一、HttpClient使用
在C#中，如果我们需要向某特定的URL地址发送Http请求的时候，通常会用到HttpClient类。会将HttpClient包裹在using内部进行声明和初始化，如下面的代码：
using (var httpClient = new HttpClient())
{
// 逻辑处理代码
}HttpClient类包含了许多有用的方法，使用上面的代码，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。
上面代码的技术范点：当你使用继承了IDisposable接口的对象时，建议在using代码块中声明和初始化，当using代码段执行完成后，会自动释放该对象而不需要手动进行显示Dispose操作。
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。具体原因有下面两点：
- 网络连接是需要耗费一定时间的，频繁开启与关闭连接，性能会受到影响。
- 开启网络连接时会占用低层socket资源，但在HttpClient调用其本身的Dispose方法时，并不能立即释放该资源，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。
看下面一段代码
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;
namespace HttpClientDemo
{
class Program
{
static void Main(string[] args)
{
//using (var httpClient = new HttpClient())
//{
// // 逻辑处理代码
//}
HttpAsync();
Console.WriteLine("Hello World!");
Console.Read();
}
public static async void HttpAsync()
{
for (int i = 0; i < 10; i++)
{
using (var client = new HttpClient())
{
var result = await client.GetAsync("http://www.baidu.com");
Console.WriteLine($"{i}:{result.StatusCode}");
}
}
}
}
}运行项目输出结果后，通过netstate查看下TCP连接情况，会发现连接依然存在，状态为“TIME_WAIT”（继续等待看是否还有延迟的包会传输过来）。
这里就会出现一个坑：在高并发的情况下，连接来不及释放，socket连接被耗尽，耗尽之后就会出现错误。就是会出现“各种套接字问题”。
那么如何解决这个问题呢？比较好的解决方法是延长HttpClient对象的使用寿命，实现HttpClient对象的复用，比如对其建一个静态的对象：
private static HttpClient Client = new HttpClient();
我们使用这种方式优化上面的代码
using System;
using System.Net.Http;
namespace HttpClientDemo
{
class Program
{
private static readonly HttpClient _client = new HttpClient();
static void Main(string[] args)
{
HttpAsync();
Console.WriteLine("Hello World");
Console.ReadKey();
}
public static async void HttpAsync()
{
for (int i = 0; i < 10; i++)
{
var result = await _client.GetAsync("http://www.baidu.com");
Console.WriteLine($"{i}:{result.StatusCode}");
}
}
}
}这样调整HttpClient的引用后，虽然可以解决一些问题，但是仍然存在一些问题：
- 因为是复用的HttpClient，那么一些公共的设置就没办法灵活的调整，如请求头的自定义。
- 因为HttpClient请求每个url时，会缓存url对应的主机ip，从而会导致DNS更新失效。
为了解决这些问题，在.NET Core 2.1中引入了新的HttpClientFactory类。
二、HttpClientFactory使用
微软在.NET Core 2.1中新引入了HttpClientFactory类，具有如下的优势：
- HttpClientFactory很高效，可以最大程度上节省系统的sock而。
- Factory，顾名思义HttpClientfactory就是HttpClient的工厂，内部已经帮我们处理好了对HttpClient的管理，不需要我们人工进行对象释放，同时，支持自定义请求头、支持DNS更新等。
我们用一个ASP.NET Core的程序作为示例，它的用法非常简单，首先是对其进行IOC注册：
public void ConfigureServices(IServiceCollection services)
{
// 注入HttpClient
services.AddHttpClient("client_1", config => //这里指定的name=client_1，可以方便我们后期服用该实例
{
config.BaseAddress = new Uri("http://www.baidu.com");
config.DefaultRequestHeaders.Add("header_1", "header_1");
});
services.AddHttpClient("client_2", config =>
{
config.BaseAddress = new Uri("https://www.qq.com/");
config.DefaultRequestHeaders.Add("header_2", "header_2");
});
services.AddHttpClient();
services.AddControllers();
}然后在控制器里面通过IHttpClientFactory创建一个HttpClient对象，之后的操作跟以前一样，但不需要担心其内部资源的释放：
using System.Net.Http;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
namespace HttpClientFactoryDemo.Controllers
{
[Route("api/[controller]")]
[ApiController]
public class DemoController : ControllerBase
{
IHttpClientFactory _httpClientFactory;
/// <summary>
/// 通过构造函数实现注入
/// </summary>
/// <param name="httpClientFactory"></param>
public DemoController(IHttpClientFactory httpClientFactory)
{
_httpClientFactory = httpClientFactory;
}
public async Task<string> Get()
{
var client = _httpClientFactory.CreateClient("client_1"); //复用在Startup中定义的client_1的httpclient
var result = await client.GetStringAsync("/page1.html");
var client2 = _httpClientFactory.CreateClient(); //新建一个HttpClient
var result2 = await client.GetAsync("http://www.baidu.com");
return result2.StatusCode.ToString();
}
}
}程序运行结果：
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;
}看下面这句代码：
services.TryAddSingleton<IHttpClientFactory, DefaultHttpClientFactory>();
这里添加依赖注入的时候为IHttpClientFactory接口绑定了DefaultHttpClientFactory类。
我们在来看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;
}从代码中我们可以看出：HttpClient的创建不在是简单的new HttpClient()，而是传入了两个参数：HttpMessageHandler handler与bool disposeHandler。
disposeHandler参数为false时表示要重用内部的handler对象。handler参数则从上一句的代码中可以看出是以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。
到此这篇关于.NET Core中的HttpClientFactory类用法的文章就介绍到这了。希望对大家的学习有所帮助，也希望大家多多支持阿兔在线工具。
