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2023-11-19 06:00:03

HttpClient和HttpClientHandler必须在请求之间被处理吗?

system.net.httpclient和system.net.httpclienthandler在。net Framework 4.5中实现了IDisposable(通过System.Net.Http.HttpMessageInvoker)。

using语句文档说:

作为规则，当使用IDisposable对象时，应该声明和 在using语句中实例化它。

这个答案使用了这个模式:

var baseAddress = new Uri("http://example.com");
var cookieContainer = new CookieContainer();
using (var handler = new HttpClientHandler() { CookieContainer = cookieContainer })
using (var client = new HttpClient(handler) { BaseAddress = baseAddress })
{
    var content = new FormUrlEncodedContent(new[]
    {
        new KeyValuePair<string, string>("foo", "bar"),
        new KeyValuePair<string, string>("baz", "bazinga"),
    });
    cookieContainer.Add(baseAddress, new Cookie("CookieName", "cookie_value"));
    var result = client.PostAsync("/test", content).Result;
    result.EnsureSuccessStatusCode();
}

但是来自Microsoft的最明显的例子既没有显式调用也没有隐式调用Dispose()。例如:

最初的博客文章宣布HttpClient的发布。 HttpClient的实际MSDN文档。 BingTranslateSample GoogleMapsSample WorldBankSample

在公告的评论中，有人问微软员工:

在检查了你的样品后，我发现你没有进行处理 对HttpClient实例的操作。我已经使用了HttpClient的所有实例 在我的应用程序上使用声明，我认为这是正确的方式 因为HttpClient实现了IDisposable接口。我是在 正确的道路?

他的回答是:

一般来说，这是正确的，尽管你必须小心 "using"和async，因为它们在。net 4和。net 4.5中不能真正混合 可以在“using”语句中使用“await”。 顺便说一句，你可以重复使用同一个HttpClient，次数不限 通常情况下，您不会一直创建/处理它们。

第二段对这个问题来说是多余的，它不关心您可以使用HttpClient实例多少次，而是关心在您不再需要它之后是否有必要处理它。

(更新:事实上，第二段是答案的关键，如下文由@DPeden提供。)

所以我的问题是:

Is it necessary, given the current implementation (.NET Framework 4.5), to call Dispose() on HttpClient and HttpClientHandler instances? Clarification: by "necessary" I mean if there are any negative consequences for not disposing, such as resource leakage or data corruption risks. If it's not necessary, would it be a "good practice" anyway, since they implement IDisposable? If it's necessary (or recommended), is this code mentioned above implementing it safely (for .NET Framework 4.5)? If these classes don't require calling Dispose(), why were they implemented as IDisposable? If they require, or if it's a recommended practice, are the Microsoft examples misleading or unsafe?

当前回答

如果你想要释放HttpClient，你可以把它设置为一个资源池。在应用程序的末尾，分配资源池。

代码:

// Notice that IDisposable is not implemented here!
public interface HttpClientHandle
{
    HttpRequestHeaders DefaultRequestHeaders { get; }
    Uri BaseAddress { get; set; }
    // ...
    // All the other methods from peeking at HttpClient
}

public class HttpClientHander : HttpClient, HttpClientHandle, IDisposable
{
    public static ConditionalWeakTable<Uri, HttpClientHander> _httpClientsPool;
    public static HashSet<Uri> _uris;

    static HttpClientHander()
    {
        _httpClientsPool = new ConditionalWeakTable<Uri, HttpClientHander>();
        _uris = new HashSet<Uri>();
        SetupGlobalPoolFinalizer();
    }

    private DateTime _delayFinalization = DateTime.MinValue;
    private bool _isDisposed = false;

    public static HttpClientHandle GetHttpClientHandle(Uri baseUrl)
    {
        HttpClientHander httpClient = _httpClientsPool.GetOrCreateValue(baseUrl);
        _uris.Add(baseUrl);
        httpClient._delayFinalization = DateTime.MinValue;
        httpClient.BaseAddress = baseUrl;

        return httpClient;
    }

    void IDisposable.Dispose()
    {
        _isDisposed = true;
        GC.SuppressFinalize(this);

        base.Dispose();
    }

    ~HttpClientHander()
    {
        if (_delayFinalization == DateTime.MinValue)
            _delayFinalization = DateTime.UtcNow;
        if (DateTime.UtcNow.Subtract(_delayFinalization) < base.Timeout)
            GC.ReRegisterForFinalize(this);
    }

    private static void SetupGlobalPoolFinalizer()
    {
        AppDomain.CurrentDomain.ProcessExit +=
            (sender, eventArgs) => { FinalizeGlobalPool(); };
    }

    private static void FinalizeGlobalPool()
    {
        foreach (var key in _uris)
        {
            HttpClientHander value = null;
            if (_httpClientsPool.TryGetValue(key, out value))
                try { value.Dispose(); } catch { }
        }

        _uris.Clear();
        _httpClientsPool = null;
    }
}

var handler = HttpClientHander.GetHttpClientHandle（new Uri（“base url”））.

HttpClient作为一个接口，不能调用Dispose()。 Dispose()将被垃圾回收器以延迟的方式调用。 或者当程序通过析构函数清理对象时。 使用弱引用+延迟清理逻辑，因此只要它被频繁重用，它就会一直被使用。 它只为传递给它的每个基本URL分配一个新的HttpClient。原因由Ohad Schneider解释，答案如下。更改基本url时的不良行为。 HttpClientHandle允许在测试中进行mock

2017-12-08 20:22:53

其他回答

如果你想要释放HttpClient，你可以把它设置为一个资源池。在应用程序的末尾，分配资源池。

代码:

// Notice that IDisposable is not implemented here!
public interface HttpClientHandle
{
    HttpRequestHeaders DefaultRequestHeaders { get; }
    Uri BaseAddress { get; set; }
    // ...
    // All the other methods from peeking at HttpClient
}

public class HttpClientHander : HttpClient, HttpClientHandle, IDisposable
{
    public static ConditionalWeakTable<Uri, HttpClientHander> _httpClientsPool;
    public static HashSet<Uri> _uris;

    static HttpClientHander()
    {
        _httpClientsPool = new ConditionalWeakTable<Uri, HttpClientHander>();
        _uris = new HashSet<Uri>();
        SetupGlobalPoolFinalizer();
    }

    private DateTime _delayFinalization = DateTime.MinValue;
    private bool _isDisposed = false;

    public static HttpClientHandle GetHttpClientHandle(Uri baseUrl)
    {
        HttpClientHander httpClient = _httpClientsPool.GetOrCreateValue(baseUrl);
        _uris.Add(baseUrl);
        httpClient._delayFinalization = DateTime.MinValue;
        httpClient.BaseAddress = baseUrl;

        return httpClient;
    }

    void IDisposable.Dispose()
    {
        _isDisposed = true;
        GC.SuppressFinalize(this);

        base.Dispose();
    }

    ~HttpClientHander()
    {
        if (_delayFinalization == DateTime.MinValue)
            _delayFinalization = DateTime.UtcNow;
        if (DateTime.UtcNow.Subtract(_delayFinalization) < base.Timeout)
            GC.ReRegisterForFinalize(this);
    }

    private static void SetupGlobalPoolFinalizer()
    {
        AppDomain.CurrentDomain.ProcessExit +=
            (sender, eventArgs) => { FinalizeGlobalPool(); };
    }

    private static void FinalizeGlobalPool()
    {
        foreach (var key in _uris)
        {
            HttpClientHander value = null;
            if (_httpClientsPool.TryGetValue(key, out value))
                try { value.Dispose(); } catch { }
        }

        _uris.Clear();
        _httpClientsPool = null;
    }
}

var handler = HttpClientHander.GetHttpClientHandle（new Uri（“base url”））.

HttpClient作为一个接口，不能调用Dispose()。 Dispose()将被垃圾回收器以延迟的方式调用。 或者当程序通过析构函数清理对象时。 使用弱引用+延迟清理逻辑，因此只要它被频繁重用，它就会一直被使用。 它只为传递给它的每个基本URL分配一个新的HttpClient。原因由Ohad Schneider解释，答案如下。更改基本url时的不良行为。 HttpClientHandle允许在测试中进行mock

2017-12-08 20:22:53

不，不要在每个请求时都创建一个新的(即使你处理了旧的)。您将导致服务器本身(不仅仅是应用程序)因为操作系统上网络级别的端口耗尽而崩溃!

请阅读下面我对一个非常类似的问题的回答。很明显，您应该将HttpClient实例视为单例，并在请求之间重用。

在WebAPI客户端中每次调用创建一个新的HttpClient的开销是多少?

2019-10-15 21:25:16

一般的共识是您不(不应该)需要处理HttpClient。

许多密切参与其工作方式的人都说过这一点。

请参阅Darrel Miller的博客文章和相关的SO文章:HttpClient爬行导致内存泄漏。

我还强烈建议你阅读《用ASP设计可进化的Web api》中的HttpClient章节。NET提供关于底层发生了什么的上下文，特别是这里引用的“生命周期”部分:

Although HttpClient does indirectly implement the IDisposable interface, the standard usage of HttpClient is not to dispose of it after every request. The HttpClient object is intended to live for as long as your application needs to make HTTP requests. Having an object exist across multiple requests enables a place for setting DefaultRequestHeaders and prevents you from having to re-specify things like CredentialCache and CookieContainer on every request as was necessary with HttpWebRequest.

甚至可以打开DotPeek。

2013-03-29 17:49:38

在构造函数中使用依赖注入使管理HttpClient的生命周期变得更容易——将生命周期管理置于需要它的代码之外，并使它在以后的日期中容易更改。

我目前的偏好是创建一个单独的http客户端类，每个目标端点域从HttpClient继承一次，然后使用依赖注入使其成为单例。公共类ExampleHttpClient: HttpClient{…｝

然后，我在需要访问该API的服务类中的自定义http客户机上获取构造函数依赖项。这解决了生存期问题，并且在连接池方面具有优势。

您可以在https://stackoverflow.com/a/50238944/3140853的相关回答中看到一个工作示例

2018-05-08 17:37:16

Dispose()调用下面的代码，关闭HttpClient实例打开的连接。代码是通过使用dotPeek进行反编译创建的。

HttpClientHandler.cs -特性

ServicePointManager.CloseConnectionGroups(this.connectionGroupName);

如果你不调用dispose，那么ServicePointManager。由定时器运行的MaxServicePointIdleTime将关闭http连接。缺省值是100秒。

ServicePointManager.cs

internal static readonly TimerThread.Callback s_IdleServicePointTimeoutDelegate = new TimerThread.Callback(ServicePointManager.IdleServicePointTimeoutCallback);
private static volatile TimerThread.Queue s_ServicePointIdlingQueue = TimerThread.GetOrCreateQueue(100000);

private static void IdleServicePointTimeoutCallback(TimerThread.Timer timer, int timeNoticed, object context)
{
  ServicePoint servicePoint = (ServicePoint) context;
  if (Logging.On)
    Logging.PrintInfo(Logging.Web, SR.GetString("net_log_closed_idle", (object) "ServicePoint", (object) servicePoint.GetHashCode()));
  lock (ServicePointManager.s_ServicePointTable)
    ServicePointManager.s_ServicePointTable.Remove((object) servicePoint.LookupString);
  servicePoint.ReleaseAllConnectionGroups();
}

如果你还没有将空闲时间设置为无限，那么不调用dispose并让空闲连接计时器启动并为你关闭连接似乎是安全的，尽管如果你知道你已经完成了HttpClient实例并更快地释放资源，那么在using语句中调用dispose会更好。

2017-10-26 22:49:51

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