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ProcessRequest UnloadAppDomain
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Note that all the methods shown in Table 5-2 have limited applicability in user applications. In particular, you re not supposed to use ProcessRequest in your own code, whereas Close is useful only if you re hosting ASP.NET in a custom application. Of the three methods in Table 5-2, only UnloadAppDomain can be considered for use if, under certain run-time conditions, you realize you need to restart the application. (See the sidebar What Causes Application Restarts later in this chapter.) Upon creation, the HttpRuntime object initializes a number of internal objects that will help carry out the page request. Helper objects include the cache manager and the file system monitor used to detect changes in the files that form the application. When the ProcessRequest method is called, the HttpRuntime object starts working to serve a page to the browser. It creates a new empty context for the request and initializes a specialized text writer object in which the markup code will be accumulated. A context is given by an instance of the HttpContext class, which encapsulates all HTTP-specific information about the request. After that, the HttpRuntime object uses the context information to either locate or create a Web application object capable of handling the request. A Web application is searched using the virtual directory information contained in the URL. The object used to find or create a new Web application is HttpApplicationFactory an internal-use object responsible for returning a valid object capable of handling the request. Before we get to discover more about the various components of the HTTP pipeline, a look at Figure 5-2 is in order.
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Part II
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default.aspx
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HttpRuntime
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Initializes the ASP.NET cache and HTTP context
HttpApplicationFactory HttpApplication PageHandlerFactory ASP .default_aspx
Based on the URL, creates/selects the application object to serve the request Determines the type of the request and invokes the proper handler factory Determines the page class required to serve the request and creates it if it doesn t exist
IHttpHandler
HttpRuntime invokes ProcessRequest on ASP.default_aspx
FIGURE 5-2 The HTTP pipeline processing for a page.
The Application Factory
During the lifetime of the application, the HttpApplicationFactory object maintains a pool of HttpApplication objects to serve incoming HTTP requests. When invoked, the application factory object verifies that an AppDomain exists for the virtual folder the request targets. If the application is already running, the factory picks an HttpApplication out of the pool of available objects and passes it the request. A new HttpApplication object is created if an existing object is not available. If the virtual folder has not yet been called for the first time, a new HttpApplication object for the virtual folder is created in a new AppDomain. In this case, the creation of an HttpApplication object entails the compilation of the global.asax application file, if one is
5 Anatomy of an ASP.NET Page
present, and the creation of the assembly that represents the actual page requested. This event is actually equivalent to the start of the application. An HttpApplication object is used to process a single page request at a time; multiple objects are used to serve simultaneous requests.
The HttpApplication Object
HttpApplication is the base class that represents a running ASP.NET application. A derived HTTP application class is dynamically generated by parsing the contents of the global.asax file, if any is present. If global.asax is available, the application class is built and named after it: ASP.global_asax. Otherwise, the base HttpApplication class is used. An instance of an HttpApplication-derived class is responsible for managing the entire lifetime of the request it is assigned to. The same instance can be reused only after the request has been completed. The HttpApplication maintains a list of HTTP module objects that can filter and even modify the content of the request. Registered modules are called during various moments of the elaboration as the request passes through the pipeline. The HttpApplication object determines the type of object that represents the resource being requested typically, an ASP.NET page, a Web service, or perhaps a user control. HttpApplication then uses the proper handler factory to get an object that represents the requested resource. The factory either instantiates the class for the requested resource from an existing assembly or dynamically creates the assembly and then an instance of the class. A handler factory object is a class that implements the IHttpHandlerFactory interface and is responsible for returning an instance of a managed class that can handle the HTTP request an HTTP handler. An ASP.NET page is simply a handler object that is, an instance of a class that implements the IHttpHandler interface. Let s see what happens when the resource requested is a page.
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