原文:循序渐进学.Net Core Web Api开发系列【13】：中间件（Middleware）
系列目录
本系列涉及到的源码下载地址：https://github.com/seabluescn/Blog_WebApi
一、概述
本篇介绍如何使用中间件（Middleware）。
二、初步演练
先写几个中间件
public class DemoAMiddleware { private readonly RequestDelegate _next; private readonly ILogger _logger;public class DemoBMiddleware</span><span style="color: #0000ff;">public</span> DemoAMiddleware(RequestDelegate next, ILogger<DemoAMiddleware><span style="color: #000000;"> logger) { _next </span>=<span style="color: #000000;"> next; _logger </span>=<span style="color: #000000;"> logger; } </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">async</span><span style="color: #000000;"> Task Invoke(HttpContext context) { _logger.LogInformation(</span><span style="color: #800000;">"</span><span style="color: #800000;">(1) DemoAMiddleware.Invoke front</span><span style="color: #800000;">"</span><span style="color: #000000;">); </span><span style="color: #0000ff;">await</span><span style="color: #000000;"> _next(context); _logger.LogInformation(</span><span style="color: #800000;">"</span><span style="color: #800000;">[1] DemoAMiddleware:Invoke back</span><span style="color: #800000;">"</span><span style="color: #000000;">); } </span><span style="color: #000000;"> }
{
private readonly RequestDelegate _next;
private readonly ILogger _logger;</span><span style="color: #0000ff;">public</span> DemoBMiddleware(RequestDelegate next, ILogger<DemoBMiddleware><span style="color: #000000;"> logger) { _next </span>=<span style="color: #000000;"> next; _logger </span>=<span style="color: #000000;"> logger; } </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">async</span><span style="color: #000000;"> Task Invoke(HttpContext context) { _logger.LogInformation(</span><span style="color: #800000;">"</span><span style="color: #800000;">(2) DemoBMiddleware.Invoke part1</span><span style="color: #800000;">"</span><span style="color: #000000;">); </span><span style="color: #0000ff;">await</span><span style="color: #000000;"> _next(context); _logger.LogInformation(</span><span style="color: #800000;">"</span><span style="color: #800000;">[2] DemoBMiddleware:Invoke part2</span><span style="color: #800000;">"</span><span style="color: #000000;">); } </span><span style="color: #000000;"> } </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">class</span><span style="color: #000000;"> RequestRecordMiddleware { </span><span style="color: #0000ff;">private</span> <span style="color: #0000ff;">readonly</span><span style="color: #000000;"> RequestDelegate _next; </span><span style="color: #0000ff;">private</span> <span style="color: #0000ff;">readonly</span><span style="color: #000000;"> ILogger _logger; </span><span style="color: #0000ff;">public</span> RequestRecordMiddleware(RequestDelegate next, ILogger<RequestRecordMiddleware><span style="color: #000000;"> logger) { _next </span>=<span style="color: #000000;"> next; _logger </span>=<span style="color: #000000;"> logger; } </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">async</span><span style="color: #000000;"> Task Invoke(HttpContext context) { _logger.LogInformation(</span><span style="color: #800000;">"</span><span style="color: #800000;">(3) RequestRecordMiddleware.Invoke</span><span style="color: #800000;">"</span><span style="color: #000000;">); String URL </span>=<span style="color: #000000;"> context.Request.Path.ToString(); _logger.LogInformation($</span><span style="color: #800000;">"</span><span style="color: #800000;">URL : {URL}</span><span style="color: #800000;">"</span><span style="color: #000000;">); </span><span style="color: #0000ff;">await</span><span style="color: #000000;"> _next(context); _logger.LogInformation(</span><span style="color: #800000;">"</span><span style="color: #800000;">[3] RequestRecordMiddleware:Invoke next</span><span style="color: #800000;">"</span><span style="color: #000000;">); _logger.LogInformation($</span><span style="color: #800000;">"</span><span style="color: #800000;">StatusCode : {context.Response.StatusCode}</span><span style="color: #800000;">"</span><span style="color: #000000;">); } </span><span style="color: #000000;"> }</span></pre>
以上中间件前两个没有做什么正经工作，就打印一些日志信息，第三个干了一点工作，它打印了用户输入的url，同时打印了返回给客户端的状态码。
要使中间件工作，需要启用中间件。
public class Startup { public Startup(IConfiguration configuration) { Configuration = configuration; }</span><span style="color: #0000ff;">public</span> IConfiguration Configuration { <span style="color: #0000ff;">get</span><span style="color: #000000;">; } </span><span style="color: #008000;">//</span><span style="color: #008000;"> This method gets called by the runtime. Use this method to add services to the container.</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> ConfigureServices(IServiceCollection services) { services.AddMvc();</span><span style="color: #000000;"> } </span><span style="color: #008000;">//</span><span style="color: #008000;"> This method gets called by the runtime. Use this method to configure the HTTP request pipeline.</span> <span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory) { app.UseUnifyException(); <span style="color: #ff00ff;">app.UseMiddleware</span></span><span style="color: #ff00ff;"><DemoAMiddleware>(); app.UseMiddleware<DemoBMiddleware>(); app.UseMiddleware<RequestRecordMiddleware></span><span style="color: #000000;"><span style="color: #ff00ff;">(); </span> app.UseStaticFiles(); app.UseMvcWithDefaultRoute(); }</span></pre>
通过扩展方法，我们对中间件的启用代码进行改造：
public static class RequestRecordMiddlewareExtensions { public static IApplicationBuilder UseRequestRecord(this IApplicationBuilder builder) { if (builder == null) { throw new ArgumentNullException("builder is null"); }</span><span style="color: #0000ff;">return</span> builder.UseMiddleware<RequestRecordMiddleware><span style="color: #000000;">(); } }</span></pre>
此时启用代码由：app.UseMiddleware<RequestRecordMiddleware>();
可以修改为： app.UseRequestRecord();
实现效果没有变化。可见下面代码都是中间件的使用。
1 2 | app.UseStaticFiles();app.UseMvcWithDefaultRoute(); |
我的理解，中间件类似车间流水线上的工人，操作的零件就是HttpContext，每个人负责两道工序，我把它们定义为“前道工序”和“后道工序”，通过代码 _next(context); 把两道工序隔离开，处理的顺序需要特别注意，按照中间件注册的顺序依次处理“前道工序”，处理完成后，再按相反的顺序处理“后道工序”，如果某个中间件没有调用_next(context)，那么就不会调用后续的中间件，所以中间件启用的顺序是需要特别考虑的。
以上代码中三个中间件输出到控制台的信息顺序如下：
（1）
（2）
（3）
【3】
【2】
【1】
个人认为，“前道工序”应重点处理Request，“后道工序”应重点处理Response。
三、做一个类似MVC的中间件
我们做一个中间件，让其返回一些内容给客户端：
public class MyMvcMiddleware { private readonly RequestDelegate _next; private readonly ILogger _logger;</span><span style="color: #0000ff;">public</span> MyMvcMiddleware(RequestDelegate next, ILogger<DemoAMiddleware><span style="color: #000000;"> logger) { _next </span>=<span style="color: #000000;"> next; _logger </span>=<span style="color: #000000;"> logger; } </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">async</span><span style="color: #000000;"> Task Invoke(HttpContext context) { </span><span style="color: #0000ff;">var</span> str = <span style="color: #800000;">"</span><span style="color: #800000;">hello,world!</span><span style="color: #800000;">"</span><span style="color: #000000;">; </span><span style="color: #0000ff;">await</span><span style="color: #000000;"> context.Response.WriteAsync(str); } }</span></pre>
这个中间件只是返回固定的字符串，我们还可以调用某个Controller的提供的方法。
public class MyMvcMiddleware { private readonly RequestDelegate _next; private readonly ILogger _logger;</span><span style="color: #0000ff;">public</span> MyMvcMiddleware(RequestDelegate next, ILogger<DemoAMiddleware><span style="color: #000000;"> logger) { _next </span>=<span style="color: #000000;"> next; _logger </span>=<span style="color: #000000;"> logger; } </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">async</span><span style="color: #000000;"> Task Invoke(HttpContext context) {<br></span><span style="color: #0000ff;"> var</span> obj = context.RequestServices.GetRequiredService<ArticleController><span style="color: #000000;">().GetArticleList(); </span><span style="color: #0000ff;">var</span> str =<span style="color: #000000;"> JsonConvert.SerializeObject(obj); </span><span style="color: #0000ff;">await</span><span style="color: #000000;"> context.Response.WriteAsync(str); } }</span></pre>
ArticleController的定义如下：
public class ArticleController : Controller { private readonly SalesContext _context; private readonly ILogger _logger; private readonly IMemoryCache _cache;</span><span style="color: #0000ff;">public</span> ArticleController(SalesContext context, ILogger<ArticleController><span style="color: #000000;"> logger, IMemoryCache memoryCache) { _context </span>=<span style="color: #000000;"> context; _logger </span>=<span style="color: #000000;"> logger; _cache </span>=<span style="color: #000000;"> memoryCache; } [HttpGet] </span><span style="color: #0000ff;">public</span><span style="color: #000000;"> ResultObject GetArticleList() { _logger.LogInformation(</span><span style="color: #800000;">"</span><span style="color: #800000;">==GetArticleList==</span><span style="color: #800000;">"</span><span style="color: #000000;">); List</span><Article> articles =<span style="color: #000000;"> _context.Articles .AsNoTracking() .ToList(); </span><span style="color: #0000ff;">return</span> <span style="color: #0000ff;">new</span><span style="color: #000000;"> ResultObject { result </span>=<span style="color: #000000;"> articles }; } </span><span style="color: #000000;"> }</span></pre>
要在中间件中通过依赖使用该Controller，需要将其注册到DI容器：
public void ConfigureServices(IServiceCollection services) { services.AddScoped<ArticleController>(); }
以上中间件实现了一个文章信息查询的功能，如果在此中间件内先判断路径，再根据不同的路径调用不同的Contorller提供的服务，就可以形成一个简单的MVC中间件了。
四、中间件的用途
中间件的使用体现了AOP（面向切片）的编程思想，在不修改现有代码的情况下，通过增加一些中间件实现一些特定逻辑，可以做的事情很多，比如：
URL重写
缓存处理
异常处理
用户认证
五、中间件的注册顺序
前文提到中间件的注册顺序是比较重要的，建议的顺序如下：
1. 异常/错误处理
2. 静态文件服务器
3. 身份验证
4. MVC