ASP.NET Core Middleware

原创

雪飞鸿

修改于 2018-09-22 18:06:38

6160

文章被收录于专栏：me的随笔me的随笔

中间件(Middleware)是ASP.NET Core中的一个重要特性。**所谓中间件就是嵌入到应用管道中用于处理请求和响应的一段代码**。ASP.NET Core Middleware可以分为两种类型：

Conventional Middleware

这种中间件没有实现特定的接口或者继承特定类，它更像是Duck Typing (你走起路来像个鸭子, 叫起来像个鸭子, 那么你就是个鸭子)。有两种表现形式：

匿名方法

这种方式又称为内联中间件（in-line middleware），可以使用Run, Map, Use ,MapWhen等扩展方法来实现。如：

public class Startup
{
    public void Configure(IApplicationBuilder app)
    {
        app.Use(async (context, next) =>
        {

            // Do work that doesn't write to the Response.
            await next.Invoke();
            // Do logging or other work that doesn't write to the Response.
        });
    }
}

IApplicationBuilder的扩展方法：Run、Map、MapWhen及

Use(this IApplicationBuilder app, Func<HttpContext, Func<Task>, Task> middleware)，最终都会调用IApplicationBuilder接口中的Use(Func<RequestDelegate, RequestDelegate> middleware)方法来实现向请求处理管道中注入中间件，后面会对源码做分析。

自定义中间件类

这种形式利于代码的复用，如：

public class XfhMiddleware
{
    private readonly RequestDelegate \_next;

    //在应用程序的生命周期中，中间件的构造函数只会被调用一次
    public XfhMiddleware(RequestDelegate next)
    {
        this.\_next = next;
    }


    public async Task InvokeAsync(HttpContext context)
    {
        // Do something...
        await \_next(context);
    }

}


public static class XfhMiddlewareExtension
{
    public static IApplicationBuilder UseXfhMiddleware(this IApplicationBuilder builder)
    {
        // 使用UseMiddleware将自定义中间件添加到请求处理管道中
        return builder.UseMiddleware<XfhMiddleware>();
    }
}

将自定义中间件配置到请求处理管道中

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseXfhMiddleware();
}

IMiddleware

IMiddleware提供了强类型约束的中间件，其默认实现是MiddlewareFactory，接口定义如下：

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

IMiddlewareFactory用于创建IMiddleware实例及对实例进行回收，接口定义：

public interface IMiddlewareFactory
{
    public IMiddleware Create (Type middlewareType);
    
    public void Release (IMiddleware middleware);
}

自定义IMiddleware类型中间件

public class MyMiddleware : IMiddleware
{
    public async Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
        await next(context);
    }
}





public static class MyMiddlewareExtensions
{
    public static IApplicationBuilder UseMyMiddleware(this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<MyMiddleware>();
    }
}

将中间件注入到请求处理管道：

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseMyMiddleware();
}

使用IMiddleware类型的中间件需要在容器中进行注册，否则抛异常，具体原因下面分析：

将中间件注入到容器中：

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<MyMiddleware>();

    services.AddMvc();
}

一段警告

下面贴一段微软文档中的警告，大意是不要试图去改变已发往客户端的响应内容，否则可能会引发异常。实在是太懒了，不想翻译就把原文贴出来了：

WarningDon't call next.Invoke after the response has been sent to the client. Changes to HttpResponse after the response has started throw an exception. For example, changes such as setting headers and a status code throw an exception. Writing to the response body after calling next:May cause a protocol violation. For example, writing more than the stated Content-Length.May corrupt the body format. For example, writing an HTML footer to a CSS file.HasStarted is a useful hint to indicate if headers have been sent or the body has been written to.

UseMiddleware

前面将自定义中间件注入到请求处理管道时用到了UseMiddleware方法，从方法签名中可以看到UserMiddleware可以接受多个参数：

public static class UseMiddlewareExtensions
{
    public static IApplicationBuilder UseMiddleware<TMiddleware>(this IApplicationBuilder app,  params object[] args);
   
    public static IApplicationBuilder UseMiddleware(this IApplicationBuilder app, Type middleware, params object[] args);
}

接下来我们看下UserMiddleware方法的具体实现，由于该方法代码量较大，所以这里只看其中的关键部分，方法整体流程如下：

public static IApplicationBuilder UseMiddleware(this IApplicationBuilder app, Type middleware, params object[] args)
{
    // IMiddleware类型
    if (typeof(IMiddleware).GetTypeInfo().IsAssignableFrom(middleware.GetTypeInfo()))
    {
        // IMiddleware doesn't support passing args directly since it's
        // activated from the containe
        if (args.Length > 0)
        {
            throw new NotSupportedException(
                Resources.FormatException\_UseMiddlewareExplicitArgumentsNotSupported(typeof(IMiddleware)));
        }

        return UseMiddlewareInterface(app, middleware);
    }
    

    // Conventional Middleware
    var applicationServices = app.ApplicationServices;
    return app.Use(next =>
    {
        // 判断传入的中间件是否符合约束
    });
}

该方法首先判断传入的middleware是否是IMiddleware类型，如果是则调用UseMiddlewareInterface

从这段代码中可以看到IMiddlewareFactory负责创建并回收IMiddleware对象

public static class UseMiddlewareExtensions
{
    private static IApplicationBuilder UseMiddlewareInterface(IApplicationBuilder app, Type middlewareType)
    {
        return app.Use(next =>
        {
            return async context =>
            {
                // 从容器中获取IMiddlewareFactory实例
                var middlewareFactory =
                    (IMiddlewareFactory) context.RequestServices.GetService(typeof(IMiddlewareFactory));
                if (middlewareFactory == null)
                {
                    // No middleware factory
                    throw new InvalidOperationException(
                 Resources.FormatException\_UseMiddlewareNoMiddlewareFactory(typeof(IMiddlewareFactory)));
                }
                
                var middleware = middlewareFactory.Create(middlewareType);
                if (middleware == null)
                {
                    // The factory returned null, it's a broken implementation
                    throw new InvalidOperationException(
                        Resources.FormatException\_UseMiddlewareUnableToCreateMiddleware(middlewareFactory.GetType(),middlewareType));
                }
                

                try
                {
                    await middleware.InvokeAsync(context, next);
                }
                finally
                {
                    middlewareFactory.Release(middleware);
                }
            };
        });
    }
}

从MiddlewareFactory的Create方法中可以看到，IMiddleware实例是从容器中获取的，若容器中找不到则会抛出异常：

public class MiddlewareFactory : IMiddlewareFactory
{
    private readonly IServiceProvider \_serviceProvider;

    public MiddlewareFactory(IServiceProvider serviceProvider)
    {
        this.\_serviceProvider = serviceProvider;
    }

    public IMiddleware Create(Type middlewareType)
    {
        return ServiceProviderServiceExtensions.GetRequiredService(this.\_serviceProvider, middlewareType) as IMiddleware;
    }

    public void Release(IMiddleware middleware)
    {

    }
}

若是Conventional Middleware则判断传入的middleware是否符合约束

首先判断传入的middleware中是否**仅包含一个名称为Invoke或InvokeAsync的公共实例方法**

// UseMiddlewareExtensions类中的两个常量

internal const string InvokeMethodName = "Invoke";

internal const string InvokeAsyncMethodName = "InvokeAsync";



// UserMiddleware方法

var methods = middleware.GetMethods(BindingFlags.Instance | BindingFlags.Public);

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));
}

其次判断方法的返回类型是否是Task：

var methodInfo = invokeMethods[0];

if (!typeof(Task).IsAssignableFrom(methodInfo.ReturnType))
{
    throw new InvalidOperationException(
        Resources.FormatException\_UseMiddlewareNonTaskReturnType(InvokeMethodName,
            InvokeAsyncMethodName, nameof(Task)));
}

然后再判断，方法的第一个参数是否是HttpContext类型：

var parameters = methodInfo.GetParameters();

if (parameters.Length == 0 || parameters[0].ParameterType != typeof(HttpContext))
{
    throw new InvalidOperationException(
        Resources.FormatException\_UseMiddlewareNoParameters(InvokeMethodName, InvokeAsyncMethodName,
            nameof(HttpContext)));
}

对于Invoke或InvokeAsync仅包含一个HttpContext类型参数的情况用到了反射(ActivatorUtilities.CreateInstance方法中)来构建RequestDelegate

var ctorArgs = new object[args.Length + 1];
ctorArgs[0] = next;
Array.Copy(args, 0, ctorArgs, 1, args.Length);
var instance = ActivatorUtilities.CreateInstance(app.ApplicationServices, middlewa
if (parameters.Length == 1)
{
    return (RequestDelegate) methodInfo.CreateDelegate(typeof(RequestDelegate), in
}

对于包含多个参数的情况，则使用了表达式树来构建RequestDelegate

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);
};

完整的代码可以在Github上看到。

Use(Func<RequestDelegate, RequestDelegate> middleware)

上述所有中间件，最终都会调用IApplicationBuilder接口中的Use(Func<RequestDelegate, RequestDelegate> middleware)方法来实现向请求处理管道中注册中间件，该方法在ApplicationBuilder类的实现如下：

public class ApplicationBuilder : IApplicationBuilde
{
    private readonly IList<Func<RequestDelegate, RequestDelegate>> \_components =
        new List<Func<RequestDelegate, RequestDelegate>>();

    public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware)
    {
        this.\_components.Add(middleware);
        return this;
    }
}

从上面代码中可以看到，中间件是一个RequestDelegate类型的委托，请求处理管道其实是一个委托列表，请求委托签名如下：

public delegate Task RequestDelegate(HttpContext context);

与ASP.NET处理管道的区别

传统的ASP.NET的处理管道是基于事件模型的，处理管道有多个IHttpModule和一个IHttpHandler组成。请求处理管道中各个模块被调用的顺序取决于两方面：

模块所注册事件被触发的先后顺序
注册同一事件的不同模块执行先后顺序有Web.config中的配置顺序决定

ASP.NET Core的请求处理管道则是有一堆中间件组成，相对ASP.NET更简单。

中间件处理请求和响应的顺序只与其在代码中的注册顺序有关：处理请求按注册顺序依次执行，处理响应按注册顺序反方向依次执行。

其次，在ASP.NET Core中只需使用代码，而无需使用Global.asax和Web.config来配置请求处理管道。

小结

所谓中间件就是嵌入到应用管道中用于处理请求和响应的一段代码，它主要有两个作用：

处理请求和响应
可以阻止请求发往请求处理管道中的下一个中间件

在ASP.NET Core中，中间件是以RequestDelegate委托的形式体现的。

ASP.NET Core中整个请求处理管道的创建是围绕这种IApplicationBuilder接口进行的，请求处理管道是一个List<RequestDelegate>类型的列表。