DotNetCore深入了解：HTTPClientFactory类

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：

1. 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。
2. 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：

1. 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。
2. 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：

1. HttpClientFactory很高效，可以最大程度上节省系统的sock而。
2. 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。

参考：https://blog.csdn.net/weixin_34321753/article/details/91362881

https://www.cnblogs.com/lizhizhang/p/9502862.html