ASP.NET Core 6框架揭祕實例演示[28]：自定義一個服務器

語言: CN / TW / HK
時間 2022-03-24 09:41:00

作為ASP.NET Core請求處理管道的“龍頭”的服務器負責監聽和接收請求並最終完成對請求的響應。它將原始的請求上下文描述為相應的特性（Feature），並以此將HttpContext上下文創建出來，中間件針對HttpContext上下文的所有操作將藉助於這些特性轉移到原始的請求上下文上。學習ASP.NET Core框架最有效的方式就是按照它的原理“再造”一個框架，瞭解服務器的本質最好的手段就是試着自定義一個服務器。現在我們自定義一個真正的服務器。在此之前，我們再來回顧一下表示服務器的IServer接口。(本篇提供的實例已經彙總到《 ASP.NET Core 6框架揭祕-實例演示版 》）

一、IServer

二、請求和響應特性

三、StreamBodyFeature

四、HttpListenerServer

一、IServer

作為服務器的IServer對象利用如下所示的Features屬性提供了與自身相關的特性。除了利用StartAsync<TContext>和StopAsync方法啟動和關閉服務器之外，它還實現了IDisposable接口，資源的釋放工作可以通過實現的Dispose方法來完成。StartAsync<TContext>方法將IHttpApplication<TContext>類型的參數作為處理請求的“應用”，該對象是對中間件管道的封裝。從這個意義上講，服務器就是傳輸層和這個IHttpApplication<TContext>對象之間的“中介”。

public interface IServer : IDisposable
{
    IFeatureCollection Features { get; }

    Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) where TContext : notnull;
    Task StopAsync(CancellationToken cancellationToken);
}

雖然不同服務器類型的定義方式千差萬別，但是背後的模式基本上與下面這個以偽代碼定義的服務器類型一致。如下這個Server利用IListener對象來監聽和接收請求，該對象是利用構造函數中注入的IListenerFactory工廠根據指定的監聽地址創建出來的。StartAsync<TContext>方法從Features特性集合中提取出IServerAddressesFeature特性，並針對它提供的每個監聽地址創建一個IListener對象。該方法為每個IListener對象開啟一個“接收和處理請求”的循環，循環中的每次迭代都會調用IListener對象的AcceptAsync方法來接收請求，我們利用RequestContext對象來表示請求上下文。

public class Server : IServer
{
    private readonly IListenerFactory _listenerFactory;
    private readonly List<IListener> _listeners = new();

    public IFeatureCollection Features { get; } = new FeatureCollection();

    public Server(IListenerFactory listenerFactory) => _listenerFactory = listenerFactory;

    public async Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) where TContext : notnull
    {
        var addressFeature = Features.Get<IServerAddressesFeature>()!;
        foreach (var address in addressFeature.Addresses)
        {
            var listener = await _listenerFactory.BindAsync(address);
            _listeners.Add(listener);
            _ = StartAcceptLoopAsync(listener);
        }

        async Task StartAcceptLoopAsync(IListener listener)
        {
            while (true)
            {
                var requestContext = await listener.AcceptAsync();
                _ = ProcessRequestAsync(requestContext);
            }
        }

        async Task ProcessRequestAsync(RequestContext requestContext)
        {
            var feature = new RequestContextFeature(requestContext);
            var contextFeatures = new FeatureCollection();
            contextFeatures.Set<IHttpRequestFeature>(feature);
            contextFeatures.Set<IHttpResponseFeature>(feature);
            contextFeatures.Set<IHttpResponseBodyFeature>(feature);

            var context = application.CreateContext(contextFeatures);
            Exception? exception = null;
            try
            {
                await application.ProcessRequestAsync(context);
            }
            catch (Exception ex)
            {
                exception = ex;
            }
            finally
            {
                application.DisposeContext(context, exception);
            }
        }
    }
    public Task StopAsync(CancellationToken cancellationToken) => Task.WhenAll(_listeners.Select(listener => listener.StopAsync()));

    public void Dispose() => _listeners.ForEach(listener => listener.Dispose());
}

public interface IListenerFactory
{
    Task<IListener> BindAsync(string listenAddress);
}

public interface IListener : IDisposable
{

    Task<RequestContext> AcceptAsync();
    Task StopAsync();
}

public class RequestContext
{
     ...
}

public class RequestContextFeature : IHttpRequestFeature, IHttpResponseFeature, IHttpResponseBodyFeature
{
    public RequestContextFeature(RequestContext requestContext);
    ...
}

StartAsync<TContext>方法接下來利用此RequestContext上下文將RequestContextFeature特性創建出來。RequestContextFeature特性類型同時實現了IHttpRequestFeature, IHttpResponseFeature和 IHttpResponseBodyFeature這三個核心接口，我們特性針對這三個接口將特性對象添加到創建的FeatureCollection集合中。特性集合隨後作為參數調用IHttpApplication<TContext>的CreateContext方法將TContext上下文創建出來，後者將進一步作為參數調用另一個ProcessRequestAsync方法將請求分發給中間件管道進行處理。待處理結束，IHttpApplication<TContext>對象的DisposeContext方法被調用，創建的TContext上下文承載的資源得以釋放。

二、請求和響應特性

接下來我們將採用類似的模式來定義一個基於HttpListener的服務器。提供的HttpListenerServer的思路就是利用自定義特性來封裝表示原始請求上下文的HttpListenerContext對象，我們使用HttpRequestFeature和HttpResponseFeature這個兩個現成特性。

public class HttpRequestFeature : IHttpRequestFeature
{
    public string 		Protocol { get; set; }
    public string 		Scheme { get; set; }
    public string 		Method { get; set; }
    public string 		PathBase { get; set; }
    public string 		Path { get; set; }
    public string 		QueryString { get; set; }

    public string 		RawTarget { get; set; }
    public IHeaderDictionary 	Headers { get; set; }
    public Stream 		Body { get; set; }
}

public class HttpResponseFeature : IHttpResponseFeature
{
    public int 		StatusCode { get; set; }
    public string? 		ReasonPhrase { get; set; }
    public IHeaderDictionary 	Headers { get; set; }
    public Stream 		Body { get; set; }
    public virtual bool 	HasStarted => false;

    public HttpResponseFeature()
    {
        StatusCode = 200;
        Headers = new HeaderDictionary();
        Body = Stream.Null;
    }

    public virtual void OnStarting(Func<object, Task> callback, object state){}
    public virtual void OnCompleted(Func<object, Task> callback, object state){}
}

如果我們使用HttpRequestFeature來描述請求，意味着HttpListener在接受到請求之後需要將請求信息從HttpListenerContext上下文轉移到該特性上。如果使用HttpResponseFeature來描述響應，待中間件管道在完成針對請求的處理後，我們還需要將該特性承載的響應數據應用到HttpListenerContext上下文上。

三、StreamBodyFeature

現在我們有了描述請求和響應的兩個特性，還需要一個描述響應主體的特性，為此我們定義瞭如下這個StreamBodyFeature特性類型。StreamBodyFeature直接使用構造函數提供的Stream對象作為響應主體的輸出流，並根據該對象創建出Writer屬性返回的PipeWriter對象。本着“一切從簡”的原則，我們並沒有實現用來發送文件的SendFileAsync方法，其他成員也採用最簡單的方式進行了實現。

public class StreamBodyFeature : IHttpResponseBodyFeature
{
    public Stream 	Stream { get; }
    public PipeWriter 	Writer { get; }

    public StreamBodyFeature(Stream stream)
    {
        Stream = stream;
        Writer = PipeWriter.Create(Stream);
    }

    public Task CompleteAsync() => Task.CompletedTask;
    public void DisableBuffering() { }
    public Task SendFileAsync(string path, long offset, long? count, CancellationToken cancellationToken = default)=> throw new NotImplementedException();
    public Task StartAsync(CancellationToken cancellationToken = default) => Task.CompletedTask;
}

四、HttpListenerServer

在如下這個自定義的HttpListenerServer服務器類型中，與傳輸層交互的HttpListener體現在_listener字段上。服務器在初始化過程中，它的Features屬性返回的IFeatureCollection對象中添加了一個ServerAddressesFeature特性，因為我們需要用它來存放註冊的監聽地址。實現StartAsync<TContext>方法將監聽地址從這個特性中取出來應用到HttpListener對象上。

public class HttpListenerServer : IServer
{
    private readonly HttpListener _listener = new();
    public IFeatureCollection Features { get; } = new FeatureCollection();

    public HttpListenerServer() => Features.Set<IServerAddressesFeature>(new ServerAddressesFeature());
    public Task StartAsync<TContext>(IHttpApplication<TContext> application,CancellationToken cancellationToken) where TContext : notnull
    {
        var pathbases = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
        var addressesFeature = Features.Get<IServerAddressesFeature>()!;
        foreach (string address in addressesFeature.Addresses)
        {
            _listener.Prefixes.Add(address.TrimEnd('/') + "/");
            pathbases.Add(new Uri(address).AbsolutePath.TrimEnd('/'));
        }
        _listener.Start();

        while (true)
        {
            var listenerContext = _listener.GetContext();
            _ = ProcessRequestAsync(listenerContext);
        }

        async Task ProcessRequestAsync( HttpListenerContext listenerContext)
        {
            FeatureCollection features = new();
            var requestFeature = CreateRequestFeature(pathbases, listenerContext);
            var responseFeature = new HttpResponseFeature();
            var body = new MemoryStream();
            var bodyFeature = new StreamBodyFeature(body);
            features.Set<IHttpRequestFeature>(requestFeature);
            features.Set<IHttpResponseFeature>(responseFeature);
            features.Set<IHttpResponseBodyFeature>(bodyFeature);

            var context = application.CreateContext(features);
            Exception? exception = null;
            try
            {
                await application.ProcessRequestAsync(context);

                var response = listenerContext.Response;
                response.StatusCode = responseFeature.StatusCode;
                if (responseFeature.ReasonPhrase is not null)
                {
                    response.StatusDescription = responseFeature.ReasonPhrase;
                }
                foreach (var kv in responseFeature.Headers)
                {
                    response.AddHeader(kv.Key, kv.Value);
                }
                body.Position = 0;
                await body.CopyToAsync(listenerContext.Response.OutputStream);
            }
            catch (Exception ex)
            {
                exception = ex;
            }
            finally
            {
                body.Dispose();
                application.DisposeContext(context, exception);
                listenerContext.Response.Close();
            }
        }
    }
    public void Dispose() => _listener.Stop();

    private static HttpRequestFeature CreateRequestFeature(HashSet<string> pathbases,HttpListenerContext listenerContext)
    {
        var request 		= listenerContext.Request;
        var url 		= request.Url!;
        var absolutePath 	= url.AbsolutePath;
        var protocolVersion 	= request.ProtocolVersion;
        var requestHeaders 	= new HeaderDictionary();
        foreach (string key in request.Headers)
        {
            requestHeaders.Add(key, request.Headers.GetValues(key));
        }

        var requestFeature = new HttpRequestFeature
        {
            Body 		= request.InputStream,
            Headers 		= requestHeaders,
            Method 		= request.HttpMethod,
            QueryString 	                = url.Query,
            Scheme 		= url.Scheme,
            Protocol 		= $"{url.Scheme.ToUpper()}/{protocolVersion.Major}.{protocolVersion.Minor}"
        };
        var pathBase = pathbases.First(it => absolutePath.StartsWith(it, StringComparison.OrdinalIgnoreCase));
        requestFeature.Path = absolutePath[pathBase.Length..];
        requestFeature.PathBase = pathBase;
        return requestFeature;
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        _listener.Stop();
        return Task.CompletedTask;
    }
}

在調用Start方法將HttpListener啟動後，StartAsync<TContext>方法開始“請求接收處理”循環。接收到的請求上下文被封裝成HttpListenerContext上下文，其承載的請求信息利用CreateRequestFeature方法轉移到創建的HttpRequestFeature特性上。StartAsync<TContext>方法創建的“空”HttpResponseFeature對象來描述響應，另一個描述響應主體的StreamBodyFeature特性則根據創建的MemoryStream對象構建而成，意味着中間件管道寫入的響應主體的內容將暫存到這個內存流中。我們將這三個特性註冊到創建的FeatureCollection集合上，並將後者作為參數調用了IHttpApplication<TContext>對象的CreateContext方法將TContext上下文創建出來。此上下文進一步作為參數調用了IHttpApplication<TContext>對象的ProcessRequestAsync方法，中間件管道得以接管請求。

待中間件管道的處理工作完成後，響應的內容還暫存在兩個特性中，我們還需要將它們應用到代表原始HttpListenerContext上下文上。StartAsync<TContext>方法從HttpResponseFeature特性提取出響應狀態碼和響應報頭轉移到HttpListenerContext上下文上，然後上述這個MemoryStream對象“拷貝”到HttpListenerContext上下文承載的響應主體輸出流中。

using App;
using Microsoft.AspNetCore.Hosting.Server;
using Microsoft.Extensions.DependencyInjection.Extensions;

var builder = WebApplication.CreateBuilder(args);
builder.Services.Replace(ServiceDescriptor.Singleton<IServer, HttpListenerServer>());
var app = builder.Build();
app.Run(context => context.Response.WriteAsync("Hello World!"));
app.Run("http://localhost:5000/foobar/");

我們採用上面的演示程序來檢測HttpListenerServer能否正常工作。我們為HttpListenerServer類型創建了一個ServiceDescriptor對象將現有的服務器的服務註冊替換掉。在調用WebApplication對象的Run方法時顯式指定了具有PathBase（“/foobar”）的監聽地址“http://localhost:5000/foobar/”，如圖1所示的瀏覽器以此地址訪問應用，會得到我們希望的結果。

圖1 HttpListenerServer返回的結果

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