Run 方法

Run 方法源码：

/// <summary>
/// Adds a terminal middleware delegate to the application's request pipeline.
/// </summary>
/// <param name="app">The <see cref="T:Microsoft.AspNetCore.Builder.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((Func<RequestDelegate, RequestDelegate>) (_ => handler));
}

不难看出 Run 方法就是专门用来添加 terminal middleware（终端中间件）的。

一个管道中通常只有一个终端中间件。

Use 方法

Use 源码：

/// <summary>
/// Adds a middleware delegate defined in-line to the application's request pipeline.
/// </summary>
/// <param name="app">The <see cref="T:Microsoft.AspNetCore.Builder.IApplicationBuilder" /> instance.</param>
/// <param name="middleware">A function that handles the request or calls the given next function.</param>
/// <returns>The <see cref="T:Microsoft.AspNetCore.Builder.IApplicationBuilder" /> instance.</returns>
public static IApplicationBuilder Use(
  this IApplicationBuilder app,
  Func<HttpContext, Func<Task>, Task> middleware)
{
  return app.Use((Func<RequestDelegate, RequestDelegate>) (next => (RequestDelegate) (context =>
  {
    Func<Task> func = (Func<Task>) (() => next(context));
    return middleware(context, func);
  })));
}

Use 用于在管道中添加 in-line（内联定义的）中间件。

请求管道

// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILogger<Startup> logger)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    app.Use(async (context, next) =>
    {
        context.Response.ContentType = "text/plain;charset=utf-8";
        logger.LogInformation("MW1: 传入请求");
        await next();
        logger.LogInformation("MW1: 传出响应");
    });
    app.Use(async (context, next) =>
    {
        logger.LogInformation("MW2: 传入请求");
        await next();
        logger.LogInformation("MW2: 传出响应");
    });
    app.Run(async (context) =>
    {
        logger.LogInformation("MW3: 传入请求");
        await context.Response.WriteAsync("MW3：处理请求，并生成响应");
        logger.LogInformation("MW3: 传出响应");
    });
}

输出：

Microsoft.AspNetCore.Hosting.Internal.WebHost:Information: Request starting HTTP/1.1 GET http://localhost:3290/  
StudentManagement.Startup:Information: MW1: 传入请求
StudentManagement.Startup:Information: MW2: 传入请求
StudentManagement.Startup:Information: MW3: 传入请求
StudentManagement.Startup:Information: MW3: 传出响应
StudentManagement.Startup:Information: MW2: 传出响应
StudentManagement.Startup:Information: MW1: 传出响应
Microsoft.AspNetCore.Hosting.Internal.WebHost:Information: Request finished in 29.7525ms 200 text/plain;charset=utf-8

• 所有的请求处理都会在每个中间件组件调用 next() 方法之前触发。请求按照途中箭头所示，依次穿过所有管道
• 当（终端）中间件处理请求并产生响应时，请求流程会在管道开始反向传递