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("
HelloWorldModule: Beginning of Request
");
}
private void Application_EndRequest(Object source, EventArgs e)
{
HttpApplication application = (HttpApplication)source;
HttpContext context = application.Context;
context.Response.Write("
HelloWorldModule: End of Request
");
}
public void Dispose()
{
}
}然后你还需要在web.config配置文件注册这个HTTP Module。
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("");
Response.Write("");
Response.Write("Hello from a synchronous custom HTTP handler.
");
Response.Write("");
Response.Write("");
}
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。
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。