當需要向某特定URL地址發送HTTP請求并得到相應響應時，通常會用到HttpClient類。該類包含了眾多有用的方法，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。
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using(var client = new HttpClient()) |
對象所占用資源應該確保及時被釋放掉，但是，對于網絡連接而言，這是錯誤的。
原因有二，網絡連接是需要耗費一定時間的，頻繁開啟與關閉連接，性能會受影響；再者，開啟網絡連接時會占用底層socket資源，但在HttpClient調用其本身的Dispose方法時，并不能立刻釋放該資源，這意味著你的程序可能會因為耗盡連接資源而產生預期之外的異常。
所以比較好的解決方法是延長HttpClient對象的使用壽命，比如對其建一個靜態的對象：
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private static HttpClient Client = new HttpClient(); |
但從程序員的角度來看，這樣的代碼或許不夠優雅。
所以在.NET Core 2.1中引入了新的HttpClientFactory類。
它的用法很簡單，首先是對其進行IoC的注冊：
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public void ConfigureServices(IServiceCollection services){ services.AddHttpClient(); services.AddMvc();} |
然后通過IHttpClientFactory創建一個HttpClient對象，之后的操作如舊，但不需要擔心其內部資源的釋放：
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public class LzzDemoController : Controller{ IHttpClientFactory _httpClientFactory; public LzzDemoController(IHttpClientFactory httpClientFactory) { _httpClientFactory = httpClientFactory; } public IActionResult Index() { var client = _httpClientFactory.CreateClient(); var result = client.GetStringAsync("http://myurl/"); return View(); }} |
AddHttpClient的源碼：
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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;} |
它的內部為IHttpClientFactory接口綁定了DefaultHttpClientFactory類。
再看IHttpClientFactory接口中關鍵的CreateClient方法：
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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;} |
HttpClient的創建不再是簡單的new HttpClient()，而是傳入了兩個參數：HttpMessageHandler handler與bool disposeHandler。disposeHandler參數為false值時表示要重用內部的handler對象。handler參數則從上一句的代碼可以看出是以name為鍵值從一字典中取出，又因為DefaultHttpClientFactory類是通過TryAddSingleton方法注冊的，也就意味著其為單例，那么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，該對象內部中包含著handler。
下一句代碼StartHandlerEntryTimer(entry); 開啟了ActiveHandlerTrackingEntry對象的過期計時處理。默認過期時間是2分鐘。
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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。
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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著，不會影響其被GC釋放。
然后新建的ExpiredHandlerTrackingEntry對象被放入專用的隊列。
最后開始清理工作，定時器的時間間隔設定為每10秒一次。
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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，會發現并沒有傳入任何name參數值。這是得益于HttpClientFactoryExtensions類的擴展方法。
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public static HttpClient CreateClient(this IHttpClientFactory factory){ if (factory == null) { throw new ArgumentNullException(nameof(factory)); } return factory.CreateClient(Options.DefaultName);} |
Options.DefaultName的值為string.Empty。
DefaultHttpClientFactory缺少無參數的構造方法，唯一的構造方法需要傳入多個參數，這也意味著構建它時需要依賴其它一些類，所以目前只適用于在ASP.NET程序中使用，還無法應用到諸如控制臺一類的程序，希望之后官方能夠對其繼續增強，使得應用范圍變得更廣。
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public DefaultHttpClientFactory( IServiceProvider services, ILoggerFactory loggerFactory, IOptionsMonitor<HttpClientFactoryOptions> optionsMonitor, IEnumerable<IHttpMessageHandlerBuilderFilter> filters) |
總結
以上就是這篇文章的全部內容了，希望本文的內容對大家的學習或者工作具有一定的參考學習價值，如果有疑問大家可以留言交流，謝謝大家對服務器之家的支持。
原文鏈接：https://www.cnblogs.com/lizhizhang/p/9502862.html
