说明：  HttpHandler：HttpHandler(HTTP处理器)是HTTP请求的处理中心，它可以完全使用HttpContext对象，真正地对客户端请求的页面进行编译和执行，并将处理后的信息附加在HTTP请求信息流中返回到HttpModule中。HttpHandler通过一个简单IHttpHandler接口实现(或者它的异步版本IHttpAsyncHandler)，此接口仅仅只有一个方法ProcessRequest()和一个属性IsReusable。
(HttpHandler: HttpHandler (HTTP processor) is the HTTP request processing center, it can be completely used HttpContext object, the client actually requested page is compiled and executed, and the processed information in the HTTP request additional information flow returns to HttpModule in. HttpHandler through a simple IHttpHandler interface (or its asynchronous version IHttpAsyncHandler), this interface has only one method ProcessRequest () and a property IsReusable.)