.NET Core开发日志——HttpClientFactory
2019-11-05

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。

博客园官方团队就遇上过这样的问题,国外博主也记录过类似的情况,YOU"RE USING HTTPCLIENT WRONG AND IT IS DESTABILIZING YOUR SOFTWARE。

究其缘由是一句看似正确的代码引起的:

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 MyController : Controller{ IHttpClientFactory _httpClientFactory; public MyController(IHttpClientFactory httpClientFactory) { _httpClientFactory = httpClientFactory; } public IActionResult Index() { var client = _httpClientFactory.CreateClient(); var result = client.GetStringAsync("http://myurl/"); return View(); }}

第一眼瞧去,可能不明白AddHttpClient方法与IHttpClientFactory有什么关系,但查到其源码后就能一目了然:

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名称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,该对象内部中就包含着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。这是受益于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);}

在传入name参数值时,还可以传入一些配置信息:

public void ConfigureServices(IServiceCollection services){ services.AddHttpClient("github", c => { c.BaseAddress = new Uri("https://api.github.com/"); c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json"); c.DefaultRequestHeaders.Add("User-Agent", "HttpClientFactory-Sample"); }); services.AddHttpClient();}

HttpClientFactory所提供的功能确实值得一用,但最后还要提一下它的缺陷,由于它的构建需要依赖其它一些类,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。