DotNetCore深入了解之HttpClientFactory类详解

技术  /  管理员 发布于 7年前   227

当需要向某特定URL地址发送HTTP请求并得到相应响应时，通常会用到HttpClient类。该类包含了众多有用的方法，可以满足绝大多数的需求。但是如果对其使用不当时，可能会出现意想不到的事情。

using(var client = new HttpClient())

对象所占用资源应该确保及时被释放掉，但是，对于网络连接而言，这是错误的。

原因有二，网络连接是需要耗费一定时间的，频繁开启与关闭连接，性能会受影响；再者，开启网络连接时会占用底层socket资源，但在HttpClient调用其本身的Dispose方法时，并不能立刻释放该资源，这意味着你的程序可能会因为耗尽连接资源而产生预期之外的异常。

所以比较好的解决方法是延长HttpClient对象的使用寿命，比如对其建一个静态的对象：

private static HttpClient Client = new HttpClient();

但从程序员的角度来看，这样的代码或许不够优雅。

所以在.NET Core 2.1中引入了新的HttpClientFactory类。

它的用法很简单，首先是对其进行IoC的注册：

 public void ConfigureServices(IServiceCollection services) {  services.AddHttpClient();  services.AddMvc(); }

然后通过IHttpClientFactory创建一个HttpClient对象，之后的操作如旧，但不需要担心其内部资源的释放：

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的源码：

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;}

它的内部为IHttpClientFactory接口绑定了DefaultHttpClientFactory类。

再看IHttpClientFactory接口中关键的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;}

HttpClient的创建不再是简单的new HttpClient()，而是传入了两个参数：HttpMessageHandler handler与bool disposeHandler。disposeHandler参数为false值时表示要重用内部的handler对象。handler参数则从上一句的代码可以看出是以name为键值从一字典中取出，又因为DefaultHttpClientFactory类是通过TryAddSingleton方法注册的，也就意味着其为单例，那么这个内部字典便是唯一的，每个键值对应的ActiveHandlerTrackingEntry对象也是唯一，该对象内部中包含着handler。

下一句代码StartHandlerEntryTimer(entry); 开启了ActiveHandlerTrackingEntry对象的过期计时处理。默认过期时间是2分钟。

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();}

先是将ActiveHandlerTrackingEntry对象传入新的ExpiredHandlerTrackingEntry对象。

public ExpiredHandlerTrackingEntry(ActiveHandlerTrackingEntry other){ Name = other.Name; _livenessTracker = new WeakReference(other.Handler); InnerHandler = other.Handler.InnerHandler;}

在其构造方法内部，handler对象通过弱引用方式关联着，不会影响其被GC释放。

然后新建的ExpiredHandlerTrackingEntry对象被放入专用的队列。

最后开始清理工作，定时器的时间间隔设定为每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(); }}

上述方法核心是判断是否handler对象已经被GC，如果是的话，则释放其内部资源，即网络连接。

回到最初创建HttpClient的代码，会发现并没有传入任何name参数值。这是得益于HttpClientFactoryExtensions类的扩展方法。

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

Options.DefaultName的值为string.Empty。

DefaultHttpClientFactory缺少无参数的构造方法，唯一的构造方法需要传入多个参数，这也意味着构建它时需要依赖其它一些类，所以目前只适用于在ASP.NET程序中使用，还无法应用到诸如控制台一类的程序，希望之后官方能够对其继续增强，使得应用范围变得更广。

 public DefaultHttpClientFactory(  IServiceProvider services,  ILoggerFactory loggerFactory,  IOptionsMonitor<HttpClientFactoryOptions> optionsMonitor,  IEnumerable<IHttpMessageHandlerBuilderFilter> filters)

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