known), application state, and session information. When an HTTP handler is requested, ASP.NET calls the ProcessRequest method of the appropriate handler. The code that you write in the handler's ProcessRequest method creates a response, which is sent back to the requesting browser.
Mapping a File Name Extension
When you create a class file as your HTTP handler, the handler can respond to any file name extension that is not already mapped in IIS and in ASP.NET. For example, if you are creating an HTTP handler for generating an RSS feed, you can map your handler to the .rss file name extension. In order for ASP.NET to know which handler to use for your custom file name extension, in IIS you must map the extension to ASP.NET. Then in the application, you must map the extension to the custom handler.
By default, ASP.NET maps the file name extension .ashx to an HTTP handler. If you add the @ WebHandler directive to a class file, ASP.NET automatically maps the .ashx file name extension to the default HTTP handler. This is similar to the way ASP.NET maps the .aspx file name extension to the ASP.NET page handler when the @ Page directive is used. Therefore, if you create an HTTP handler class that has the file name extension .ashx, the handler is automatically registered with IIS and ASP.NET.
If you want to create a custom file name extension for a handler, you must explicitly register the extension with IIS and ASP.NET. The advantage of not using the .ashx file name extension is that the handler is then reusable for different extension mappings. For example, in one application the custom handler might respond to requests that end in .rss. In another application, it might respond to requests that end in .feed. As another example, the handler might be mapped to both file name extensions in the same application, but might create different responses based on the extension.
The process for registering a handler's custom file name extension is different in IIS 7.0 and in earlier versions of IIS. For more information, see How to: Register HTTP Handlers and How to: Configure an HTTP Handler Extension in IIS.
Asynchronous and Synchronous HTTP Handlers
An HTTP handler can be either synchronous or asynchronous. A synchronous handler does not return until it finishes processing the HTTP request for which it is called. An asynchronous handler runs a process independently of sending a response to the user. Asynchronous handlers are useful when you must start an application process that might be lengthy and the user does not have to wait until it finishes before receiving a response from the server.
Asynchronous HTTP handlers enable you to start an external process, such as a method call to a remote server. The handler can then continue processing without waiting for the external process to finish. While an asynchronous HTTP handler is processing, ASP.NET puts the thread that would ordinarily be used for the external process back into the thread pool until the handler receives a callback from the external process. This can prevent thread blocking and can improve performance because only a limited number of threads can execute at the same time. If many users request synchronous HTTP handlers that rely on external processes, the operating system can run out of threads, because many threads are blocked and are waiting for an external process.
When you create an asynchronous handler, you must implement the IHttpAsyncHandler interface. You must also implement the BeginProcessRequest method in order to initiate an asynchronous call that processes individual HTTP requests. In addition, you must implement the EndProcessRequest method to run cleanup code when the process ends.
Custom IHttpHandlerFactory Classes
The IHttpHandlerFactory class receives requests and is responsible for forwarding a request to the appropriate HTTP handler. You can create a custom HTTP handler factory by creating a class that implements the IHttpHandlerFactory interface. A custom handler factory can give you finer control over how HTTP requests are processed by creating different handlers based on run-time conditions. For example, with a custom HTTP handler factory, you can instantiate one HTTP handler for a file type if the HTTP request method is PUT, and another if the method is GET.
To register a custom extension for a handler factory, follow the steps for registering a custom extension for a handler. For an example of creating and registering a handler factory, see Walkthrough: Creating and Registering HTTP Handler Factories.
HTTP Modules
An HTTP module is an assembly that is called on every request that is made to your application. HTTP modules are called as part of the request pipeline and have access to life-cycle events throughout the request. HTTP modules therefore let you examine incoming requests and take action based on the request. They also let you examine the outgoing response and modify it.
In IIS 6.0, the ASP.NET request pipeline is separate from the Web server request pipeline. In IIS 7.0, the ASP.NET request pipeline and the Web server request pipeline can be integrated into a common request pipeline. In IIS 7.0, this is referred to as Integrated mode. The unified pipeline has several benefits for ASP.NET developers. For example, it lets managed-code modules receive pipeline notifications for all requests, even if the requests are not for ASP.NET resources. However, if you want, you can run IIS 7.0 in Classic mode, which emulates ASP.NET running in IIS 6.0. For more information, see ASP.NET Application Life Cycle Overview for IIS 7.0.
ASP.NET HTTP modules are like ISAPI filters because they are invoked for all requests. However, they are written in managed code and are fully integrated with the life cycle of an ASP.NET application. You can put custom module source code in the App_Code folder of your application, or you can put compiled custom modules as assemblies in the Bin folder of an application.
ASP.NET uses modules to implement various application features, which includes forms authentication, caching, session state, and client script services. In each case, when those services are enabled, the module is called as part of a request and performs tasks that are outside the scope of any single page request. Modules can consume application events and can raise events that can be handled in the Global.asax file. For more information about application events, see ASP.NET Application Life Cycle Overview for IIS 5.0 and 6.0 and ASP.NET Application Life Cycle Overview for IIS 7.0.
 Note
HTTP modules differ from HTTP handlers. An HTTP handler returns a response to a request that is identified by a file name extension or family of file name extensions. In contrast, an HTTP module is invoked for all requests and responses. It subscribes to event notifications in the request pipeline and lets you run code in registered event handlers. The tasks that a module is used for are general to an application and to all requests for resources in the application.
How HTTP Modules Work
Modules must be registered to receive notifications from the request pipeline. The most common way to register an HTTP module is in the application's Web.config file. In IIS 7.0, the unified request pipeline also enables you to register a module in other ways, which includes through IIS Manager and through the Appcmd.exe command-line tool. For more information, see Configuring Handler Mappings in IIS 7.0 and Start Appcmd.exe.
When ASP.NET creates an instance of the HttpApplication class that represents your application, instances of any modules that have been registered are created. When a module is created, its Init method is called and the module initializes itself. For more information, see ASP.NET Application Life Cycle Overview for IIS 5.0 and 6.0 and ASP.NET ASP.NET Application Life Cycle Overview for IIS 7.0.
In a module's Init method, you can subscribe to various application events such as BeginRequest or EndRequest by binding the events to methods in the module.
 Note
For modules that operate in the IIS 7.0 integrated pipeline, you should register event handlers in the Init method.

When application events are raised, the appropriate method in your module is called. The method can perform whatever logic is required, such as checking authentication or logging request information. During event handling, the module has access to the Context property of the current request. This enables you to redirect the request to an alternative page, modify the request, or perform any other request manipulation. For example, if the module checks authentication, the module might redirect to a login or error page if the credentials are not correct. Otherwise, when the module's event handler has finished running, ASP.NET calls the next process in the pipeline. This might be another module or might be the appropriate HTTP handler (such as an .aspx file) for the request.
HTTP Modules versus Global.asax Files
You can implement much of the functionality of a module in the application's Global.asax file, which enables you to respond to application events. However, modules have an advantage over the Global.asax file because they are encapsulated and can be created one time and used in many different applications. By adding them to the global assembly cache and registering them in the Machine.config file, you can reuse them across applications. For more information, see Global Assembly Cache.
 Note
In IIS 7.0, the integrated pipeline enables managed modules to subscribe to pipeline notifications for all requests, not just requests for ASP.NET resources. Event handlers in the Global.asax file are invoked only for notifications during requests for resources in the application. Custom modules in Integrated mode can be explicitly scoped to receive event notifications only for requests to the application. Otherwise, custom modules receive event notification for all requests to the application. If the precondition attribute of the add element of the modules section is set to "managedHandler", the module is scoped to the application.
The advantage of using the Global.asax file is that you can handle other registered events such as Session_Start and Session_End. In addition, the Global.asax file enables you to instantiate global objects that are available throughout the application.
You should use a module whenever you must create code that depends on application events, and when the following conditions are true:
You want to re-use the module in other applications.
You want to avoid putting complex code in the Global.asax file.
The module applies to all requests in the pipeline (IIS 7.0 Integrated mode only).
You should add code in the Global.asax file whenever you must create code that depends on application events and you do not have to reuse it across applications. You can also use the Global.asax file when you have to subscribe to events that are not available to modules, such as Session_Start.
Creating an HTTP Module
The general process for writing an HTTP module is as follows:
Create a class that implements the IHttpModule interface.
Write a handler for the Init method. The initialization method should initialize your module and subscribe to any application events you need. For example, if you want to append something to responses, you might subscribe to the EndRequest event. If you want to perform custom authentication logic, you might subscribe to the AuthenticateRequest event. For more information about application events, see ASP.NET Application Life Cycle Overview for IIS 5.0 and 6.0.
Write code for the events that you have subscribed to.
Optionally implement the Dispose method if the module requires cleanup.
Register the module in the Web.config file. For more information, see Walkthrough: Creating and Registering a Custom HTTP Module
For information about how to move a module from an application that is running in IIS 6.0 or earlier versions to an application that runs on in IIS 7.0, see Moving an ASP.NET Application from IIS 6.0 to IIS 7.0.

 

HTTP 处理程序和 HTTP 模块概述
.NET Framework 4
ASP.NET HTTP 处理程序是响应对 ASP.NET Web 应用程序的请求而运行的过程（通常称为“终结点”）。 最常用的处理程序是处理 .aspx 文件的 ASP.NET 页处理程序。 用户请求 .aspx 文件时，页通过页处理程序来处理请求。 您可以创建自己的 HTTP 处理程序将自定义输出呈现给浏览器。
HTTP 模块是一个在每次针对应用程序发出请求时调用的程序集。 HTTP 模块作为 ASP.NET 请求管道的一部分调用，它们能够在整个请求过程中访问生命周期事件。 HTTP 模块使您可以检查传入和传出的请求并根据该请求进行操作。
 方案
自定义 HTTP 处理程序通常具有以下用途：
RSS 源 若要为网站创建 RSS 源，可以创建一个可发出 RSS 格式 XML 的处理程序。 然后可以将文件扩展名（如 .rss）绑定到此自定义处理程序。 当用户向站点发送以 .rss 结尾的请求时，ASP.NET 将调用您的处理程序来处理请求。
图像服务器 如果希望 Web 应用程序能够提供不同大小的图像，可以编写一个自定义处理程序来调整图像大小，然后将调整后的图像作为处理程序的响应发送给用户。
HTTP 模块通常具有以下用途：
安全 因为您可以检查传入的请求，所以 HTTP 模块可以在调用请求页、XML Web services 或处理程序之前执行自定义的身份验证或其他安全检查。 在以集成模式运行的 Internet 信息服务 (IIS) 7.0 中，可以将 Forms 身份验证扩展到应用程序中的所有内容类型。
统计信息和日志记录 因为 HTTP 模块是在每次请求时调用的，所以，您可以将请求统计信息和日志信息收集到一个集中的模块中，而不是收集到各页中。
自定义的页眉或页脚 因为您可以修改传出响应，所以可以在每一个页面或 XML Web services 响应中插入内容，如自定义的标头信息。
功能：
HTTP 处理程序和模块功能包括：
IHttpHandler 和 IHttpModule 接口是开发处理程序和模块的起始点。
IHttpAsyncHandler 接口是开发异步处理程序的起始点。
自定义处理程序和模块源代码可以放在应用程序的 App_Code 文件夹中，也可以在应用程序的 Bin 文件夹中编译和存放。
为在 IIS 6.0 中使用而开发的处理程序和模块经过少许更改或不经更改即可在 IIS 7.0 中使用。 有关更多信息，请参见将 ASP.NET 应用程序从 IIS 6.0 迁移到 IIS 7.0。
模块可以订阅多种请求管道通知。 模块可以接收 HttpApplication 对象的事件通知。
在 IIS 7.0 中，请求管道与 Web 服务器请求管道集成在一起。 HTTP 模块可用于对 Web 服务器的任何请求，而不仅仅是 ASP.NET 请求。

背景
HTTP 处理程序
ASP.NET HTTP 处理程序是响应对 ASP.NET Web 应用程序的请求而运行的过程。 最常用的处理程序是处理 .aspx 文件的 ASP.NET 页处理程序。 用户请求 .aspx 文件时，通过页处理程序来处理请求。
ASP.NET 页处理程序仅仅是一种类型的处理程序。 ASP.NET 包括其他几种内置的处理程序，例如用于 .asmx 文件的 Web 服务处理程序。
ASP.NET 中的内置 HTTP 处理程序
ASP.NET 根据文件扩展名将 HTTP 请求映射到 HTTP 处理程序。 每个 HTTP 处理程序都可以处理应用程序中的单个 HTTP URL 或 URL 扩展名组。 ASP.NET 包括几种内置的 HTTP 处理程序，如下表所列。
处理程序  说明
ASP.NET 页处理程序 (*.aspx)  用于所有 ASP.NET 页的默认 HTTP 处理程序。
Web 服务处理程序 (*.asmx)  在 ASP.NET 中作为 .asmx 文件创建的 Web 服务页的默认 HTTP 处理程序。
泛型 Web 处理程序 (*.ashx)  不含 UI 和包括 @ WebHandler 指令的所有 Web 处理程序的默认 HTTP 处理程序。
跟踪处理程序 (trace.axd)  显示当前页跟踪信息的处理程序。 有关详细信息，请参见 如何：使用跟踪查看器查看 ASP.NET 跟踪信息。

创建自定义 HTTP 处理程序
若要创建自定义 HTTP 处理程序，请创建实现 IHttpHandler 接口的类来创建一个同步处理程序。 或者，可以实现 IHttpAsyncHandler 来创建一个异步处理程序。 两种处理程序接口都要求您实现 IsReusable 属性和 ProcessRequest 方法。 IsReusable 属性指定 IHttpHandlerFactory 对象（实际调用适当处理程序的对象）是否可以将处理程序放置在池中，并且重新使用它以提高性能。 如果处理程序不能放在池中，则在每次需要处理程序时工厂都必须创建处理程序的新实例。
ProcessRequest 方法负责处理单个 HTTP 请求。 在此方法中，将编写生成处理程序输出的代码。
HTTP 处理程序有权访问应用程序上下文。 其中包括请求用户的标识（如果已知）、应用程序状态和会话信息。 当请求 HTTP 处理程序时，ASP.NET 将调用相应处理程序的 ProcessRequest 方法。 您在处理程序的 ProcessRequest 方法中编写的代码将创建一个响应，此响应随后发送回请求浏览器。
映射文件扩展名
当创建一个类文件作为 HTTP 处理程序时，该处理程序可以响应尚未在 IIS 和 ASP.NET 中映射的任何文件扩展名。 例如，如果要创建用于生成 RSS 源的 HTTP 处理程序，则可以将您的处理程序映射到 .rss 文件扩展名。 为了使 ASP.NET 了解为您的自定义文件扩展名使用哪个处理程序，在 IIS 中必须将该扩展名映射到 ASP.NET。 然后在应用程序中必须将该扩展名映射到自定义处理程序。
默认情况下，ASP.NET 将文件扩展名 .ashx 映射到 HTTP 处理程序。 如果将 @ WebHandler 指令添加到某个类文件，则 ASP.NET 会自动将 .ashx 文件扩展名映射到默认的 HTTP 处理程序。 这与在使用 @ Page 指令时，ASP.NET 将 .aspx 文件扩展名映射到 ASP.NET 页处理程序的方式类似。 因此，如果您创建具有文件扩展名 .ashx 的 HTTP 处理程序类，该处理程序将自动注册到 IIS 和 ASP.NET。
如果要为处理程序创建自定义文件扩展名，则必须使用 IIS 和 ASP.NET 显式注册该扩展名。 不使用 .ashx 文件扩展名的优点在于处理程序可为不同的扩展名映射重用。 例如，在一个应用程序中，自定义处理程序可能响应以 .rss 结尾的请求。 在另一个应用程序中，该自定义处理程序可能响应以 .feed 结尾的请求。 再举一例，该处理程序可能映射到同一应用程序中的两个文件扩展名，但可能基于扩展名创建两个不同的响应。
在 IIS 7.0 中注册处理程序的自定义文件扩展名的过程与在早期版本的 IIS 中不同。有关更多信息，请参见如何：注册 HTTP 处理程序和如何：在 IIS 中配置 HTTP 处理程序扩展名。
异步和同步 HTTP 处理程序
HTTP 处理程序可以是同步的也可以是异步的。 同步处理程序在完成对为其调用该处理程序的 HTTP 请求的处理后才会返回。 异步处理程序运行进程的行为与向用户发送响应无关。 当您必须启动一个可能耗费很长时间的应用程序进程，而用户又无需等候进程完成以便从服务器接收响应时，异步处理程序非常有用。
使用异步 HTTP 处理程序可以启动一个外部进程，如对远程服务器的方法调用。 该处理程序可以继续处理，而不必等待外部进程结束。 在异步 HTTP 处理程序正在处理时，ASP.NET 将通常用于外部进程的线程放回线程池中，直到处理程序收到来自外部进程的回调。 由于只能同时执行有限数量的线程，因此这样可以避免阻止线程并改善性能。 如果许多用户都在请求依赖于外部进程的同步 HTTP 处理程序，那么操作系统可能用完所有线程，因为大量线程被阻止，正在等待外部进程。
在创建异步处理程序时，必须实现 IHttpAsyncHandler 接口。 为了启动处理单个 HTTP 请求的异步调用，还必须实现 BeginProcessRequest 方法。 此外，必须实现 EndProcessRequest 方法，以便在进程结束时运行清理代码。
自定义 IHttpHandlerFactory 类
IHttpHandlerFactory 类接收请求并负责向相应的 HTTP 处理程序转发请求。 您可以通过创建一个实现了 IHttpHandlerFactory 接口的类来创建自定义 HTTP 处理程序工厂。 自定义处理程序工厂通过创建不同的基于运行时条件的处理程序，使您可以更好地控制对 HTTP 请求的处理方式。 例如，使用自定义 HTTP 处理程序工厂，可以在 HTTP 请求方法为 PUT 时为某个文件类型实例化一个 HTTP 处理程序，而在该方法为 GET 时实例化另一个 HTTP 处理程序。
若要为处理程序工厂注册自定义扩展名，请遵循为处理程序注册自定义扩展名的步骤。 有关创建和注册处理程序工厂的示例，请参见演练：创建和注册 HTTP 处理程序工厂。
HTTP 模块
HTTP 模块是一个在每次针对应用程序发出请求时调用的程序集。 HTTP 模块作为请求管道的一部分调用，它们能够在整个请求过程中访问生命周期事件。 因此，HTTP 模块使您可以检查传入的请求并根据该请求进行操作。 它们还使您可以检查传出的响应并修改它。
在 IIS 6.0 中，ASP.NET 请求管道与 Web 服务器请求管道分离。 在 IIS 7.0 中，ASP.NET 请求管道和 Web 服务器请求管道可以集成到通用请求管道中。 在 IIS 7.0 中，此操作称为“集成”模式。 对于 ASP.NET 开发人员，统一的管道有多个优点。 例如，它使托管代码模块可以接收所有请求的管道通知，即使这些请求不是针对 ASP.NET 资源。 但是，如果需要，可以在经典模式下运行 IIS 7.0，该模式模拟在 IIS 6.0 中运行 ASP.NET。 有关更多信息，请参见 IIS 7.0 的 ASP.NET 应用程序生命周期概述。
ASP.NET HTTP 模块针对所有请求调用，这与 ISAPI 筛选器类似。 但是它们是用托管代码编写的，而且可以与 ASP.NET 应用程序的生命周期完全集成。 可以将自定义模块源代码放在应用程序的 App_Code 文件夹中，也可以将经过编译的自定义模块作为程序集放在应用程序的 Bin 文件夹中。
ASP.NET 使用模块来实现各个应用程序功能，其中包括 Forms 身份验证、缓存、会话状态和客户端脚本服务。 在每种情况下，如果这些服务处于启用状态，模块会作为请求的一部分调用，并执行在任何单一页请求范围之外的任务。 模块可以使用应用程序事件，可能会引发可在 Global.asax 文件中处理的事件。 有关应用程序事件的更多信息，请参见 IIS 5.0 和 6.0 的 ASP.NET 应用程序生命周期概述和 IIS 7.0 的 ASP.NET 应用程序生命周期概述。
 注意
HTTP 模块不同于 HTTP 处理程序。 HTTP 处理程序通过文件扩展名或文件扩展名系列来识别请求，并返回对请求的响应。 相反，HTTP 模块是为所有请求和响应调用的。 它订阅请求管道中的事件通知，并使您可以运行已注册事件处理程序中的代码。 使用模块的任务对于应用程序和针对应用程序中的资源的所有请求都是通用的。
HTTP 模块的工作方式
模块必须经过注册才能从请求管道接收通知。 注册 HTTP 模块的最常用方法是在应用程序的 Web.config 文件中进行注册。 在 IIS 7.0 中，统一的请求管道使您还可以通过其他方式注册模块，其中包括通过 IIS 管理器和 Appcmd.exe 命令行工具。 有关更多信息，请参见 Configuring Handler Mappings in IIS 7.0（在 IIS 7.0 中配置处理程序映射）和 Start Appcmd.exe（启动 Appcmd.exe）。
当 ASP.NET 创建表示您的应用程序的 HttpApplication 类的实例时，将创建已注册的任何模块的实例。 在创建模块时，将调用它的 Init 方法，并且模块会自行初始化。 有关更多信息，请参见 IIS 5.0 和 6.0 的 ASP.NET 应用程序生命周期概述和 IIS 7.0 的 ASP.NET 应用程序生命周期概述。
在模块的 Init 方法中，可以订阅各种应用程序事件（如 BeginRequest 或 EndRequest），这可以通过将事件绑定到模块中的方法来完成。
 注意
对于在 IIS 7.0 集成管道中运行的模块，应该在 Init 方法中注册事件处理程序。
当这些应用程序事件被引发时，将调用模块中的相应方法。 该方法可以执行所需的任何逻辑，如检查身份验证或记录请求信息。 在事件处理过程中，模块能够访问当前请求的 Context 属性。 这使您可以将请求重定向到其他页、修改请求或者执行任何其他请求操作。 例如，如果模块检查身份验证，则在凭据不正确的情况下，模块可能重定向到登录页或错误页。 否则，当模块的事件处理程序完成运行时，ASP.NET 会调用管道中的下一个进程。 这可能是另一个模块，也可能是用于该请求的 HTTP 处理程序（如 .aspx 文件）。
HTTP 模块与 Global.asax 文件
可以在应用程序的 Global.asax 文件中实现模块的许多功能，这使您可以响应应用程序事件。 但是，模块相对于 Global.asax 文件具有如下优点：模块可以进行封装，因此可以在创建一次后在许多不同的应用程序中使用。 通过将它们添加到全局程序集缓存并将它们注册到 Machine.config 文件中，可以跨应用程序重新使用它们。 有关更多信息，请参见全局程序集缓存。
 注意
在 IIS 7.0 中，集成管道使托管模块可以订阅所有请求的管道通知，而不仅仅是对 ASP.NET 资源的请求。 Global.asax 文件中的事件处理程序仅为请求应用程序的资源过程中的通知调用。 可以显式指定集成模式下的自定义模块的作用范围，以只接收针对应用程序的请求的事件通知。 否则，自定义模块将接收对应用程序的所有请求的事件通知。 如果 modules 部分的 add 元素的 precondition 特性设置为“managedHandler”，则模块的作用范围是应用程序。
使用 Global.asax 文件的好处在于可以处理其他已注册事件，如 Session_Start 和 Session_End。 此外，Global.asax 文件还允许您实例化可在整个应用程序中使用的全局对象。
当您必须创建依赖应用程序事件的代码，并且符合以下条件时，都应该使用模块：
希望在其他应用程序中重用该模块。
希望避免将复杂代码放在 Global.asax 文件中。
该模块应用于管道中的所有请求（仅限 IIS 7.0 集成模式）。
当您必须创建依赖应用程序事件的代码并且不需要跨应用程序重用代码时，应该将代码添加到 Global.asax 文件中。 当必须订阅对于模块不可用的事件（如 Session_Start）时，也可以使用 Global.asax 文件。
创建 HTTP 模块
编写 HTTP 模块的一般过程如下：
创建实现 IHttpModule 接口的类。
为 Init 方法编写一个处理程序。 初始化方法应该初始化模块并订阅所需的任何应用程序事件。 例如，如果希望为响应附加一些内容，可以订阅 EndRequest 事件。 如果希望执行自定义身份验证逻辑，可以订阅 AuthenticateRequest 事件。 有关应用程序事件的更多信息，请参见 IIS 5.0 和 6.0 的 ASP.NET 应用程序生命周期概述。
为已经订阅的事件编写代码。
如果模块需要清理，还可选择实现 Dispose 方法。
在 Web.config 文件中注册模块。 有关更多信息，请参见演练：创建和注册自定义 HTTP 模块
有关如何将模块从正在运行于 IIS 6.0 或更早版本 IIS 的应用程序移到运行于 IIS 7.0 的应用程序，请参见将 ASP.NET 应用程序从 IIS 6.0 迁移到 IIS 7.0。