.net core HttpClient 使用之掉坑解析（一）
- 2020 年 5 月 13 日
- 筆記
- Asp.Net Core, Asp.Net Core 源碼, HttpClient, IHttpClientFactory
一、前言
在我們開發當中經常需要向特定URL地址發送Http請求操作，在.net core 中對httpClient使用不當會造成災難性的問題，這篇文章主要來分享.net core中通過IHttpClientFactory 工廠來使用HttpClient的正確打開方式。
二、HttpClient使用中的那些坑
2.1 錯誤使用
using(var client = new HttpClient())
我們可以先來做一個簡單的測試，程式碼如下：
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;
}
運行項目輸出結果後，通過netstate查看下TCP連接情況：
雖然項目已經運行結束，但是連接依然存在，狀態為” TIME_WAIT”（繼續等待看是否還有延遲的包會傳輸過來；默認在windows下，TIME_WAIT狀態將會使系統將會保持該連接 240s。
在高並發的情況下，連接來不及釋放，socket被耗盡，耗盡之後就會出現喜聞樂見的一個錯誤：
錯誤原因:
對象所佔用資源應該確保及時被釋放掉，但是，對於網路連接而言，這是錯誤的,原因有如下：
- 網路連接是需要耗費一定時間的，頻繁開啟與關閉連接，性能會受影響；
- 開啟網路連接時會佔用底層socket資源，但在HttpClient調用其本身的Dispose方法時，並不能立刻釋放該資源，這意味著你的程式可能會因為耗盡連接資源而產生災難性的問題。
對於上面的錯誤原因，大家可能會想到使用靜態單例模式的HttpClient，如下：
private static HttpClient Client = new HttpClient();
靜態單例模式雖然可以解決上面問題，但是會帶來另外一個問題：
- DNS變更會導致不能解析，DNS不會重新載入，需要重啟才能變更（有興趣的大佬可以去嘗試一下）
三、正確使用及源碼分析
HttpClientFactory 以模組化、可命名、可配置、彈性方式重建了 HttpClient 的使用方式： 由 DI 框架注入 IHttpClientFactory 工廠；由工廠創建 HttpClient 並從內部的 Handler 池分配請求 Handler。
.net core 2.1 開始引入了IHttpClientFactory 工廠類來自動管理IHttpClientFactory 類的創建和資源釋放，可以通過Ioc 注入方式進行使用，程式碼如下：
services.AddControllers();
services.AddHttpClient();
調用程式碼如下：
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;
}
程式碼中通過IHttpClientFactory 中的CreateClient()方法進行創建一個HttpClient 對象,但是沒有看到有釋放資源的動作，那它是怎麼釋放的呢？
我們來看看它的主要源程式碼
/// <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;
}
程式碼中可以看到創建HttpClent 時會先創建HttpMessageHandler對象,而CreateHandler 方法中調用了StartHandlerEntryTimer方法，該方法主要時啟動清理釋放定時器方法，核心程式碼如下：
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);
}
從微軟源碼分析，HttpClient繼承自HttpMessageInvoker，而HttpMessageInvoker實質就是HttpClientHandler。
HttpClientFactory 創建的HttpClient，也即是HttpClientHandler，只是這些個HttpClient被放到了「池子」中，工廠每次在create的時候會自動判斷是新建還是復用。(默認生命周期為2min)。
希望這篇文章對你有幫助，如果對你有幫助請點個推薦，感謝!
