.NET Core開發日誌——Middleware

時間 2019-12-01

標籤 core 開發日誌 middleware 简体版

原文原文鏈接

熟悉ASP.NET架構的開發者必定對於HTTP Modules與HTTP Handlers不陌生。二者的做用主要是對網絡請求執行特定的處理工做。而在.NET Core中，它們都被Middleware(中件間)取代了。html

以前的Http Modules和HTTP Handlers是以下圖般處理請求的：web

如今變成了這樣：網絡

一言歸納之，Middleware完成了HTTP Modules與HTTP Handlers的原有工做，但又不是簡單的化二爲一的減法做用。架構

Middleware減去的實際上是與原來ASP.NET中重要的基礎——應用程序生命週期事件(application life cycle event)的綁定。app

HTTP Modules在初始化時就須要針對HttpApplication的事件做綁定處理，這樣當HttpApplication的各項事件被觸發時，已綁定的相應處理程序纔會按照預期的那樣被執行。async

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()
    {
    }
}

而後你還須要在web.config配置文件註冊這個HTTP Module。ui

<configuration>
    <system.web>
        <httpModules>
           <add name="HelloWorldModule" type="HelloWorldModule"/>
        </httpModules>
    </system.web>
</configuration>

若是是用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做。this

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");
    });
}

類似的，對於HTTP Handlers，雖然不用取消對HttpApplication事件的依賴，但以二者的代碼實現方式做比較，Middleware亳無疑問勝出一籌。3d

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; }
    }
}

仍須要在web.config文件中註冊HTTP handler。code

<configuration>
    <system.web>
        <httpHandlers>
            <add verb="*" path="*.sample" 
                  type="HelloWorldHandler"/>
        </httpHandlers>
    </system.web>
</configuration>

換做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的優勢：

沒有對HttpApplication的依賴
沒有對IHttpModule與IHttpHandler接口的依賴
無需在web.config文件中添加各類配置
代碼簡潔

最後須要補充Middleware與HTTP Modules的一點差別。各Middleware中處理請求與響應的順序是恰好相反的，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，由於每一個HTTP Module請求與響應事件的綁定都是在同一階段完成的。

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