HttpWebRequest 二三事

转自:http://www.cnblogs.com/1971ruru/archive/2012/04/11/2442589.html

随着REST风格的流行，直接通过 HttpWebRequest 进行服务调用的客户端应用越来越多。这里总结一些可能需要费时调查的经验，希望能帮助大家。

1. 用完的HttpWebRequest要Abort()或者要把 Response.Close()
否则会导致请求Timeout。 (HttpWebRequest.Method默认是GET)

static void Main( string [] args)
{
for ( int i = 0; i < 10; i++)
{
Console.Write( "[{0}] Request - " , i + 1);
TryGet( "https://login.live.com/" );
}
Console.Read();
}
static void TryGet( object obj)
{
try
{
HttpWebRequest webReq = null ;
string url = (  string )obj;
webReq = (HttpWebRequest)HttpWebRequest.Create(url);
webReq.Timeout = 20 * 1000;
var resp = webReq.GetResponse() as HttpWebResponse;
resp.Close();
Console.WriteLine( "Get Response StatusCode: {0}({1})" ,
resp.StatusCode, ( int )resp.StatusCode);
}
catch (WebException we)
{
Console.WriteLine( "Get Response StatusCode: {0}({1})" ,
we.Status, ( int )we.Status);
}
catch (Exception ex)
{
Console.WriteLine(ex);
}
}

static void Main(string[] args) { for (int i = 0; i < 10; i++) { Console.Write("[{0}] Request - ", i + 1); TryGet("https://login.live.com/"); } Console.Read(); } static void TryGet(object obj) { try { HttpWebRequest webReq = null; string url = (string)obj; webReq = (HttpWebRequest)HttpWebRequest.Create(url); webReq.Timeout = 20 * 1000; var resp = webReq.GetResponse() as HttpWebResponse; resp.Close(); Console.WriteLine("Get Response StatusCode: {0}({1})", resp.StatusCode, (int)resp.StatusCode); } catch (WebException we) { Console.WriteLine("Get Response StatusCode: {0}({1})", we.Status, (int)we.Status); } catch (Exception ex) { Console.WriteLine(ex); } }

上面的代码，会从第3次Request开始出现Timeout，因为GetResponse 后 Stream打开未关闭。

解决方法：上面的代码中加上 resp.Close(); 或者 webReq.Abort(); 就能解决。

2. 多线程中调用 HttpWebRequest 时，需要设置 ServicePointManager.DefaultConnectionLimit 数(默认连接数是 2)。
当多线程请求时，同时的连接数超过Limit时，GetResponse会抛出 Timeout WebException。

// 用多线程同时发出4个请求
WaitCallback methodTarget = new WaitCallback(TryGet);
ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/" );
ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/" );
ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/" );
ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/" );

// 用多线程同时发出4个请求 WaitCallback methodTarget = new WaitCallback(TryGet); ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/"); ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/"); ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/"); ThreadPool.QueueUserWorkItem(methodTarget, "https://login.live.com/");

解决方法：在GetResponse()之前设置 ServicePointManager.DefaultConnectionLimit = 100;

3. 当请求一个基于SSL的服务时，默认的验证行为都在 ServicePointManager 定义：
ServicePointManager.CheckCertificateRevocationList = true;

如果请求的服务端证书没有第三方的认证支持，则请求会失败，如果要完全信任服务端证书，则可以将
CheckCertificateRevocationList 设为 false。

4. 可以在 <system.net> 配置节中配置 HttpWebRequest 的属性，包括 WebProxy

<system.net>
<connectionManagement>
</connectionManagement>
<defaultProxy>
<proxy proxyaddress= "http://xxx.xxx.xxx.xxx:xxx" bypassonlocal= "False" />
</defaultProxy>
<settings>
<httpWebRequest useUnsafeHeaderParsing= "true" />
<servicePointManager checkCertificateName= "true"
checkCertificateRevocationList= "true"
enableDnsRoundRobin= "true"
expect100Continue= "true"
useNagleAlgorithm= "true" />
</settings>
</system.net>