ASP.NET Core 中間件(Middleware)的使用及其源碼解析（一）

本文轉載自查看原文 2022-04-17 20:30 1044 AspNetCore源碼解析/ 中間件/ Middleware/ .NET Core

中間件是一種裝配到應用管道以處理請求和響應的軟件。每個組件：

1、選擇是否將請求傳遞到管道中的下一個組件。

2、可在管道中的下一個組件前后執行工作。

請求委托用於生成請求管道。請求委托處理每個 HTTP 請求。

請求管道中的每個中間件組件負責調用管道中的下一個組件，或使管道短路。當中間件短路時，它被稱為“終端中間件”，因為它阻止中間件進一步處理請求。

廢話不多說，我們直接來看一個Demo，Demo的目錄結構如下所示：

本Demo的Web項目為ASP.NET Core Web 應用程序（目標框架為.NET Core 3.1） MVC項目。

其中 Home 控制器代碼如下：

using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Logging;

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace NETCoreMiddleware.Controllers
{
    public class HomeController : Controller
    {
        private readonly ILogger<HomeController> _logger;

        public HomeController(ILogger<HomeController> logger)
        {
            _logger = logger;
        }

        public IActionResult Index()
        {
            Console.WriteLine("");
            Console.WriteLine($"This is {typeof(HomeController)} Index");
            Console.WriteLine("");
            return View();
        }
    }
}

其中 Startup.cs 類的代碼如下：

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace NETCoreMiddleware
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        //服務注冊（往容器中添加服務）
        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllersWithViews();
        }

        /// <summary>
        /// 配置Http請求處理管道
        /// Http請求管道模型---就是Http請求被處理的步驟
        /// 所謂管道，就是拿着HttpContext，經過多個步驟的加工，生成Response，這就是管道
        /// </summary>
        /// <param name="app"></param>
        /// <param name="env"></param>
        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            #region 環境參數

            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
            }

            #endregion 環境參數

            //靜態文件中間件
            app.UseStaticFiles();

            #region Use中間件

            //中間件1
            app.Use(next =>
            {
                Console.WriteLine("middleware 1");
                return async context =>
                {
                    await Task.Run(() =>
                    {
                        Console.WriteLine("");
                        Console.WriteLine("===================================Middleware===================================");
                        Console.WriteLine($"This is middleware 1 Start");
                    });
                    await next.Invoke(context);
                    await Task.Run(() =>
                    {
                        Console.WriteLine($"This is middleware 1 End");
                        Console.WriteLine("===================================Middleware===================================");
                    });
                };
            });

            //中間件2
            app.Use(next =>
            {
                Console.WriteLine("middleware 2");
                return async context =>
                {
                    await Task.Run(() => Console.WriteLine($"This is middleware 2 Start"));
                    await next.Invoke(context); //可通過不調用 next 參數使請求管道短路
                    await Task.Run(() => Console.WriteLine($"This is middleware 2 End"));
                };
            });

            //中間件3
            app.Use(next =>
            {
                Console.WriteLine("middleware 3");
                return async context =>
                {
                    await Task.Run(() => Console.WriteLine($"This is middleware 3 Start"));
                    await next.Invoke(context);
                    await Task.Run(() => Console.WriteLine($"This is middleware 3 End"));
                };
            });

            #endregion Use中間件

            #region 最終把請求交給MVC

            app.UseRouting();
            app.UseAuthorization();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "areas",
                    pattern: "{area:exists}/{controller=Home}/{action=Index}/{id?}");

                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller=Home}/{action=Index}/{id?}");
            });

            #endregion 最終把請求交給MVC
        }
    }
}

用 Use 將多個請求委托鏈接在一起，next 參數表示管道中的下一個委托（下一個中間件）。

下面我們使用命令行（CLI）方式啟動我們的網站，如下所示：

可以發現控制台依次輸出了“middleware 3” 、“middleware 2”、“middleware 1”，這是怎么回事呢？此處我們先留個疑問，該點在后面的講解中會再次提到。

啟動成功后，我們來訪問一下 “/home/index” ，控制台輸出結果如下所示：

請求管道包含一系列請求委托，依次調用，下圖演示了這一過程：

每個委托均可在下一個委托前后執行操作。應盡早在管道中調用異常處理委托，這樣它們就能捕獲在管道的后期階段發生的異常。

此外，可通過不調用 next 參數使請求管道短路，如下所示：

/// <summary>
/// 配置Http請求處理管道
/// Http請求管道模型---就是Http請求被處理的步驟
/// 所謂管道，就是拿着HttpContext，經過多個步驟的加工，生成Response，這就是管道
/// </summary>
/// <param name="app"></param>
/// <param name="env"></param>
// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    #region 環境參數

    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
    }

    #endregion 環境參數

    //靜態文件中間件
    app.UseStaticFiles();

    #region Use中間件

    //中間件1
    app.Use(next =>
    {
        Console.WriteLine("middleware 1");
        return async context =>
        {
            await Task.Run(() =>
            {
                Console.WriteLine("");
                Console.WriteLine("===================================Middleware===================================");
                Console.WriteLine($"This is middleware 1 Start");
            });
            await next.Invoke(context);
            await Task.Run(() =>
            {
                Console.WriteLine($"This is middleware 1 End");
                Console.WriteLine("===================================Middleware===================================");
            });
        };
    });

    //中間件2
    app.Use(next =>
    {
        Console.WriteLine("middleware 2");
        return async context =>
        {
            await Task.Run(() => Console.WriteLine($"This is middleware 2 Start"));
            //await next.Invoke(context); //可通過不調用 next 參數使請求管道短路
            await Task.Run(() => Console.WriteLine($"This is middleware 2 End"));
        };
    });

    //中間件3
    app.Use(next =>
    {
        Console.WriteLine("middleware 3");
        return async context =>
        {
            await Task.Run(() => Console.WriteLine($"This is middleware 3 Start"));
            await next.Invoke(context);
            await Task.Run(() => Console.WriteLine($"This is middleware 3 End"));
        };
    });

    #endregion Use中間件

    #region 最終把請求交給MVC

    app.UseRouting();
    app.UseAuthorization();
    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllerRoute(
            name: "areas",
            pattern: "{area:exists}/{controller=Home}/{action=Index}/{id?}");

        endpoints.MapControllerRoute(
            name: "default",
            pattern: "{controller=Home}/{action=Index}/{id?}");
    });

    #endregion 最終把請求交給MVC
}

此處我們注釋掉了中間件2 的 next 參數調用，使請求管道短路。下面我們重新編譯后再次訪問 “/home/index” ，控制台輸出結果如下所示：

當委托不將請求傳遞給下一個委托時，它被稱為“讓請求管道短路”。通常需要短路，因為這樣可以避免不必要的工作。例如，靜態文件中間件可以處理對靜態文件的請求，並讓管道的其余部分短路，從而起到終端中間件的作用。

對於終端中間件，框架專門為我們提供了一個叫 app.Run(...) 的擴展方法，其實該方法的內部也是調用 app.Use(...) 這個方法的，下面我們來看個示例：

/// <summary>
/// 配置Http請求處理管道
/// Http請求管道模型---就是Http請求被處理的步驟
/// 所謂管道，就是拿着HttpContext，經過多個步驟的加工，生成Response，這就是管道
/// </summary>
/// <param name="app"></param>
/// <param name="env"></param>
// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    #region 環境參數

    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
    }

    #endregion 環境參數

    //靜態文件中間件
    app.UseStaticFiles();

    #region Use中間件

    //中間件1
    app.Use(next =>
    {
        Console.WriteLine("middleware 1");
        return async context =>
        {
            await Task.Run(() =>
            {
                Console.WriteLine("");
                Console.WriteLine("===================================Middleware===================================");
                Console.WriteLine($"This is middleware 1 Start");
            });
            await next.Invoke(context);
            await Task.Run(() =>
            {
                Console.WriteLine($"This is middleware 1 End");
                Console.WriteLine("===================================Middleware===================================");
            });
        };
    });

    //中間件2
    app.Use(next =>
    {
        Console.WriteLine("middleware 2");
        return async context =>
        {
            await Task.Run(() => Console.WriteLine($"This is middleware 2 Start"));
            await next.Invoke(context); //可通過不調用 next 參數使請求管道短路
            await Task.Run(() => Console.WriteLine($"This is middleware 2 End"));
        };
    });

    //中間件3
    app.Use(next =>
    {
        Console.WriteLine("middleware 3");
        return async context =>
        {
            await Task.Run(() => Console.WriteLine($"This is middleware 3 Start"));
            await next.Invoke(context);
            await Task.Run(() => Console.WriteLine($"This is middleware 3 End"));
        };
    });

    #endregion Use中間件

    #region 終端中間件

    //app.Use(_ => handler);
    app.Run(async context =>
    {
        await Task.Run(() => Console.WriteLine($"This is Run"));
    });

    #endregion 終端中間件

    #region 最終把請求交給MVC

    app.UseRouting();
    app.UseAuthorization();
    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllerRoute(
            name: "areas",
            pattern: "{area:exists}/{controller=Home}/{action=Index}/{id?}");

        endpoints.MapControllerRoute(
            name: "default",
            pattern: "{controller=Home}/{action=Index}/{id?}");
    });

    #endregion 最終把請求交給MVC
}

Run 委托不會收到 next 參數。第一個 Run 委托始終為終端，用於終止管道。Run 是一種約定。某些中間件組件可能會公開在管道末尾運行 Run[Middleware] 方法。

我們重新編譯后再次訪問 “/home/index” ，控制台輸出結果如下所示：

此外，app.Use(...) 方法還有另外一個重載，如下所示（中間件4）：

/// <summary>
/// 配置Http請求處理管道
/// Http請求管道模型---就是Http請求被處理的步驟
/// 所謂管道，就是拿着HttpContext，經過多個步驟的加工，生成Response，這就是管道
/// </summary>
/// <param name="app"></param>
/// <param name="env"></param>
// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    #region 環境參數

    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
    }

    #endregion 環境參數

    //靜態文件中間件
    app.UseStaticFiles();

    #region Use中間件

    //中間件1
    app.Use(next =>
    {
        Console.WriteLine("middleware 1");
        return async context =>
        {
            await Task.Run(() =>
            {
                Console.WriteLine("");
                Console.WriteLine("===================================Middleware===================================");
                Console.WriteLine($"This is middleware 1 Start");
            });
            await next.Invoke(context);
            await Task.Run(() =>
            {
                Console.WriteLine($"This is middleware 1 End");
                Console.WriteLine("===================================Middleware===================================");
            });
        };
    });

    //中間件2
    app.Use(next =>
    {
        Console.WriteLine("middleware 2");
        return async context =>
        {
            await Task.Run(() => Console.WriteLine($"This is middleware 2 Start"));
            await next.Invoke(context); //可通過不調用 next 參數使請求管道短路
            await Task.Run(() => Console.WriteLine($"This is middleware 2 End"));
        };
    });

    //中間件3
    app.Use(next =>
    {
        Console.WriteLine("middleware 3");
        return async context =>
        {
            await Task.Run(() => Console.WriteLine($"This is middleware 3 Start"));
            await next.Invoke(context);
            await Task.Run(() => Console.WriteLine($"This is middleware 3 End"));
        };
    });

    //中間件4
    //Use方法的另外一個重載
    app.Use(async (context, next) =>
    {
        await Task.Run(() => Console.WriteLine($"This is middleware 4 Start"));
        await next();
        await Task.Run(() => Console.WriteLine($"This is middleware 4 End"));
    });

    #endregion Use中間件

    #region 終端中間件

    //app.Use(_ => handler);
    app.Run(async context =>
    {
        await Task.Run(() => Console.WriteLine($"This is Run"));
    });

    #endregion 終端中間件

    #region 最終把請求交給MVC

    app.UseRouting();
    app.UseAuthorization();
    app.UseEndpoints(endpoints =>
    {
        endpoints.MapControllerRoute(
            name: "areas",
            pattern: "{area:exists}/{controller=Home}/{action=Index}/{id?}");

        endpoints.MapControllerRoute(
            name: "default",
            pattern: "{controller=Home}/{action=Index}/{id?}");
    });

    #endregion 最終把請求交給MVC
}

我們重新編譯后再次訪問 “/home/index” ，控制台輸出結果如下所示：

下面我們結合ASP.NET Core源碼來分析下其實現原理：

首先我們通過調試來看下 IApplicationBuilder 的實現類到底是啥？如下所示：

可以看出它的實現類是 Microsoft.AspNetCore.Builder.ApplicationBuilder ，我們找到 ApplicationBuilder 類的源碼，如下所示：

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Http.Features;
using Microsoft.Extensions.Internal;

namespace Microsoft.AspNetCore.Builder
{
    public class ApplicationBuilder : IApplicationBuilder
    {
        private const string ServerFeaturesKey = "server.Features";
        private const string ApplicationServicesKey = "application.Services";

        private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>();

        public ApplicationBuilder(IServiceProvider serviceProvider)
        {
            Properties = new Dictionary<string, object>(StringComparer.Ordinal);
            ApplicationServices = serviceProvider;
        }

        public ApplicationBuilder(IServiceProvider serviceProvider, object server)
            : this(serviceProvider)
        {
            SetProperty(ServerFeaturesKey, server);
        }

        private ApplicationBuilder(ApplicationBuilder builder)
        {
            Properties = new CopyOnWriteDictionary<string, object>(builder.Properties, StringComparer.Ordinal);
        }

        public IServiceProvider ApplicationServices
        {
            get
            {
                return GetProperty<IServiceProvider>(ApplicationServicesKey);
            }
            set
            {
                SetProperty<IServiceProvider>(ApplicationServicesKey, value);
            }
        }

        public IFeatureCollection ServerFeatures
        {
            get
            {
                return GetProperty<IFeatureCollection>(ServerFeaturesKey);
            }
        }

        public IDictionary<string, object> Properties { get; }

        private T GetProperty<T>(string key)
        {
            object value;
            return Properties.TryGetValue(key, out value) ? (T)value : default(T);
        }

        private void SetProperty<T>(string key, T value)
        {
            Properties[key] = value;
        }

        public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware)
        {
            _components.Add(middleware); return this;
        }

        public IApplicationBuilder New()
        {
            return new ApplicationBuilder(this);
        }

        public RequestDelegate Build()
        {
            RequestDelegate app = context =>
            {
                // If we reach the end of the pipeline, but we have an endpoint, then something unexpected has happened.
                // This could happen if user code sets an endpoint, but they forgot to add the UseEndpoint middleware.
                var endpoint = context.GetEndpoint();
                var endpointRequestDelegate = endpoint?.RequestDelegate;
                if (endpointRequestDelegate != null)
                {
                    var message =
                        $"The request reached the end of the pipeline without executing the endpoint: '{endpoint.DisplayName}'. " +
                        $"Please register the EndpointMiddleware using '{nameof(IApplicationBuilder)}.UseEndpoints(...)' if using " +
                        $"routing.";
                    throw new InvalidOperationException(message);
                }

                context.Response.StatusCode = 404;
                return Task.CompletedTask;
            };

            foreach (var component in _components.Reverse())
            {
                app = component(app);
            }

            return app;
        }
    }
}

其中 RequestDelegate 委托的聲明，如下：

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System.Threading.Tasks;

namespace Microsoft.AspNetCore.Http
{
    /// <summary>
    /// A function that can process an HTTP request.
    /// </summary>
    /// <param name="context">The <see cref="HttpContext"/> for the request.</param>
    /// <returns>A task that represents the completion of request processing.</returns>
    public delegate Task RequestDelegate(HttpContext context);
}

仔細閱讀后可以發現其實 app.Use(...) 這個方法就只是將 Func<RequestDelegate, RequestDelegate> 類型的委托參數添加到 _components 這個集合中。

最終程序會調用 ApplicationBuilder 類的 Build() 方法去構建Http請求處理管道，接下來我們就重點來關注一下這個 Build() 方法，如下：

public RequestDelegate Build()
{
    RequestDelegate app = context =>
    {
        // If we reach the end of the pipeline, but we have an endpoint, then something unexpected has happened.
        // This could happen if user code sets an endpoint, but they forgot to add the UseEndpoint middleware.
        var endpoint = context.GetEndpoint();
        var endpointRequestDelegate = endpoint?.RequestDelegate;
        if (endpointRequestDelegate != null)
        {
            var message =
                $"The request reached the end of the pipeline without executing the endpoint: '{endpoint.DisplayName}'. " +
                $"Please register the EndpointMiddleware using '{nameof(IApplicationBuilder)}.UseEndpoints(...)' if using " +
                $"routing.";
            throw new InvalidOperationException(message);
        }

        context.Response.StatusCode = 404;
        return Task.CompletedTask;
    };

    foreach (var component in _components.Reverse())
    {
        app = component(app);
    }

    return app;
}

仔細觀察上面的源碼后我們可以發現：

1、首先它是將 _components 這個集合反轉（即：_components.Reverse()），然后依次調用里面的中間件（Func<RequestDelegate, RequestDelegate>委托），這也就解釋了為什么網站啟動時我們的控制台會依次輸出 “middleware 3” 、“middleware 2”、“middleware 1” 的原因。

2、調用反轉后的第一個中間件（即：注冊的最后一個中間件）時傳入的參數是狀態碼為404的 RequestDelegate 委托，作為默認處理步驟。

3、在調用反轉后的中間件時，它是用第一個中間件的返回值作為調用第二個中間件的參數，用第二個中間件的返回值作為調用第三個中間件的參數，依次類推。這也就是為什么說注冊時的那個 next 參數是指向注冊時下一個中間件的原因。

4、Build() 方法最終返回的是調用反轉后最后一個中間件（即：注冊的第一個中間件）的返回值。

下面我們來看一下Use方法的另外一個重載，如下所示：

//中間件4
//Use方法的另外一個重載
app.Use(async (context, next) =>
{
    await Task.Run(() => Console.WriteLine($"This is middleware 4 Start"));
    await next();
    await Task.Run(() => Console.WriteLine($"This is middleware 4 End"));
});

我們找到它的源碼，如下：

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;

namespace Microsoft.AspNetCore.Builder
{
    /// <summary>
    /// Extension methods for adding middleware.
    /// </summary>
    public static class UseExtensions
    {
        /// <summary>
        /// Adds a middleware delegate defined in-line to the application's request pipeline.
        /// </summary>
        /// <param name="app">The <see cref="IApplicationBuilder"/> instance.</param>
        /// <param name="middleware">A function that handles the request or calls the given next function.</param>
        /// <returns>The <see cref="IApplicationBuilder"/> instance.</returns>
        public static IApplicationBuilder Use(this IApplicationBuilder app, Func<HttpContext, Func<Task>, Task> middleware)
        {
            return app.Use(next =>
            {
                return context =>
                {
                    Func<Task> simpleNext = () => next(context);
                    return middleware(context, simpleNext);
                };
            });
        }
    }
}

可以發現其實它是個擴展方法，主要就是對 app.Use(...) 這個方法包裝了一下，最終調用的還是 app.Use(...) 這個方法。

最后我們來看一下 app.Run(...) 這個擴展方法，如下所示：

//app.Use(_ => handler);
app.Run(async context =>
{
    await Task.Run(() => Console.WriteLine($"This is Run"));
});

我們找到它的源碼，如下：

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using Microsoft.AspNetCore.Http;

namespace Microsoft.AspNetCore.Builder
{
    /// <summary>
    /// Extension methods for adding terminal middleware.
    /// </summary>
    public static class RunExtensions
    {
        /// <summary>
        /// Adds a terminal middleware delegate to the application's request pipeline.
        /// </summary>
        /// <param name="app">The <see cref="IApplicationBuilder"/> instance.</param>
        /// <param name="handler">A delegate that handles the request.</param>
        public static void Run(this IApplicationBuilder app, RequestDelegate handler)
        {
            if (app == null)
            {
                throw new ArgumentNullException(nameof(app));
            }

            if (handler == null)
            {
                throw new ArgumentNullException(nameof(handler));
            }

            app.Use(_ => handler);
        }
    }
}

可以發現，其實 app.Run(...) 這個擴展方法最終也是調用 app.Use(...) 這個方法，只不過它直接丟棄了 next 參數，故調用這個方法會終止管道，它屬於終端中間件。

更多關於ASP.NET Core 中間件的相關知識可參考微軟官方文檔： https://docs.microsoft.com/zh-cn/aspnet/core/fundamentals/middleware/?view=aspnetcore-6.0

至此本文就全部介紹完了，如果覺得對您有所啟發請記得點個贊哦！！！

Demo源碼：

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提取碼：7rt1

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