概念
ASP.NET Core Middleware是在应用程序处理管道pipeline中用于处理请求和操作响应的组件。
每个组件：
特性和行为
ASP.NET Core处理管道由一系列请求委托组成，一环接一环的被调用， 下面给出自己绘制的Middleware pipeline流程图：
从上图可以看出，请求自进入处理管道，经历了四个中间件，每个中间件都包含后续紧邻中间件 执行委托（next）的引用，同时每个中间件在交棒之前和交棒之后可以自行决定参与一些Http请求和响应的逻辑处理。
每个中间件还可以决定不将请求转发给下一个委托，这称为请求管道的短路（短路是有必要的，某些专有中间件比如 StaticFileMiddleware 可以在完成功能之后，避免请求被转发到其他动态处理过程）。
源码实现
观察一个标准的中间件代码的写法和用法：
using System.Threading.Tasks;
using Alyio.AspNetCore.ApiMessages;
using Gridsum.WebDissector.Common;
using Microsoft.AspNetCore.Http;
namespace Gridsum.WebDissector
{
sealed class AuthorizationMiddleware
{
private readonly RequestDelegate _next; // 下一个中间件执行委托的引用
public AuthorizationMiddleware(RequestDelegate next)
{
_next = next;
}
public Task Invoke(HttpContext context) // 贯穿始终的HttpContext对象
{
if (context.Request.Path.Value.StartsWith("/api/"))
{
return _next(context);
}
if (context.User.Identity.IsAuthenticated && context.User().DisallowBrowseWebsite)
{
throw new ForbiddenMessage("You are not allow to browse the website.");
}
return _next(context);
}
}
}
public static IApplicationBuilder UserAuthorization(this IApplicationBuilder app)
{
return app.UseMiddleware<AuthorizationMiddleware>();
}
// 启用该中间件，也就是注册该中间件
app.UserAuthorization();
标准的中间件使用方式是如此简单明了，带着几个问题探究一下源码实现
(1).中间件传参是怎样完成的： app.UseMiddleware<Authorization>(AuthOption); 我们传参的时候，为什么能自动注入中间件构造函数非第1个参数
(2).编写中间件的时候，为什么必须要定义特定的 Invoke/InvokeAsync 函数？
(3).设定中间件的顺序很重要，中间件的嵌套顺序是怎么确定的 ？
思考以上标准中间件的行为： 输入下一个中间件的执行委托Next， 定义当前中间件的执行委托Invoke/InvokeAsync；
每个中间件完成了 Func<RequestDelegate，RequestDelegate>这样的行为；
通过参数next与下一个中间件的执行委托Invoke/InvokeAsync 建立"链式"关系。
public delegate Task RequestDelegate(HttpContext context);
//-----------------节选自 Microsoft.AspNetCore.Builder.UseMiddlewareExtensions------------------
/// <summary>
/// Adds a middleware type to the application's request pipeline.
/// </summary>
/// <typeparam name="TMiddleware">The middleware type.</typeparam>
/// <param name="app">The <see cref="IApplicationBuilder"/> instance.</param>
/// <param name="args">The arguments to pass to the middleware type instance's constructor.</param>
/// <returns>The <see cref="IApplicationBuilder"/> instance.</returns>
public static IApplicationBuilder UseMiddleware<TMiddleware>(this IApplicationBuilder app, params object[] args)
{
return app.UseMiddleware(typeof(TMiddleware), args);
}
/// <summary>
/// Adds a middleware type to the application's request pipeline.
/// </summary>
/// <param name="app">The <see cref="IApplicationBuilder"/> instance.</param>
/// <param name="middleware">The middleware type.</param>
/// <param name="args">The arguments to pass to the middleware type instance's constructor.</param>
/// <returns>The <see cref="IApplicationBuilder"/> instance.</returns>
public static IApplicationBuilder UseMiddleware(this IApplicationBuilder app, Type middleware, params object[] args)
{
if (typeof(IMiddleware).GetTypeInfo().IsAssignableFrom(middleware.GetTypeInfo()))
{
// IMiddleware doesn't support passing args directly since it's
// activated from the container
if (args.Length > 0)
{
throw new NotSupportedException(Resources.FormatException_UseMiddlewareExplicitArgumentsNotSupported(typeof(IMiddleware)));
}
return UseMiddlewareInterface(app, middleware);
}
var applicationServices = app.ApplicationServices;
return app.Use(next =>
{
var methods = middleware.GetMethods(BindingFlags.Instance | BindingFlags.Public);
// 执行委托名称被为Invoke/InvokeAsync
var invokeMethods = methods.Where(m =>
string.Equals(m.Name, InvokeMethodName, StringComparison.Ordinal)
|| string.Equals(m.Name, InvokeAsyncMethodName, StringComparison.Ordinal)
).ToArray();
if (invokeMethods.Length > 1)
{
throw new InvalidOperationException(Resources.FormatException_UseMiddleMutlipleInvokes(InvokeMethodName, InvokeAsyncMethodName));
}
if (invokeMethods.Length == 0)
{
throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoInvokeMethod(InvokeMethodName, InvokeAsyncMethodName, middleware));
}
var methodInfo = invokeMethods[0];
if (!typeof(Task).IsAssignableFrom(methodInfo.ReturnType))
{
throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNonTaskReturnType(InvokeMethodName, InvokeAsyncMethodName, nameof(Task)));
}
var parameters = methodInfo.GetParameters();
if (parameters.Length == 0 || parameters[0].ParameterType != typeof(HttpContext))
{
throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoParameters(InvokeMethodName, InvokeAsyncMethodName, nameof(HttpContext)));
}
var ctorArgs = new object[args.Length + 1];
ctorArgs[0] = next;
Array.Copy(args, 0, ctorArgs, 1, args.Length);
// 通过反射形成中间件实例的时候，构造函数第一个参数被指定为 下一个中间件的执行委托
var instance = ActivatorUtilities.CreateInstance(app.ApplicationServices, middleware, ctorArgs);
if (parameters.Length == 1)
{
return (RequestDelegate)methodInfo.CreateDelegate(typeof(RequestDelegate), instance);
}
// 当前执行委托除了可指定HttpContext参数以外， 还可以注入更多的依赖参数
var factory = Compile<object>(methodInfo, parameters);
return context =>
{
var serviceProvider = context.RequestServices ?? applicationServices;
if (serviceProvider == null)
{
throw new InvalidOperationException(Resources.FormatException_UseMiddlewareIServiceProviderNotAvailable(nameof(IServiceProvider)));
}
return factory(instance, context, serviceProvider);
};
});
}
//-------------------节选自 Microsoft.AspNetCore.Builder.Internal.ApplicationBuilder-------------------
private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>();
publicIApplicationBuilder Use(Func<RequestDelegate,RequestDelegate> middleware)
{
this._components.Add(middleware);
return this;
}
public RequestDelegate Build()
{
RequestDelegate app = context =>
{
context.Response.StatusCode = 404;
return Task.CompletedTask;
};
foreach (var component in _components.Reverse())
{
app = component(app);
}
return app;
}
通过以上代码我们可以看出：
分析源码：回答上面的问题：
附：非标准中间件的用法
短路中间件、 分叉中间件、条件中间件
整个处理管道的形成，存在一些管道分叉或者临时插入中间件的行为，一些重要方法可供使用