DotNetCore深入了解之HttpClientFactory类详解

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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