熟悉ASP.NET架構的開發者一定對於HTTP Modules與HTTP Handlers不陌生。兩者的作用主要是對網絡請求執行特定的處理工作。而在.NET Core中，它們都被Middleware(中件間)取代了。
之前的Http Modules和HTTP Handlers是如下圖般處理請求的：
現在變成了這樣：
一言概括之，Middleware完成了HTTP Modules與HTTP Handlers的原有工作，但又不是簡單的化二為一的減法作用。
Middleware減去的其實是與原來ASP.NET中重要的基礎——應用程序生命周期事件(application life cycle event)的綁定。
HTTP Modules在初始化時就需要針對HttpApplication的事件作綁定處理，這樣當HttpApplication的各項事件被觸發時，已綁定的相應處理程序才會按照預期的那樣被執行。
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()
{
}
}
然后你還需要在web.config配置文件注冊這個HTTP Module。
<configuration>
<system.web>
<httpModules>
<add name="HelloWorldModule" type="HelloWorldModule"/>
</httpModules>
</system.web>
</configuration>
如果是用Middleware的話，事情就變得很簡單了。拋棄IHttpModule接口及HttpModule實現類，不用再關心HttpApplication的任何事件，還有煩人的web.config配置。直接在代碼中以最簡潔的方式完成工作。
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，對於HTTP Handlers，雖然不用取消對HttpApplication事件的依賴，但以兩者的代碼實現方式作比較，Middleware亳無疑問勝出一籌。
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文件中注冊HTTP handler。
<configuration>
<system.web>
<httpHandlers>
<add verb="*" path="*.sample"
type="HelloWorldHandler"/>
</httpHandlers>
</system.web>
</configuration>
換作Middleware的寫法：
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);
}
總結下使用Middleware的優點：
- 沒有對HttpApplication的依賴
- 沒有對IHttpModule與IHttpHandler接口的依賴
- 無需在web.config文件中添加各種配置
- 代碼簡潔
最后需要補充Middleware與HTTP Modules的一點差異。各Middleware中處理請求與響應的順序是剛好相反的，越早處理請求的Middleware越晚處理響應。而HTTP Modules中處理請求與響應的順序則保持一致，因為每個HTTP Module請求與響應事件的綁定都是在同一階段完成的。