熟悉ASP.NET架构的开发者一定对于HTTP Modules与HTTP Handlers不陌生。两者的作用主要是对网络请求执行特定的处理工作。而在.NET Core中，它们都被Middleware(中件间)取代了。
之前的Http Modules和HTTP Handlers是如下图般处理请求的：
现在变成了这样：
一言概括之，Middleware完成了HTTP Modules与HTTP Handlers的原有工作，但又不是简单的化二为一的减法作用。
Middleware减去的其实是与原来ASP.NET中重要的基础——应用程序生命周期事件(application life cycle event)的绑定。
HTTP Modules在初始化时就需要针对HttpApplication的事件作绑定处理，这样当HttpApplication的各项事件被触发时，已绑定的相应处理程序才会按照预期的那样被执行。
public class HelloWorldModule : IHttpModule{ public HelloWorldModule() {} public String ModuleName{ get { return "HelloWorldModule"; }} // In the Init function, register for HttpApplication // events by adding your handlers.public void Init(HttpApplication application) {application.BeginRequest += (new EventHandler(this.Application_BeginRequest));application.EndRequest += (new EventHandler(this.Application_EndRequest));} private void Application_BeginRequest(Object source, EventArgs e) { // Create HttpApplication and HttpContext objects to access// request and response properties.HttpApplication application = (HttpApplication)source;HttpContext context = application.Context;context.Response.Write("<h1><font color=red>HelloWorldModule: Beginning of Request</font></h1><hr>");} private void Application_EndRequest(Object source, EventArgs e) {HttpApplication application = (HttpApplication)source;HttpContext context = application.Context;context.Response.Write("<hr><h1><font color=red>HelloWorldModule: End of Request</font></h1>");} public void Dispose() {}
}然后你还需要在web.config配置文件注册这个HTTP Module。
<configuration><system.web><httpModules><add name="HelloWorldModule" type="HelloWorldModule"/></httpModules></system.web></configuration>如果是用Middleware的话，事情就变得很简单了。抛弃IHttpModule接口及HttpModule实现类，不用再关心HttpApplication的任何事件，还有烦人的web.config配置。直接在代码中以最简洁的方式完成工作。
public void Configure(IApplicationBuilder app, IHostingEnvironment env){app.Use(async(context, next) =>{
await context.Response.WriteAsync("Beginning of Request\n");
await next.Invoke();
await context.Response.WriteAsync("End of Request\n");});app.Run(async (context) =>{ await context.Response.WriteAsync("Hello World!\n");});
}
相似的，对于HTTP Handlers，虽然不用取消对HttpApplication事件的依赖，但以两者的代码实现方式作比较，Middleware亳无疑问胜出一筹。
public class HelloWorldHandler : IHttpHandler{
public HelloWorldHandler() {}
public void ProcessRequest(HttpContext context) {HttpRequest Request = context.Request;HttpResponse Response = context.Response; // This handler is called whenever a file ending // in .sample is requested. A file with that extension// does not need to exist.Response.Write("<html>");Response.Write("<body>");Response.Write("<h1>Hello from a synchronous custom HTTP handler.</h1>");Response.Write("</body>");Response.Write("</html>");}
public bool IsReusable{ // To enable pooling, return true here.// This keeps the handler in memory.get { return false; }}
}仍需要在web.config文件中注册HTTP handler。
<configuration><system.web><httpHandlers><add verb="*" path="*.sample" type="HelloWorldHandler"/></httpHandlers></system.web></configuration>换作Middleware的写法：
private static void HandleSample(IApplicationBuilder app){app.Run(async context =>{ await context.Response.WriteAsync("Hello Sample");});
}
// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IHostingEnvironment env){app.MapWhen(context => context.Request.Path.Value.EndsWith("sample"), HandleSample);
}
总结下使用Middleware的优点：
没有对HttpApplication的依赖
没有对IHttpModule与IHttpHandler接口的依赖
无需在web.config文件中添加各种配置
代码简洁
最后需要补充Middleware与HTTP Modules的一点差异。各Middleware中处理请求与响应的顺序是刚好相反的，越早处理请求的Middleware越晚处理响应。而HTTP Modules中处理请求与响应的顺序则保持一致，因为每个HTTP Module请求与响应事件的绑定都是在同一阶段完成的。
