前言

简单介绍一下.net core的中间件。

正文

官方文档已经给出了中间件的概念图:

和其密切相关的是下面这两个东西:

IApplicationBuilder 和 RequestDelegate(HttpContext context)

IApplicationBuilder :

public interface IApplicationBuilder
{
IServiceProvider ApplicationServices { get; set; }

IFeatureCollection ServerFeatures { get; }

IDictionary<string, object> Properties { get; }

IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware);

IApplicationBuilder New();

RequestDelegate Build();
}

RequestDelegate:

namespace Microsoft.AspNetCore.Http
{
  public delegate Task RequestDelegate(HttpContext context);
}

举一个 中间件的例子:

app.Use(async (context, next) => {
                await context.Response.WriteAsync("hello word");
});

效果:

这里我没有执行next，故而在这里就终止了。

来看下这个Use,干了什么:

public static class UseExtensions
{
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);
  })));
}
}

是的，他是对IApplicationBuilder 的一个扩展。

如果不想使用这个扩展方法，那么你要这么写:

app.Use((Func<RequestDelegate, RequestDelegate>)  (next=> (RequestDelegate)((context)=>
{
	Func<Task> func = (Func<Task>)(() => next(context));
	Func<HttpContext, Func<Task>, Task> middleware = async (context1, next2) =>
	{
		await context1.Response.WriteAsync("hello word");
	};
	return middleware(context, func);
})));

有些人可能看不惯这样写哈,换一种写法:

app.Use((Func<RequestDelegate, RequestDelegate>)  (next =>
{
	return (RequestDelegate)((context) =>
	{
		Func<Task> func = (Func<Task>)(() => next(context));
		Func<HttpContext, Func<Task>, Task> middleware = async (context1, next2) =>
		{
			await context1.Response.WriteAsync("hello word");
		};
		return middleware(context, func);
	});
}));

又或者，这样写:

app.Use((Func<RequestDelegate, RequestDelegate>)  (next =>
{
	return (RequestDelegate)((context) =>
	{
		Func<HttpContext, RequestDelegate, Task> middleware = async (context1, next2) =>
		{
			await context1.Response.WriteAsync("hello word");
		};
		return middleware(context, next);
	});
}));

还可以这样写:

public async Task WriteAsync(HttpContext context, RequestDelegate requestDelegate)
{
	await context.Response.WriteAsync("hello word");
}
app.Use((Func<RequestDelegate, RequestDelegate>)  (next =>
            {
                return (RequestDelegate)((context) => WriteAsync(context, next));
            }));

上面没有用到这个next，那么这个next是干什么的呢？从上面的传参推断出，就是我们的下一步。

如果没有执行下一步，那么下一步是不会执行的。

来看一下IApplicationBuilder的实现类ApplicationBuilder的use方法:

private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = (IList<Func<RequestDelegate, RequestDelegate>>) new List<Func<RequestDelegate, RequestDelegate>>();
public IApplicationBuilder Use(
  Func<RequestDelegate, RequestDelegate> middleware)
{
  this._components.Add(middleware);
  return (IApplicationBuilder) this;
}

会将我们传入的middleware，加入到_components 中。

ApplicationBuilder看下build 方法:

public RequestDelegate Build()
{
  RequestDelegate requestDelegate = (RequestDelegate) (context =>
  {
	Endpoint endpoint = context.GetEndpoint();
	if (endpoint?.RequestDelegate != null)
	  throw new InvalidOperationException("The request reached the end of the pipeline without executing the endpoint: ‘" + endpoint.DisplayName + "‘. Please register the EndpointMiddleware using ‘IApplicationBuilder.UseEndpoints(...)‘ if using routing.");
	context.Response.StatusCode = 404;
	return Task.CompletedTask;
  });
  foreach (Func<RequestDelegate, RequestDelegate> func in this._components.Reverse<Func<RequestDelegate, RequestDelegate>>())
	requestDelegate = func(requestDelegate);
  return requestDelegate;
}

这个就是套娃工程，把后面一个的requestDelegate，作为前面一个requestDelegate的参数。最后返回第一个requestDelegate。

断点验证,我打了两个断点，下面是断点的顺序。

第一个断点停留的位置:

第二个断点停留的位置:

第二个断点里面的next就是第一个断点返回的结果。

因为返回的是第一个中间件的返回的RequestDelegate，那么运行。

那么运行顺序就是第一个返回的RequestDelegate开始运行，且参数是第二个中间件返回的RequestDelegate。

返回的RequestDelegate运行顺序如下:

这大概就是中间件的原理了。

下面看一下动态中间件:

app.Map("/abc", builder =>
{
	app.Use((Func<RequestDelegate, RequestDelegate>)(next =>
	{
		
		return (RequestDelegate)((context) => WriteAsync(context, next));
	}));
});

如上面这样，如果匹配到了/abc,那么就走里面的中间件。

看下源码吧，Map的。

public static class MapExtensions
{
public static IApplicationBuilder Map(
  this IApplicationBuilder app,
  PathString pathMatch,
  Action<IApplicationBuilder> configuration)
{
  if (app == null)
	throw new ArgumentNullException(nameof (app));
  if (configuration == null)
	throw new ArgumentNullException(nameof (configuration));
  if (pathMatch.HasValue && pathMatch.Value.EndsWith("/", StringComparison.Ordinal))
	throw new ArgumentException("The path must not end with a ‘/‘", nameof (pathMatch));
  IApplicationBuilder applicationBuilder = app.New();
  configuration(applicationBuilder);
  RequestDelegate requestDelegate = applicationBuilder.Build();
  MapOptions options = new MapOptions()
  {
	Branch = requestDelegate,
	PathMatch = pathMatch
  };
  return app.Use((Func<RequestDelegate, RequestDelegate>) (next => new RequestDelegate(new MapMiddleware(next, options).Invoke)));
}
}

里面做的主要是两件事，一件事是另外 app.New();弄出一条分支出来。然后调用Build()独立走出一条新的中间件链。

New方法如下:

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

第二件事就是返回了一个新的中间件RequestDelegate，传入了两个参数一个是next，这个是用来走老的分支，估摸着不匹配的时候走旧的分支。

还有一个参数是options，这个参数有两个属性，一个是Branch 就是新的分支。一个是PathMatch 是匹配字符，那么就是如果是匹配的话，就走新的分支。

事实证明果然如此:

public class MapMiddleware
{
private readonly RequestDelegate _next;
private readonly MapOptions _options;

public MapMiddleware(RequestDelegate next, MapOptions options)
{
  if (next == null)
	throw new ArgumentNullException(nameof (next));
  if (options == null)
	throw new ArgumentNullException(nameof (options));
  this._next = next;
  this._options = options;
}

public async Task Invoke(HttpContext context)
{
  if (context == null)
	throw new ArgumentNullException(nameof (context));
  PathString matched;
  PathString remaining;
  if (context.Request.Path.StartsWithSegments(this._options.PathMatch, out matched, out remaining))
  {
	PathString path = context.Request.Path;
	PathString pathBase = context.Request.PathBase;
	context.Request.PathBase = pathBase.Add(matched);
	context.Request.Path = remaining;
	try
	{
	  await this._options.Branch(context);
	}
	finally
	{
	  context.Request.PathBase = pathBase;
	  context.Request.Path = path;
	}
	path = new PathString();
	pathBase = new PathString();
  }
  else
	await this._next(context);
}
}

还可以这样自定义:

app.MapWhen(context =>
{
	return context.Request.Query.Keys.Contains("abc");
}, builder =>
{
	app.Use((Func<RequestDelegate, RequestDelegate>)(next =>
	{

		return (RequestDelegate)((context) => WriteAsync(context, next));
	}));
});

有了上面的简单分析，应该不难理解哈。

我这里直接贴了:
MapWhen:

public static class MapWhenExtensions
{
public static IApplicationBuilder MapWhen(
  this IApplicationBuilder app,
  Func<HttpContext, bool> predicate,
  Action<IApplicationBuilder> configuration)
{
  if (app == null)
	throw new ArgumentNullException(nameof (app));
  if (predicate == null)
	throw new ArgumentNullException(nameof (predicate));
  if (configuration == null)
	throw new ArgumentNullException(nameof (configuration));
  IApplicationBuilder applicationBuilder = app.New();
  configuration(applicationBuilder);
  RequestDelegate requestDelegate = applicationBuilder.Build();
  MapWhenOptions options = new MapWhenOptions()
  {
	Predicate = predicate,
	Branch = requestDelegate
  };
  return app.Use((Func<RequestDelegate, RequestDelegate>) (next => new RequestDelegate(new MapWhenMiddleware(next, options).Invoke)));
}
}

MapWhenMiddleware:

public class MapWhenMiddleware
{
private readonly RequestDelegate _next;
private readonly MapWhenOptions _options;

public MapWhenMiddleware(RequestDelegate next, MapWhenOptions options)
{
  if (next == null)
	throw new ArgumentNullException(nameof (next));
  if (options == null)
	throw new ArgumentNullException(nameof (options));
  this._next = next;
  this._options = options;
}

public async Task Invoke(HttpContext context)
{
  if (context == null)
	throw new ArgumentNullException(nameof (context));
  if (this._options.Predicate(context))
	await this._options.Branch(context);
  else
	await this._next(context);
}
}

上面都是异曲同工，就不做解释了。

这里再介绍一个方法，run:

app.Run(async context =>
{
	await context.Response.WriteAsync("hello word");
});

这个这个Run方法，没有传入next。

如下:

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));
}

表示这是末端。

那么下面介绍一下，将我们的中间件写入到一个独立的类里面去。

定义一个扩展类:

public static class SelfBuilderExtensions
{
	public static IApplicationBuilder UseSelfSelfMiddleware(this IApplicationBuilder app)
	{
		return app.UseMiddleware<SelfMiddleware>();
	}
}

具体的实现:

public class SelfMiddleware
{
	private readonly RequestDelegate _next;

	public SelfMiddleware(RequestDelegate next)
	{
		this._next = next;
	}

	public async Task InvokeAsync(HttpContext context)
	{
		Console.WriteLine("request handle");
		await this._next(context);
		Console.WriteLine("response handle");
	}
}

使用:

app.UseSelfSelfMiddleware();

简单看一下UseMiddleware这个方法:

public static IApplicationBuilder UseMiddleware<TMiddleware>(
  this IApplicationBuilder app,
  params object[] args)
{
  return app.UseMiddleware(typeof (TMiddleware), args);
}

继续看app.UseMiddleware:

public static IApplicationBuilder UseMiddleware(
  this IApplicationBuilder app,
  Type middleware,
  params object[] args)
{
  if (typeof (IMiddleware).GetTypeInfo().IsAssignableFrom(middleware.GetTypeInfo()))
  {
	if (args.Length != 0)
	  throw new NotSupportedException(Resources.FormatException_UseMiddlewareExplicitArgumentsNotSupported((object) typeof (IMiddleware)));
	return UseMiddlewareExtensions.UseMiddlewareInterface(app, middleware);
  }
  IServiceProvider applicationServices = app.ApplicationServices;
  return app.Use((Func<RequestDelegate, RequestDelegate>) (next =>
  {
	MethodInfo[] array = ((IEnumerable<MethodInfo>) middleware.GetMethods(BindingFlags.Instance | BindingFlags.Public)).Where<MethodInfo>((Func<MethodInfo, bool>) (m => string.Equals(m.Name, "Invoke", StringComparison.Ordinal) || string.Equals(m.Name, "InvokeAsync", StringComparison.Ordinal))).ToArray<MethodInfo>();
	if (array.Length > 1)
	  throw new InvalidOperationException(Resources.FormatException_UseMiddleMutlipleInvokes((object) "Invoke", (object) "InvokeAsync"));
	if (array.Length == 0)
	  throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoInvokeMethod((object) "Invoke", (object) "InvokeAsync", (object) middleware));
	MethodInfo methodInfo = array[0];
	if (!typeof (Task).IsAssignableFrom(methodInfo.ReturnType))
	  throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNonTaskReturnType((object) "Invoke", (object) "InvokeAsync", (object) "Task"));
	ParameterInfo[] parameters = methodInfo.GetParameters();
	if (parameters.Length == 0 || parameters[0].ParameterType != typeof (HttpContext))
	  throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoParameters((object) "Invoke", (object) "InvokeAsync", (object) "HttpContext"));
	object[] objArray = new object[args.Length + 1];
	objArray[0] = (object) next;
	Array.Copy((Array) args, 0, (Array) objArray, 1, args.Length);
	object instance = ActivatorUtilities.CreateInstance(app.ApplicationServices, middleware, objArray);
	if (parameters.Length == 1)
	  return (RequestDelegate) methodInfo.CreateDelegate(typeof (RequestDelegate), instance);
	Func<object, HttpContext, IServiceProvider, Task> factory = UseMiddlewareExtensions.Compile<object>(methodInfo, parameters);
	return (RequestDelegate) (context =>
	{
	  IServiceProvider serviceProvider = context.RequestServices ?? applicationServices;
	  if (serviceProvider == null)
		throw new InvalidOperationException(Resources.FormatException_UseMiddlewareIServiceProviderNotAvailable((object) "IServiceProvider"));
	  return factory(instance, context, serviceProvider);
	});
  }));
}

一段一段分析：

if (typeof (IMiddleware).GetTypeInfo().IsAssignableFrom(middleware.GetTypeInfo()))
{
if (args.Length != 0)
  throw new NotSupportedException(Resources.FormatException_UseMiddlewareExplicitArgumentsNotSupported((object) typeof (IMiddleware)));
return UseMiddlewareExtensions.UseMiddlewareInterface(app, middleware);
}

如果middleware 继承IMiddleware，那么将会调用UseMiddlewareExtensions.UseMiddlewareInterface.
IMiddleware如下:

public interface IMiddleware
{
Task InvokeAsync(HttpContext context, RequestDelegate next);
}

然后UseMiddlewareExtensions.UseMiddlewareInterface:

private static IApplicationBuilder UseMiddlewareInterface(
  IApplicationBuilder app,
  Type middlewareType)
{
  return app.Use((Func<RequestDelegate, RequestDelegate>) (next => (RequestDelegate) (async context =>
  {
	IMiddlewareFactory middlewareFactory = (IMiddlewareFactory) context.RequestServices.GetService(typeof (IMiddlewareFactory));
	if (middlewareFactory == null)
	  throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoMiddlewareFactory((object) typeof (IMiddlewareFactory)));
	IMiddleware middleware = middlewareFactory.Create(middlewareType);
	if (middleware == null)
	  throw new InvalidOperationException(Resources.FormatException_UseMiddlewareUnableToCreateMiddleware((object) middlewareFactory.GetType(), (object) middlewareType));
	try
	{
	  await middleware.InvokeAsync(context, next);
	}
	finally
	{
	  middlewareFactory.Release(middleware);
	}
  })));

上面的大意就是封装一个中间件，里面调用的方法就InvokeAsync。这个很好理解。

MethodInfo[] array = ((IEnumerable<MethodInfo>) middleware.GetMethods(BindingFlags.Instance | BindingFlags.Public)).Where<MethodInfo>((Func<MethodInfo, bool>) (m => string.Equals(m.Name, "Invoke", StringComparison.Ordinal) || string.Equals(m.Name, "InvokeAsync", StringComparison.Ordinal))).ToArray<MethodInfo>();
if (array.Length > 1)
  throw new InvalidOperationException(Resources.FormatException_UseMiddleMutlipleInvokes((object) "Invoke", (object) "InvokeAsync"));
if (array.Length == 0)
  throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoInvokeMethod((object) "Invoke", (object) "InvokeAsync", (object) middleware));

获取Invoke和InvokeAsync方法。

如果这两个方法同时存在，抛出异常。

如果一个都没有抛出异常。

if (!typeof (Task).IsAssignableFrom(methodInfo.ReturnType))
  throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNonTaskReturnType((object) "Invoke", (object) "InvokeAsync", (object) "Task"));
ParameterInfo[] parameters = methodInfo.GetParameters();
if (parameters.Length == 0 || parameters[0].ParameterType != typeof (HttpContext))
  throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoParameters((object) "Invoke", (object) "InvokeAsync", (object) "HttpContext"));
object[] objArray = new object[args.Length + 1];
objArray[0] = (object) next;
Array.Copy((Array) args, 0, (Array) objArray, 1, args.Length);

如果返回结果不是一个Task报错。

如果里面的第一个参数不是HttpContext 报错。

object[] objArray = new object[args.Length + 1];
objArray[0] = (object) next;
Array.Copy((Array) args, 0, (Array) objArray, 1, args.Length);
object instance = ActivatorUtilities.CreateInstance(app.ApplicationServices, middleware, objArray);
if (parameters.Length == 1)
  return (RequestDelegate) methodInfo.CreateDelegate(typeof (RequestDelegate), instance);
Func<object, HttpContext, IServiceProvider, Task> factory = UseMiddlewareExtensions.Compile<object>(methodInfo, parameters);
return (RequestDelegate) (context =>
{
  IServiceProvider serviceProvider = context.RequestServices ?? applicationServices;
  if (serviceProvider == null)
	throw new InvalidOperationException(Resources.FormatException_UseMiddlewareIServiceProviderNotAvailable((object) "IServiceProvider"));
  return factory(instance, context, serviceProvider);
});

上面表示含义是实例化函数的第一个参数应该是RequestDelegate。

然后通过反射生成具体的对象。

如果Invoke或者InvokeAsync 只有一个参数的话，也就是只有HttpContext参数，直接通过CreateDelegate，创建委托。

如果不止的话，就通过一系列操作进行转换，这里就不介绍了，细节篇介绍了。毕竟是实践篇。

结

以上只是个人整理，如果有错误，望请指点。

下一节异常处理中间件。

重新整理 .net core 实践篇—————中间件[十九]