一、HttpClient使用

在C#中，如果我們需要向某特定的URL地址發送Http請求的時候，通常會用到HttpClient類。會將HttpClient包裹在using內部進行聲明和初始化，如下面的代碼：

using (var httpClient = new HttpClient())
{
    // 邏輯處理代碼
}

HttpClient類包含了許多有用的方法，使用上面的代碼，可以滿足絕大多數的需求，但是如果對其使用不當時，可能會出現意想不到的事情。

上面代碼的技術範點：當你使用繼承了IDisposable接口的對象時，建議在using代碼塊中聲明和初始化，當using代碼段執行完成後，會自動釋放該對象而不需要手動進行顯示Dispose操作。

對象所佔用資源應該確保及時被釋放掉，但是，對於網絡連接而言，這是錯誤的。具體原因有下面兩點：

網絡連接是需要耗費一定時間的，頻繁開啓與關閉連接，性能會受到影響。
開啓網絡連接時會佔用低層socket資源，但在HttpClient調用其本身的Dispose方法時，並不能立即釋放該資源，這意味着你的程序可能會因爲耗盡連接資源而產生預期之外的異常。

看下面一段代碼

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;

namespace HttpClientDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            //using (var httpClient = new HttpClient())
            //{
            //    // 邏輯處理代碼
            //}

            HttpAsync();
            Console.WriteLine("Hello World!");
            Console.Read();

        }

        public static async void HttpAsync()
        {
            for (int i = 0; i < 10; i++)
            {
                using (var client = new HttpClient())
                {
                    var result = await client.GetAsync("http://www.baidu.com");
                    Console.WriteLine($"{i}:{result.StatusCode}");
                }
            }
        }
    }
}

運行項目輸出結果後，通過netstate查看下TCP連接情況，會發現連接依然存在，狀態爲“TIME_WAIT”（繼續等待看是否還有延遲的包會傳輸過來）。

這裏就會出現一個坑：在高併發的情況下，連接來不及釋放，socket連接被耗盡，耗盡之後就會出現錯誤。就是會出現“各種套接字問題”。

那麼如何解決這個問題呢？比較好的解決方法是延長HttpClient對象的使用壽命，實現HttpClient對象的複用，比如對其建一個靜態的對象：

private static HttpClient Client = new HttpClient();

我們使用這種方式優化上面的代碼

using System;
using System.Net.Http;

namespace HttpClientDemo
{
    class Program
    {
        private static readonly HttpClient _client = new HttpClient();
        static void Main(string[] args)
        {
            HttpAsync();
            Console.WriteLine("Hello World");
            Console.ReadKey();
        }

        public static async void HttpAsync()
        {
            for (int i = 0; i < 10; i++)
            {
                var result = await _client.GetAsync("http://www.baidu.com");
                Console.WriteLine($"{i}:{result.StatusCode}");
            }
        }

    }
}

這樣調整HttpClient的引用後，雖然可以解決一些問題，但是仍然存在一些問題：

因爲是複用的HttpClient，那麼一些公共的設置就沒辦法靈活的調整，如請求頭的自定義。
因爲HttpClient請求每個url時，會緩存url對應的主機ip，從而會導致DNS更新失效。

爲了解決這些問題，在.NET Core 2.1中引入了新的HttpClientFactory類。

二、HttpClientFactory使用

微軟在.NET Core 2.1中新引入了HttpClientFactory類，具有如下的優勢：

HttpClientFactory很高效，可以最大程度上節省系統的sock而。
Factory，顧名思義HttpClientfactory就是HttpClient的工廠，內部已經幫我們處理好了對HttpClient的管理，不需要我們人工進行對象釋放，同時，支持自定義請求頭、支持DNS更新等。

我們用一個ASP.NET Core的程序作爲示例，它的用法非常簡單，首先是對其進行IOC註冊：

public void ConfigureServices(IServiceCollection services)
{
    // 注入HttpClient
    services.AddHttpClient("client_1", config =>  //這裏指定的name=client_1，可以方便我們後期服用該實例
    {
        config.BaseAddress = new Uri("http://www.baidu.com");
        config.DefaultRequestHeaders.Add("header_1", "header_1");
    });
    services.AddHttpClient("client_2", config =>
    {
        config.BaseAddress = new Uri("https://www.qq.com/");
        config.DefaultRequestHeaders.Add("header_2", "header_2");
    });
    services.AddHttpClient();
    services.AddControllers();
}

然後在控制器裏面通過IHttpClientFactory創建一個HttpClient對象，之後的操作跟以前一樣，但不需要擔心其內部資源的釋放：

using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;

namespace HttpClientFactoryDemo.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class DemoController : ControllerBase
    {
        IHttpClientFactory _httpClientFactory;

        /// <summary>
        /// 通過構造函數實現注入
        /// </summary>
        /// <param name="httpClientFactory"></param>
        public DemoController(IHttpClientFactory httpClientFactory)
        {
            _httpClientFactory = httpClientFactory;
        }

        public async Task<string> Get()
        {
            var client = _httpClientFactory.CreateClient("client_1"); //複用在Startup中定義的client_1的httpclient
            var result = await client.GetStringAsync("/page1.html");

            var client2 = _httpClientFactory.CreateClient(); //新建一個HttpClient
            var result2 = await client.GetAsync("http://www.baidu.com");



            return result2.StatusCode.ToString();
        }
    }
}

程序運行結果：

AddHttpClient的源碼：

public static IServiceCollection AddHttpClient(this IServiceCollection services)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    services.AddLogging();
    services.AddOptions();

    //
    // Core abstractions
    //
    services.TryAddTransient<HttpMessageHandlerBuilder, DefaultHttpMessageHandlerBuilder>();
    services.TryAddSingleton<IHttpClientFactory, DefaultHttpClientFactory>();

    //
    // Typed Clients
    //
    services.TryAdd(ServiceDescriptor.Singleton(typeof(ITypedHttpClientFactory<>), typeof(DefaultTypedHttpClientFactory<>)));

    //
    // Misc infrastructure
    //
    services.TryAddEnumerable(ServiceDescriptor.Singleton<IHttpMessageHandlerBuilderFilter, LoggingHttpMessageHandlerBuilderFilter>());

    return services;
}

看下面這句代碼：

services.TryAddSingleton<IHttpClientFactory, DefaultHttpClientFactory>();

這裏添加依賴注入的時候爲IHttpClientFactory接口綁定了DefaultHttpClientFactory類。

我們在來看IHttpClientFactory接口中關鍵的CreateClient方法：

public HttpClient CreateClient(string name)
{
    if (name == null)
    {
        throw new ArgumentNullException(nameof(name));
    }

    var entry = _activeHandlers.GetOrAdd(name, _entryFactory).Value;
    var client = new HttpClient(entry.Handler, disposeHandler: false);

    StartHandlerEntryTimer(entry);

    var options = _optionsMonitor.Get(name);
    for (var i = 0; i < options.HttpClientActions.Count; i++)
    {
        options.HttpClientActions[i](client);
    }

    return client;
}

從代碼中我們可以看出：HttpClient的創建不在是簡單的new HttpClient()，而是傳入了兩個參數：HttpMessageHandler handler與bool disposeHandler。

disposeHandler參數爲false時表示要重用內部的handler對象。handler參數則從上一句的代碼中可以看出是以name爲鍵值從一字典中取出，又因爲DefaultHttpClientFactory類是通過TryAddSingleton方法註冊的，也就意味着其爲單例，那麼這個內部字典便是唯一的，每個鍵值對應的ActiveHandlerTrackingEntry對象也是唯一，該對象內部中包含着handler。

下一句代碼StartHandlerEntryTimer(entry); 開啓了ActiveHandlerTrackingEntry對象的過期計時處理。默認過期時間是2分鐘。

internal void ExpiryTimer_Tick(object state)
{
    var active = (ActiveHandlerTrackingEntry)state;

    // The timer callback should be the only one removing from the active collection. If we can't find
    // our entry in the collection, then this is a bug.
    var removed = _activeHandlers.TryRemove(active.Name, out var found);
    Debug.Assert(removed, "Entry not found. We should always be able to remove the entry");
    Debug.Assert(object.ReferenceEquals(active, found.Value), "Different entry found. The entry should not have been replaced");

    // At this point the handler is no longer 'active' and will not be handed out to any new clients.
    // However we haven't dropped our strong reference to the handler, so we can't yet determine if
    // there are still any other outstanding references (we know there is at least one).
    //
    // We use a different state object to track expired handlers. This allows any other thread that acquired
    // the 'active' entry to use it without safety problems.
    var expired = new ExpiredHandlerTrackingEntry(active);
    _expiredHandlers.Enqueue(expired);

    Log.HandlerExpired(_logger, active.Name, active.Lifetime);

    StartCleanupTimer();
}

先是將ActiveHandlerTrackingEntry對象傳入新的ExpiredHandlerTrackingEntry對象。

public ExpiredHandlerTrackingEntry(ActiveHandlerTrackingEntry other)
{
    Name = other.Name;

    _livenessTracker = new WeakReference(other.Handler);
    InnerHandler = other.Handler.InnerHandler;
}

在其構造方法內部，handler對象通過弱引用方式關聯着，不會影響其被GC釋放。

然後新建的ExpiredHandlerTrackingEntry對象被放入專用的隊列。

最後開始清理工作，定時器的時間間隔設定爲每10秒一次。

internal void CleanupTimer_Tick(object state)
{
    // Stop any pending timers, we'll restart the timer if there's anything left to process after cleanup.
    //
    // With the scheme we're using it's possible we could end up with some redundant cleanup operations.
    // This is expected and fine.
    //
    // An alternative would be to take a lock during the whole cleanup process. This isn't ideal because it
    // would result in threads executing ExpiryTimer_Tick as they would need to block on cleanup to figure out
    // whether we need to start the timer.
    StopCleanupTimer();

    try
    {
        if (!Monitor.TryEnter(_cleanupActiveLock))
        {
            // We don't want to run a concurrent cleanup cycle. This can happen if the cleanup cycle takes
            // a long time for some reason. Since we're running user code inside Dispose, it's definitely
            // possible.
            //
            // If we end up in that position, just make sure the timer gets started again. It should be cheap
            // to run a 'no-op' cleanup.
            StartCleanupTimer();
            return;
        }

        var initialCount = _expiredHandlers.Count;
        Log.CleanupCycleStart(_logger, initialCount);

        var stopwatch = ValueStopwatch.StartNew();

        var disposedCount = 0;
        for (var i = 0; i < initialCount; i++)
        {
            // Since we're the only one removing from _expired, TryDequeue must always succeed.
            _expiredHandlers.TryDequeue(out var entry);
            Debug.Assert(entry != null, "Entry was null, we should always get an entry back from TryDequeue");

            if (entry.CanDispose)
            {
                try
                {
                    entry.InnerHandler.Dispose();
                    disposedCount++;
                }
                catch (Exception ex)
                {
                    Log.CleanupItemFailed(_logger, entry.Name, ex);
                }
            }
            else
            {
                // If the entry is still live, put it back in the queue so we can process it
                // during the next cleanup cycle.
                _expiredHandlers.Enqueue(entry);
            }
        }

        Log.CleanupCycleEnd(_logger, stopwatch.GetElapsedTime(), disposedCount, _expiredHandlers.Count);
    }
    finally
    {
        Monitor.Exit(_cleanupActiveLock);
    }

    // We didn't totally empty the cleanup queue, try again later.
    if (_expiredHandlers.Count > 0)
    {
        StartCleanupTimer();
    }
}

上述方法核心是判斷是否handler對象已經被GC，如果是的話，則釋放其內部資源，即網絡連接。

回到最初創建HttpClient的代碼，會發現並沒有傳入任何name參數值。這是得益於HttpClientFactoryExtensions類的擴展方法。

public static HttpClient CreateClient(this IHttpClientFactory factory)
{
    if (factory == null)
    {
        throw new ArgumentNullException(nameof(factory));
    }

    return factory.CreateClient(Options.DefaultName);
}

Options.DefaultName的值爲string.Empty。

參考：https://blog.csdn.net/weixin_34321753/article/details/91362881

https://www.cnblogs.com/lizhizhang/p/9502862.html

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