ASP.NET Core 中间件(Middleware)的使用及其源码解析（一）

中间件是一种装配到应用管道以处理请求和响应的软件。每个组件：

1、选择是否将请求传递到管道中的下一个组件。

2、可在管道中的下一个组件前后执行工作。

请求委托用于生成请求管道。请求委托处理每个 HTTP 请求。

请求管道中的每个中间件组件负责调用管道中的下一个组件，或使管道短路。当中间件短路时，它被称为“终端中间件”，因为它阻止中间件进一步处理请求。

废话不多说，我们直接来看一个Demo，Demo的目录结构如下所示：

本Demo的Web项目为ASP.NET Core Web 应用程序（目标框架为.NET Core 3.1） MVC项目。  

其中 Home 控制器代码如下：

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 类的代码如下：

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; }

        //服务注册（往容器中添加服务）
        // 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请求处理管道
        /// Http请求管道模型---就是Http请求被处理的步骤
        /// 所谓管道，就是拿着HttpContext，经过多个步骤的加工，生成Response，这就是管道
        /// </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 环境参数

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

            #endregion 环境参数

            //静态文件中间件
            app.UseStaticFiles();

            #region Use中间件

            //中间件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===================================");
                    });
                };
            });

            //中间件2
            app.Use(next =>
            {
                Console.WriteLine("middleware 2");
                return async context =>
                {
                    await Task.Run(() => Console.WriteLine($"This is middleware 2 Start"));
                    await next.Invoke(context); //可通过不调用 next 参数使请求管道短路
                    await Task.Run(() => Console.WriteLine($"This is middleware 2 End"));
                };
            });

            //中间件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中间件

            #region 最终把请求交给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 最终把请求交给MVC
        }
    }
}

用 Use 将多个请求委托链接在一起，next 参数表示管道中的下一个委托（下一个中间件）。

下面我们使用命令行（CLI）方式启动我们的网站，如下所示：

可以发现控制台依次输出了“middleware 3” 、“middleware 2”、“middleware 1”，这是怎么回事呢？此处我们先留个疑问，该点在后面的讲解中会再次提到。

启动成功后，我们来访问一下 “/home/index” ，控制台输出结果如下所示：

请求管道包含一系列请求委托，依次调用，下图演示了这一过程：

每个委托均可在下一个委托前后执行操作。应尽早在管道中调用异常处理委托，这样它们就能捕获在管道的后期阶段发生的异常。

此外，可通过不调用 next 参数使请求管道短路，如下所示：

/// <summary>
/// 配置Http请求处理管道
/// Http请求管道模型---就是Http请求被处理的步骤
/// 所谓管道，就是拿着HttpContext，经过多个步骤的加工，生成Response，这就是管道
/// </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 环境参数

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

    #endregion 环境参数

    //静态文件中间件
    app.UseStaticFiles();

    #region Use中间件

    //中间件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===================================");
            });
        };
    });

    //中间件2
    app.Use(next =>
    {
        Console.WriteLine("middleware 2");
        return async context =>
        {
            await Task.Run(() => Console.WriteLine($"This is middleware 2 Start"));
            //await next.Invoke(context); //可通过不调用 next 参数使请求管道短路
            await Task.Run(() => Console.WriteLine($"This is middleware 2 End"));
        };
    });

    //中间件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中间件

    #region 最终把请求交给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 最终把请求交给MVC
}

此处我们注释掉了 中间件2 的 next 参数调用，使请求管道短路。下面我们重新编译后再次访问 “/home/index” ，控制台输出结果如下所示：

当委托不将请求传递给下一个委托时，它被称为“让请求管道短路”。 通常需要短路，因为这样可以避免不必要的工作。 例如，静态文件中间件可以处理对静态文件的请求，并让管道的其余部分短路，从而起到终端中间件的作用。 

对于终端中间件，框架专门为我们提供了一个叫 app.Run(...) 的扩展方法，其实该方法的内部也是调用 app.Use(...) 这个方法的，下面我们来看个示例：

/// <summary>
/// 配置Http请求处理管道
/// Http请求管道模型---就是Http请求被处理的步骤
/// 所谓管道，就是拿着HttpContext，经过多个步骤的加工，生成Response，这就是管道
/// </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 环境参数

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

    #endregion 环境参数

    //静态文件中间件
    app.UseStaticFiles();

    #region Use中间件

    //中间件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===================================");
            });
        };
    });

    //中间件2
    app.Use(next =>
    {
        Console.WriteLine("middleware 2");
        return async context =>
        {
            await Task.Run(() => Console.WriteLine($"This is middleware 2 Start"));
            await next.Invoke(context); //可通过不调用 next 参数使请求管道短路
            await Task.Run(() => Console.WriteLine($"This is middleware 2 End"));
        };
    });

    //中间件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中间件

    #region 终端中间件

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

    #endregion 终端中间件

    #region 最终把请求交给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 最终把请求交给MVC
}

Run 委托不会收到 next 参数。第一个 Run 委托始终为终端，用于终止管道。Run 是一种约定。某些中间件组件可能会公开在管道末尾运行 Run[Middleware] 方法。

我们重新编译后再次访问 “/home/index” ，控制台输出结果如下所示：

此外，app.Use(...) 方法还有另外一个重载，如下所示（中间件4）：

/// <summary>
/// 配置Http请求处理管道
/// Http请求管道模型---就是Http请求被处理的步骤
/// 所谓管道，就是拿着HttpContext，经过多个步骤的加工，生成Response，这就是管道
/// </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 环境参数

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

    #endregion 环境参数

    //静态文件中间件
    app.UseStaticFiles();

    #region Use中间件

    //中间件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===================================");
            });
        };
    });

    //中间件2
    app.Use(next =>
    {
        Console.WriteLine("middleware 2");
        return async context =>
        {
            await Task.Run(() => Console.WriteLine($"This is middleware 2 Start"));
            await next.Invoke(context); //可通过不调用 next 参数使请求管道短路
            await Task.Run(() => Console.WriteLine($"This is middleware 2 End"));
        };
    });

    //中间件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"));
        };
    });

    //中间件4
    //Use方法的另外一个重载
    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中间件

    #region 终端中间件

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

    #endregion 终端中间件

    #region 最终把请求交给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 最终把请求交给MVC
}

我们重新编译后再次访问 “/home/index” ，控制台输出结果如下所示：

下面我们结合ASP.NET Core源码来分析下其实现原理： 

首先我们通过调试来看下 IApplicationBuilder 的实现类到底是啥？如下所示：

可以看出它的实现类是  Microsoft.AspNetCore.Builder.ApplicationBuilder ，我们找到 ApplicationBuilder 类的源码，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 委托的声明，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);
}

仔细阅读后可以发现其实 app.Use(...) 这个方法就只是将 Func<RequestDelegate, RequestDelegate> 类型的委托参数添加到 _components 这个集合中。

最终程序会调用 ApplicationBuilder 类的 Build() 方法去构建Http请求处理管道，接下来我们就重点来关注一下这个 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;
}

仔细观察上面的源码后我们可以发现： 

1、首先它是将 _components 这个集合反转（即：_components.Reverse()），然后依次调用里面的中间件（Func<RequestDelegate, RequestDelegate>委托），这也就解释了为什么网站启动时我们的控制台会依次输出 “middleware 3” 、“middleware 2”、“middleware 1” 的原因。 

2、调用反转后的第一个中间件（即：注册的最后一个中间件）时传入的参数是状态码为404的 RequestDelegate 委托，作为默认处理步骤。

3、在调用反转后的中间件时，它是用第一个中间件的返回值作为调用第二个中间件的参数，用第二个中间件的返回值作为调用第三个中间件的参数，依次类推。这也就是为什么说注册时的那个 next 参数是指向注册时下一个中间件的原因。 

4、Build() 方法最终返回的是调用反转后最后一个中间件（即：注册的第一个中间件）的返回值。

下面我们来看一下Use方法的另外一个重载，如下所示：

//中间件4
//Use方法的另外一个重载
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"));
});

我们找到它的源码，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);
                };
            });
        }
    }
}

可以发现其实它是个扩展方法，主要就是对 app.Use(...) 这个方法包装了一下，最终调用的还是 app.Use(...) 这个方法。

最后我们来看一下 app.Run(...) 这个扩展方法，如下所示：

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

我们找到它的源码，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);
        }
    }
}

可以发现，其实 app.Run(...) 这个扩展方法最终也是调用 app.Use(...) 这个方法，只不过它直接丢弃了 next 参数，故调用这个方法会终止管道，它属于终端中间件。

更多关于ASP.NET Core 中间件的相关知识可参考微软官方文档： https://docs.microsoft.com/zh-cn/aspnet/core/fundamentals/middleware/?view=aspnetcore-6.0

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