.net core HttpClient 使用之掉坑解析（一）

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!