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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();
services.TryAddSingleton();
//
// Typed Clients
//
services.TryAdd(ServiceDescriptor.Singleton(typeof(ITypedHttpClientFactory<>), typeof(DefaultTypedHttpClientFactory<>)));
//
// Misc infrastructure
//
services.TryAddEnumerable(ServiceDescriptor.Singleton());
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的，每个键值对应的ActiveHandlerTrackingEntry对象也是，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，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, IOptionsMonitoroptionsMonitor, IEnumerable filters)
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