.NET Core HttpClient原始碼探究

前言

    在之前的文章我們介紹過HttpClient相關的服務發現，確實HttpClient是目前.NET Core進行Http網路程式設計的的主要手段。在之前的介紹中也看到了，我們使用了一個很重要的抽象HttpMessageHandler，接下來我們就探究一下HttpClient原始碼，並找尋它和HttpMessageHandler的關係究竟是怎麼樣的。

HttpClient原始碼解析

    首先我們找到HttpClient原始碼的位置，微軟也提供了專門的網站可以查詢.Net Core原始碼有興趣的同學可以自行查閱。接下來我們查閱一下HttpClient的核心程式碼。首先，我們可以看到HttpClient繼承自HttpMessageInvoker這個類，待會我們在探究這個類。

public class HttpClient : HttpMessageInvoker
{
}

然後我們看下幾個核心的建構函式

public HttpClient()
    : this(new HttpClientHandler())
{
}

public HttpClient(HttpMessageHandler handler)
    : this(handler, true)
{
}

public HttpClient(HttpMessageHandler handler, bool disposeHandler)
    : base(handler, disposeHandler)
{
    _timeout = s_defaultTimeout;
    _maxResponseContentBufferSize = HttpContent.MaxBufferSize;
    _pendingRequestsCts = new CancellationTokenSource();
}

    通過這幾個建構函式我們看出，我們可以傳遞自定義的HttpMessageHandler。我們再看無參預設的構造，其實也是例項化了HttpClientHandler傳遞給了自己的另一個建構函式，我們之前講解過HttpClientHandler是繼承自了HttpMessageHandler，通過最後一個建構函式可知最終HttpMessageHandler，傳給了父類HttpMessageInvoker。到了這裡我們基本上就可以感受到HttpMessageHandler在HttpClient中存在的意義。
    接下來，我們從一個最簡單，而且最常用的方法為入口開始探索HttpClient的工作原理。這種方式可能是我們最常用而且最有效的的探索原始碼的方式了。個人建議沒看過原始碼，或者剛開始入門看原始碼的小夥伴們，找原始碼的入口一定是你最有把握的的一個，然後逐步深入瞭解。接下來我們選用HttpClient的GetAsync開始入手，而且是隻傳遞Url的那一個。

public Task<HttpResponseMessage> GetAsync(string? requestUri)
{
    return GetAsync(CreateUri(requestUri));
}

public Task<HttpResponseMessage> GetAsync(Uri? requestUri)
{
    return GetAsync(requestUri, defaultCompletionOption);
}

通過這裡我們可以大致瞭解到。其實大部分最簡單的呼叫方式，往往都是從最複雜的呼叫方式，一步步的封裝起來的，自是系統幫我們初始化了一部分引數，讓我們按需使用。順著方法一直向下找，最後找到了這裡。

public Task<HttpResponseMessage> GetAsync(Uri? requestUri, HttpCompletionOption completionOption,
            CancellationToken cancellationToken)
{
    return SendAsync(CreateRequestMessage(HttpMethod.Get, requestUri), completionOption, cancellationToken);
}

由此可以看出這裡是所有GetAsync方法的執行入口，我們通過查詢SendAsync引用可以發現。不僅僅是GetAsync, PostAsync，PutAsync，DeleteAsync最終都是呼叫了這個方法。也就是說SendAsync是所有傳送請求的真正執行者。接下來我們就檢視SendAsync方法，部分邊角料程式碼我貼上的時候將會做刪減。

public Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, HttpCompletionOption completionOption,
            CancellationToken cancellationToken)
{
    if (request == null)
    {
        throw new ArgumentNullException(nameof(request));
    }
    CheckDisposed();
    CheckRequestMessage(request);

    SetOperationStarted();
    //這裡會把傳送請求的HttpRequestMessage準備妥當
    PrepareRequestMessage(request);

    CancellationTokenSource cts;
    bool disposeCts;
    bool hasTimeout = _timeout != s_infiniteTimeout;
    long timeoutTime = long.MaxValue;
    if (hasTimeout || cancellationToken.CanBeCanceled)
    {
        disposeCts = true;
        cts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, _pendingRequestsCts.Token);
        if (hasTimeout)
        {
            timeoutTime = Environment.TickCount64 + (_timeout.Ticks / TimeSpan.TicksPerMillisecond);
            cts.CancelAfter(_timeout);
        }
    }
    else
    {
        disposeCts = false;
        cts = _pendingRequestsCts;
    }
    Task<HttpResponseMessage> sendTask;
    try
    {
        //***這裡是核心，最終執行呼叫的地方！！！
        sendTask = base.SendAsync(request, cts.Token);
    }
    catch (Exception e)
    {
        HandleFinishSendAsyncCleanup(cts, disposeCts);
        if (e is OperationCanceledException operationException && TimeoutFired(cancellationToken, timeoutTime))
        {
            throw CreateTimeoutException(operationException);
        }
        throw;
    }
    //這裡處理輸出的唯一型別HttpResponseMessage
    return completionOption == HttpCompletionOption.ResponseContentRead && !string.Equals(request.Method.Method, "HEAD", StringComparison.OrdinalIgnoreCase) ?
        FinishSendAsyncBuffered(sendTask, request, cts, disposeCts, cancellationToken, timeoutTime) :
        FinishSendAsyncUnbuffered(sendTask, request, cts, disposeCts, cancellationToken, timeoutTime);
}

通過分析這段程式碼可以得知，HttpClient類中最終執行的是父類的SendAsync的方法。看來是時候檢視父類HttpMessageInvoker的原始碼了。

HttpMessageInvoker原始碼解析

public class HttpMessageInvoker : IDisposable
{
    private volatile bool _disposed;
    private readonly bool _disposeHandler;
    private readonly HttpMessageHandler _handler;

    public HttpMessageInvoker(HttpMessageHandler handler)
        : this(handler, true)
    {
    }

    public HttpMessageInvoker(HttpMessageHandler handler, bool disposeHandler)
    {
        if (NetEventSource.IsEnabled) NetEventSource.Enter(this, handler);

        if (handler == null)
        {
            throw new ArgumentNullException(nameof(handler));
        }

        if (NetEventSource.IsEnabled) NetEventSource.Associate(this, handler);

        _handler = handler;
        _disposeHandler = disposeHandler;

        if (NetEventSource.IsEnabled) NetEventSource.Exit(this);
    }

    public virtual Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,
        CancellationToken cancellationToken)
    {
        if (request == null)
        {
            throw new ArgumentNullException(nameof(request));
        }
        CheckDisposed();

        if (NetEventSource.IsEnabled) NetEventSource.Enter(this, request);

        //***這裡是HttpClient呼叫的本質，其實傳送請求的根本是HttpMessageHandler的SendAsync
        Task<HttpResponseMessage> task = _handler.SendAsync(request, cancellationToken);

        if (NetEventSource.IsEnabled) NetEventSource.Exit(this, task);
        return task;
    }

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (disposing && !_disposed)
        {
            _disposed = true;

            if (_disposeHandler)
            {
                _handler.Dispose();
            }
        }
    }

    private void CheckDisposed()
    {
        if (_disposed)
        {
            throw new ObjectDisposedException(GetType().ToString());
        }
    }
}

    是的，你並沒有看錯，整個HttpMessageInvoker就這麼多程式碼，而且還是靠子類初始化過來的基本屬性。找到SendAsync方法，這裡基本上可以總結一點，負責呼叫輸入輸出的類只有兩個。一個是提供請求引數的HttpRequestMessage，另一個是接收輸出的HttpResponseMessage。這裡也給我們日常工作編碼中提供了一個很好的思路。針對具體某個功能的操作方法，最好只保留一個，其外圍呼叫，都是基於該方法的封裝。然後我們找到了傳送請求的地方_handler.SendAsync(request, cancellationToken)，而handler正是我們通過HttpClient傳遞下來的HttpMessageHandler.由此可知，HttpClient的本質是HttpMessageHandler的包裝類。

自定義HttpClient

    探究到這裡我們也差不多大概瞭解到HttpClient類的本質是什麼了。其實到這裡我們可以藉助HttpMessageHandler的相關子類，封裝一個簡單的Http請求類.接下來我將動手實現一個簡單的Http請求類，我們定義一個類叫MyHttpClient,實現程式碼如下

public class MyHttpClient : IDisposable
{
    private readonly MyHttpClientHandler _httpClientHandler;
    private readonly bool _disposeHandler;
    private volatile bool _disposed;

    public MyHttpClient()
        :this(true)
    {
    }

    public MyHttpClient(bool disposeHandler)
    {
        _httpClientHandler = new MyHttpClientHandler();
        _disposeHandler = disposeHandler;
    }

    public Task<HttpResponseMessage> GetAsync(string url)
    {
        return GetAsync(new Uri(url));
    }

    public Task<HttpResponseMessage> GetAsync(Uri uri)
    {
        HttpRequestMessage httpRequest = new HttpRequestMessage
        {
            Method = HttpMethod.Get,
            RequestUri = uri
        };
        return SendAsync(httpRequest,CancellationToken.None);
    }

    public Task<HttpResponseMessage> PostAsync(string url, HttpContent content)
    {
        return PostAsync(new Uri(url),content,null);
    }

    public Task<HttpResponseMessage> PostAsync(Uri uri, HttpContent content,Dictionary<string,string> headers)
    {
        HttpRequestMessage httpRequest = new HttpRequestMessage
        {
            Method = HttpMethod.Post,
            RequestUri = uri,
            Content = content
        };
        if (headers != null && headers.Any())
        {
            foreach (var head in headers)
            {
                httpRequest.Headers.Add(head.Key,head.Value);
            }
        }
        return SendAsync(httpRequest, CancellationToken.None);
    }

    private Task<HttpResponseMessage> SendAsync(HttpRequestMessage httpRequest, CancellationToken cancellationToken)
    {
        if (httpRequest.RequestUri == null || string.IsNullOrWhiteSpace(httpRequest.RequestUri.OriginalString))
        {
            throw new ArgumentNullException("RequestUri");
        }
        return _httpClientHandler.SendRequestAsync(httpRequest, cancellationToken);
    }

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (disposing && !_disposed)
        {
            _disposed = true;

            if (_disposeHandler)
            {
                _httpClientHandler.Dispose();
            }
        }
    }
}

由於HttpMessageHandler的SendAsync是protected非子類無法直接呼叫，所以我封裝了一個MyHttpClientHandler繼承自HttpClientHandler在MyHttpClient中呼叫，具體實現如下

public class MyHttpClientHandler : HttpClientHandler
{
    public Task<HttpResponseMessage> SendRequestAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        return this.SendAsync(request, cancellationToken);
    }

    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        return base.SendAsync(request, cancellationToken);
    }
}

最後寫了一段測試程式碼

using (MyHttpClient httpClient = new MyHttpClient())
{
    Task<HttpResponseMessage> httpResponse = httpClient.GetAsync("http://localhost:5000/Person/GetPerson?userId=1");
    HttpResponseMessage responseMessage = httpResponse.Result;
    if (responseMessage.StatusCode == HttpStatusCode.OK)
    {
        string content = responseMessage.Content.ReadAsStringAsync().Result;
        if (!string.IsNullOrWhiteSpace(content))
        {
            System.Console.WriteLine(content);
        }
    }
}

到這裡自己實現MyHttpClient差不多到此結束了，因為只是講解大致思路，所以方法封裝的相對簡單，只是封裝了Get和Post相關的方法。

總結

    通過本文分析HttpClient的原始碼，我們大概知道了HttpClient本質還是HttpMessageHandler的包裝類。最終的傳送還是呼叫的HttpMessageHandler的SendAsync方法。最後，我根據HttpClientHandler實現了一個MyHttpClient。以上只是本人理解，如果處在理解不正確或者不恰當的地方，忘多多包涵，同時也期望能指出理解不周的地方。我寫文章的主要一部分是想把我的理解傳遞給大家，歡迎大家多多交流。