9102年了，汇总下HttpClient问题，封印一个

如果找的是core的HttpClientFactory 出门右转。

1. 官方写法，高并发下，TCP连接不能快速释放，导致端口占完，无法连接

   Dispose 不是马上关闭tcp连接

   主动关闭的一方为什么不能马上close而是进入timewait状态：TCP四次挥手客户端关闭链接为什么要等待2倍MSL

   正确写法一个域(一个地址) 保证一个静态httpclient操作，保证重用tcp连接。

2. 如果HttpClient唯一，如果请求头内容需要变化怎么办，异常："集合已修改；可能无法执行枚举操作"

   HttpClient有个接口SendAsync。看源码知道其实HttpClient内部get,post,put,delete最终都是调用SendAsync。

   这个方法可以允许用户传递HttpRequestMessage,内部包含(HttpRequestHeaders)

3. HttpClient 死锁

   关于Task同步上下文 造成死锁问题就不多解释。避免方法就是ConfigureAwait(false)或者await always。最好是await always。传送门

   说下不用await 而使用类似HttpClient.GetStringAsync(uri).Result 直接同步获取为什么没有死锁

   因为HttpClient源码里面用到async await的地方几乎都加了ConfigureAwait(false)。233333

4. 预热和长连接

   其实这是嘴巴dudu园长大人在很久以前就做分析过 传送门

5. HttpClient 利用stream和json.net 减少内存开销，加快反序列化过程

   我们一般的请求流程：发起请求，获取返回的string对象，然后反序列化成我们想要的对象。而其实可以利用stream直接反序列化成我们想要的对象。

   而且可以是在HttpCompletionOption.ResponseHeadersRead的情况下。传送门

   封印：

    /// <summary>
       /// 异步http请求者。
       /// 注意：杜绝new的形式，可以IOC容器单例注入
       /// 如果多个address地址，通过name key形式注入多个单例
       /// </summary>
       public class HttpAsyncSender
       {
           private  readonly ILogger _logger = LoggerManager.GetLogger(typeof(HttpAsyncSender));
           //静态 重用tcp连接 长连接（not dispose）https://aspnetmonsters.com/2016/08/2016-08-27-httpclientwrong/
           private  readonly HttpClient _httpClient = new HttpClient();
   
           public HttpAsyncSender(Uri baseUri, int timeoutSeconds = 0, bool keepAlive = true, bool preheating = false, long maxResponseContentBufferSize = 0)
           {
               //基础地址
               _httpClient.BaseAddress = baseUri;
               //超时
               if (timeoutSeconds != 0)
               {
                   _httpClient.Timeout = TimeSpan.FromSeconds(timeoutSeconds);
               }
               //response最大接收字节 默认2gbmstsc
               if (maxResponseContentBufferSize != 0)
               {
                   _httpClient.MaxResponseContentBufferSize = maxResponseContentBufferSize;
               }
               //长连接 //https://www.cnblogs.com/lori/p/7692152.html  http 1.1 default set keep alive
               if (keepAlive)
               {
                   _httpClient.DefaultRequestHeaders.Connection.Add("keep-alive");
               }
               //httpclient 预热 the first request  //https://www.cnblogs.com/dudu/p/csharp-httpclient-attention.html 
               if (preheating)
               {
                   _httpClient.SendAsync(new HttpRequestMessage
                   {
                       Method = new HttpMethod("HEAD"),
                       RequestUri = new Uri(baseUri + "/")
                   }).Result.EnsureSuccessStatusCode();
               }
           }
   
           #region 异步 
   
           /// <summary>
           /// GetAsync 注意 await always
           /// </summary>
           /// <param name="url"></param>
           /// <param name="cancellationToken"></param>
           /// <param name="jsonSerializerSettings"></param>
           /// <returns></returns>
           public async Task<T> GetAsync<T>(string url, CancellationToken cancellationToken, JsonSerializerSettings jsonSerializerSettings = null)
           {
               try
               {
                   //https://johnthiriet.com/efficient-api-calls/ 减少内存开销 利用stream特性 加快反序列化
                   using (var request = new HttpRequestMessage(HttpMethod.Get, url))
                   {
                       var res = await SendAsync(request, HttpCompletionOption.ResponseHeadersRead, cancellationToken).ConfigureAwait(false);
                       var resStesam = await res.Content.ReadAsStreamAsync().ConfigureAwait(false);
                       if (res.IsSuccessStatusCode)
                       {
                           return DeserializeJsonFromStream<T>(resStesam, jsonSerializerSettings);
                       }
                       var resStr = await StreamToStringAsync(resStesam).ConfigureAwait(false);
                       _logger.Error($"HttpAsyncSender, GetAsync ,response fail StatusCode:{res.StatusCode} resStr:{resStr}  BaseAddress:{_httpClient.BaseAddress},Url:{url}");
                   }
               }
               catch (AggregateException ae)
               {
                   _logger.Error($"HttpAsyncSender,GetAsync AggregateException,BaseAddress:{_httpClient.BaseAddress},Url:{url} ae:{ae.Flatten()}");
                   throw;
   
               }
               catch (Exception e)
               {
                   _logger.Error($"HttpAsyncSender,GetAsync Exception,BaseAddress:{_httpClient.BaseAddress},Url:{url}",e);
                   throw;
               }
   
               return default(T);
           }
   
           /// <summary>
           /// PostAsync 注意 await always
           /// </summary>
           /// <param name="url"></param>
           /// <param name="content"></param>
           /// <param name="cancellationToken"></param>
           /// <param name="jsonSerializerSettings"></param>
           /// <returns></returns>
           public async Task<TRes> PostAsync<TReq, TRes>(string url, TReq content, CancellationToken cancellationToken, JsonSerializerSettings jsonSerializerSettings = null)
           {
               try
               {
                   //https://johnthiriet.com/efficient-api-calls/ 减少内存开销 利用stream特性 加快反序列化
                   using (var request = new HttpRequestMessage(HttpMethod.Post, url))
                   using (var httpContent = CreateHttpContent(content, jsonSerializerSettings))
                   {
                       request.Content = httpContent;
                       var res = await SendAsync(request, HttpCompletionOption.ResponseHeadersRead, cancellationToken).ConfigureAwait(false);
                       var resStesam = await res.Content.ReadAsStreamAsync().ConfigureAwait(false);
                       if (res.IsSuccessStatusCode)
                       {
                           return DeserializeJsonFromStream<TRes>(resStesam, jsonSerializerSettings);
                       }
                       var resStr = await StreamToStringAsync(resStesam).ConfigureAwait(false);
                       _logger.Error($"HttpAsyncSender, PostAsync ,response fail StatusCode:{res.StatusCode} resStr:{resStr}  BaseAddress:{_httpClient.BaseAddress},Url:{url}");
                   }
               }
               catch (AggregateException ae)
               {
                   _logger.Error($"HttpAsyncSender,PostAsync AggregateException,BaseAddress:{_httpClient.BaseAddress},Url:{url} ae:{ae.Flatten()}");
                   throw;
   
               }
               catch (Exception e)
               {
                   _logger.Error($"HttpAsyncSender,PostAsync Exception,BaseAddress:{_httpClient.BaseAddress},Url:{url}",e);
                   throw;
               }
   
               return default(TRes);
           }
   
           /// <summary>
           /// SendAsync 注意 await always 当需要动态改变request head的时候 调用此方法。 解决 "集合已修改；可能无法执行枚举操作"
           /// </summary>
           /// <param name="httpRequestMessage"></param>
           /// <param name="completionOption"></param>
           /// <param name="cancellationToken"></param>
           /// <returns></returns>
           public async Task<HttpResponseMessage> SendAsync(HttpRequestMessage httpRequestMessage, HttpCompletionOption completionOption, CancellationToken cancellationToken)
           {
               try
               {
                   return await _httpClient.SendAsync(httpRequestMessage, completionOption, cancellationToken).ConfigureAwait(false);
               }
               catch (AggregateException ae)
               {
                   _logger.Error(
                       $"HttpAsyncSender,SendAsync AggregateException,BaseAddress:{_httpClient.BaseAddress},Url:{httpRequestMessage.RequestUri} ae:{ae.Flatten()}");
                   throw;
   
               }
               catch (Exception e)
               {
                   _logger.Error($"HttpAsyncSender,SendAsync Exception,BaseAddress:{_httpClient.BaseAddress},Url:{httpRequestMessage.RequestUri}",e);
                   throw;
               }
           }
   
           #endregion
   
   
           private  T DeserializeJsonFromStream<T>(Stream stream, JsonSerializerSettings jsonSerializerSettings = null)
           {
               if (stream == null || stream.CanRead == false)
                   return default(T);
   
               using (var sr = new StreamReader(stream))
               using (var jtr = new JsonTextReader(sr))
               {
                   var js = jsonSerializerSettings == null ? new JsonSerializer() : JsonSerializer.Create(jsonSerializerSettings);
                   var searchResult = js.Deserialize<T>(jtr);
                   return searchResult;
               }
           }
   
           private  async Task<string> StreamToStringAsync(Stream stream)
           {
               string content = null;
   
               if (stream != null)
                   using (var sr = new StreamReader(stream))
                       content = await sr.ReadToEndAsync();
   
               return content;
           }
   
           public  void SerializeJsonIntoStream(object value, Stream stream, JsonSerializerSettings jsonSerializerSettings = null)
           {
               using (var sw = new StreamWriter(stream, new UTF8Encoding(false), 1024, true))
               using (var jtw = new JsonTextWriter(sw) { Formatting = Formatting.None })
               {
                   var js = jsonSerializerSettings==null?new JsonSerializer():JsonSerializer.Create(jsonSerializerSettings);
                   js.Serialize(jtw, value);
                   jtw.Flush();
               }
           }
   
           private  HttpContent CreateHttpContent<T>(T content, JsonSerializerSettings jsonSerializerSettings = null)
           {
               HttpContent httpContent = null;
               if (content != null)
               {
                   var ms = new MemoryStream();
                   SerializeJsonIntoStream(content, ms, jsonSerializerSettings);
                   ms.Seek(0, SeekOrigin.Begin);
                   httpContent = new StreamContent(ms);
                   httpContent.Headers.ContentType = new MediaTypeHeaderValue("application/json");
               }
   
               return httpContent;
           }
       }