.net core HttpClient 使用之掉坑解析（一）
在我们开发当中经常需要向特定URL地址发送Http请求操作，在.net core 中对httpClient使用不当会造成灾难性的问题，这篇文章主要来分享.net core中通过HttpFactory 工厂来使用HttpClient的正确打开方式。
一、前言
在我们开发当中经常需要向特定URL地址发送Http请求操作，在.net core 中对httpClient使用不当会造成灾难性的问题，这篇文章主要来分享.net core中通过IHttpClientFactory 工厂来使用HttpClient的正确打开方式。
二、HttpClient使用中的那些坑
2.1 错误使用
using(var client = new HttpClient())我们可以先来做一个简单的测试，代码如下：
public async Task<string> GetBaiduListAsync(string url) { var html = ""; for (var i = 0; i < 10; i++) { using (var client = new System.Net.Http.HttpClient()) { var result=await client.GetStringAsync(url); html += result; } } return html; }运行项目输出结果后，通过netstate查看下TCP连接情况：
虽然项目已经运行结束，但是连接依然存在，状态为" TIME_WAIT"（继续等待看是否还有延迟的包会传输过来；默认在windows下，TIME_WAIT状态将会使系统将会保持该连接 240s。
在高并发的情况下，连接来不及释放，socket被耗尽，耗尽之后就会出现喜闻乐见的一个错误：
错误原因:
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,原因有如下：
- 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；
- 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灾难性的问题。
对于上面的错误原因，大家可能会想到使用静态单例模式的HttpClient，如下：
private static HttpClient Client = new HttpClient();静态单例模式虽然可以解决上面问题，但是会带来另外一个问题：
- DNS变更会导致不能解析，DNS不会重新加载，需要重启才能变更（有兴趣的大佬可以去尝试一下）
三、正确使用及源码分析
HttpClientFactory 以模块化、可命名、可配置、弹性方式重建了 HttpClient 的使用方式： 由 DI 框架注入 IHttpClientFactory 工厂；由工厂创建 HttpClient 并从内部的 Handler 池分配请求 Handler。
.net core 2.1 开始引入了IHttpClientFactory 工厂类来自动管理IHttpClientFactory 类的创建和资源释放，可以通过Ioc 注入方式进行使用，代码如下：
services.AddControllers();services.AddHttpClient();调用代码如下：
private readonly IHttpClientFactory _clientFactory;public FirstController(IHttpClientFactory clientFactory){ _clientFactory = clientFactory;}/// <summary>/// /// </summary>/// <param name="url"></param>/// <returns></returns>public async Task<string> GetBaiduAsync(string url){ var client = _clientFactory.CreateClient(); var result = await client.GetStringAsync(url); return result;}代码中通过IHttpClientFactory 中的CreateClient()方法进行创建一个HttpClient 对象,但是没有看到有释放资源的动作，那它是怎么释放的呢？
我们来看看它的主要源代码
/// <summary>/// Creates a new <see cref="HttpClient"/> using the default configuration./// </summary>/// <param name="factory">The <see cref="IHttpClientFactory"/>.</param>/// <returns>An <see cref="HttpClient"/> configured using the default configuration.</returns>public static HttpClient CreateClient(this IHttpClientFactory factory){ if (factory == null) { throw new ArgumentNullException(nameof(factory)); } return factory.CreateClient(Options.DefaultName);}public HttpClient CreateClient(string name){ if (name == null) { throw new ArgumentNullException(nameof(name)); } var handler = CreateHandler(name); var client = new HttpClient(handler, disposeHandler: false); var options = _optionsMonitor.Get(name); for (var i = 0; i < options.HttpClientActions.Count; i++) { options.HttpClientActions[i](client); } return client;}public HttpMessageHandler CreateHandler(string name){ if (name == null) { throw new ArgumentNullException(nameof(name)); } var entry = _activeHandlers.GetOrAdd(name, _entryFactory).Value; StartHandlerEntryTimer(entry); return entry.Handler;}代码中可以看到创建HttpClent 时会先创建HttpMessageHandler对象,而CreateHandler 方法中调用了StartHandlerEntryTimer方法，该方法主要时启动清理释放定时器方法，核心代码如下：
public DefaultHttpClientFactory( IServiceProvider services, IServiceScopeFactory scopeFactory, ILoggerFactory loggerFactory, IOptionsMonitor<HttpClientFactoryOptions> optionsMonitor, IEnumerable<IHttpMessageHandlerBuilderFilter> filters) { if (services == null) { throw new ArgumentNullException(nameof(services)); } if (scopeFactory == null) { throw new ArgumentNullException(nameof(scopeFactory)); } if (loggerFactory == null) { throw new ArgumentNullException(nameof(loggerFactory)); } if (optionsMonitor == null) { throw new ArgumentNullException(nameof(optionsMonitor)); } if (filters == null) { throw new ArgumentNullException(nameof(filters)); } _services = services; _scopeFactory = scopeFactory; _optionsMonitor = optionsMonitor; _filters = filters.ToArray(); _logger = loggerFactory.CreateLogger<DefaultHttpClientFactory>(); // case-sensitive because named options is. _activeHandlers = new ConcurrentDictionary<string, Lazy<ActiveHandlerTrackingEntry>>(StringComparer.Ordinal); _entryFactory = (name) => { return new Lazy<ActiveHandlerTrackingEntry>(() => { return CreateHandlerEntry(name); }, LazyThreadSafetyMode.ExecutionAndPublication); }; _expiredHandlers = new ConcurrentQueue<ExpiredHandlerTrackingEntry>(); _expiryCallback = ExpiryTimer_Tick; _cleanupTimerLock = new object(); _cleanupActiveLock = new object(); } // Internal for tests 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(); } // Internal so it can be overridden in tests internal virtual void StartHandlerEntryTimer(ActiveHandlerTrackingEntry entry) { entry.StartExpiryTimer(_expiryCallback); }从微软源码分析，HttpClient继承自HttpMessageInvoker，而HttpMessageInvoker实质就是HttpClientHandler。
HttpClientFactory 创建的HttpClient，也即是HttpClientHandler，只是这些个HttpClient被放到了"池子"中，工厂每次在create的时候会自动判断是新建还是复用。(默认生命周期为2min)。
希望这篇文章对你有帮助，如果对你有帮助请点个推荐，感谢!
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