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.NET Core HttpClient源码探究

前言

在之前的文章我们介绍过HttpClient相关的服务发现,确实HttpClient是目前.NET Core进行Http网络编程的的主要手段。在之前的介绍中也看到了,我们使用了一个很重要的抽象HttpMessageHandler,接下来我们就探究一下HttpClient源码,并找寻它和HttpMessageHandler的关系究竟是怎么样的。

HttpClient源码解析

首先我们找到HttpClient源码的位置,微软也提供了专门的网站可以查找.Net Core源码有兴趣的同学可以自行查阅。接下来我们查阅一下HttpClient的核心代码。首先,我们可以看到HttpClient继承自HttpMessageInvoker这个类,待会我们在探究这个类。

public class HttpClient : HttpMessageInvoker

{

}

然后我们看下几个核心的构造函数

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();

}

通过这几个构造函数我们看出,我们可以传递自定义的HttpMessageHandler。我们再看无参默认的构造,其实也是实例化了HttpClientHandler传递给了自己的另一个构造函数,我们之前讲解过HttpClientHandler是继承自了HttpMessageHandler,通过最后一个构造函数可知最终HttpMessageHandler,传给了父类HttpMessageInvoker。到了这里我们基本上就可以感受到HttpMessageHandler在HttpClient中存在的意义。

    接下来,我们从一个最简单,而且最常用的方法为入口开始探索HttpClient的工作原理。这种方式可能是我们最常用而且最有效的的探索源码的方式了。 个人建议没看过源码,或者刚开始入门看源码的小伙伴们,找源码的入口一定是你最有把握的的一个,然后逐步深入了解。

接下来我们选用HttpClient的GetAsync开始入手,而且是只传递Url的那一个。

public Task<HttpResponseMessage> GetAsync(string? requestUri)

{

return GetAsync(CreateUri(requestUri));

}


public Task<HttpResponseMessage> GetAsync(Uri? requestUri)

{

return GetAsync(requestUri, defaultCompletionOption);

}

通过这里我们可以大致了解到。 其实大部分最简单的调用方式,往往都是从最复杂的调用方式,一步步的封装起来的,只是系统帮我们初始化了一部分参数,让我们按需使用。 顺着方法一直向下找,最后找到了这里。

public Task<HttpResponseMessage> GetAsync(Uri? requestUri, HttpCompletionOption completionOption,

CancellationToken cancellationToken)

{

return SendAsync(CreateRequestMessage(HttpMethod.Get, requestUri), completionOption, cancellationToken);

}

由此可以看出这里是所有GetAsync方法的执行入口,我们通过查找SendAsync引用可以发现。不仅仅是GetAsync, PostAsync,PutAsync,DeleteAsync最终都是调用了这个方法。也就是说SendAsync是所有发送请求的真正执行者。接下来我们就查看SendAsync方法,部分边角料代码我粘贴的时候将会做删减。

public Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, HttpCompletionOption completionOption,

CancellationToken cancellationToken)

{

if (request == null)

{

throw new ArgumentNullException(nameof(request));

}

CheckDisposed();

CheckRequestMessage(request);


SetOperationStarted();

//这里会把发送请求的HttpRequestMessage准备妥当

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

{

//***这里是核心,最终执行调用的地方!!!

sendTask = base.SendAsync(request, cts.Token);

}

catch (Exception e)

{

HandleFinishSendAsyncCleanup(cts, disposeCts);

if (e is OperationCanceledException operationException && TimeoutFired(cancellationToken, timeoutTime))

{

throw CreateTimeoutException(operationException);

}

throw;

}

//这里处理输出的唯一类型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);

}

通过分析这段代码可以得知,HttpClient类中最终执行的是父类的SendAsync的方法。看来是时候查看父类HttpMessageInvoker的源码了。

HttpMessageInvoker源码解析

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);


//***这里是HttpClient调用的本质,其实发送请求的根本是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,你并没有看错,整个HttpMessageInvoker就这么多代码,而且还是靠子类初始化过来的基本属性。找到SendAsync方法,这里基本上可以总结一点,负责调用输入输出的类只有两个。一个是提供请求参数的HttpRequestMessage,另一个是接收输出的HttpResponseMessage。 这里也给我们日常工作编码中提供了一个很好的思路。针对具体某个功能的操作方法,最好只保留一个,其外围调用,都是基于该方法的封装。 然后我们找到了发送请求的地方_handler.SendAsync(request, cancellationToken),而handler正是我们通过HttpClient传递下来的HttpMessageHandler.由此可知, HttpClient的本质是HttpMessageHandler的包装类。

自定义HttpClient

探究到这里我们也差不多大概了解到HttpClient类的本质是什么了。其实到这里我们可以借助HttpMessageHandler的相关子类,封装一个简单的Http请求类.接下来我将动手实现一个简单的Http请求类,我们定义一个类叫MyHttpClient,实现代码如下

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();

}

}

}

}

由于HttpMessageHandler的SendAsync是protected非子类无法直接调用,所以我封装了一个MyHttpClientHandler继承自HttpClientHandler在MyHttpClient中调用,具体实现如下

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);

}

}

最后写了一段测试代码

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);

}

}

}

到这里自己实现MyHttpClient差不多到此结束了,因为只是讲解大致思路,所以方法封装的相对简单,只是封装了Get和Post相关的方法。

总结

通过本文分析HttpClient的源码,我们大概知道了HttpClient本质还是HttpMessageHandler的包装类。最终的发送还是调用的HttpMessageHandler的SendAsync方法。最后,我根据HttpClientHandler实现了一个MyHttpClient。以上只是本人理解,如果处在理解不正确或者不恰当的地方,望多多包涵,同时也期望能指出理解不周的地方。我写文章的主要一部分是想把我的理解传递给大家,欢迎大家多多交流。

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.NET Core HttpClient源码探究

原文  http://mp.weixin.qq.com/s?__biz=MzAwNTMxMzg1MA==&mid=2654078970&idx=2&sn=66c80bca541a3a0dc99cfa3226675cce
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