ServicePoint 類別

參考

定義

命名空間:: System.Net

組件:: System.Net.ServicePoint.dll

組件:: System.Net.Requests.dll

組件:: System.dll

組件:: netstandard.dll

來源:: ServicePoint.cs

來源:: ServicePoint.cs

來源:: ServicePoint.cs

重要

部分資訊涉及發行前產品，在發行之前可能會有大幅修改。 Microsoft 對此處提供的資訊，不做任何明確或隱含的瑕疵擔保。

提供 HTTP 連線的連線管理。

public ref class ServicePoint

public class ServicePoint

type ServicePoint = class

Public Class ServicePoint

繼承: Object
ServicePoint

範例

下列程式代碼範例會建立連線至 URI www.contoso.comServicePoint 物件。

// This example shows how to use the ServicePoint and ServicePointManager classes.
// The ServicePointManager class uses the ServicePoint class to manage connections
// to a remote host. The networking classes reuse service points for all 
// requests to a given URI. In fact, the same ServicePoint object 
// is used to issue requests to Internet resources identified by the same
// scheme identifier (for example,  HTTP) and host fragment (for example,  www.contoso.com).  
// This should improve your application performance. 
// Reusing service points in this way can help improve application performance.
#using <System.dll>

using namespace System;
using namespace System::Net;
using namespace System::Threading;
using namespace System::Text::RegularExpressions;
void ShowProperties( ServicePoint^ sp )
{
   Console::WriteLine( "Done calling FindServicePoint()..." );
   
   // Display the ServicePoint Internet resource address.
   Console::WriteLine( "Address = {0}", sp->Address );
   
   // Display the date and time that the ServicePoint was last 
   // connected to a host.
   Console::WriteLine( "IdleSince = {0}", sp->IdleSince );
   
   // Display the maximum length of time that the ServicePoint instance  
   // is allowed to maintain an idle connection to an Internet  
   // resource before it is recycled for use in another connection.
   Console::WriteLine( "MaxIdleTime = {0}", sp->MaxIdleTime );
   
   Console::WriteLine( "ConnectionName = {0}", sp->ConnectionName );
   
   // Display the maximum number of connections allowed on this 
   // ServicePoint instance.
   Console::WriteLine( "ConnectionLimit = {0}", sp->ConnectionLimit );
   
   // Display the number of connections associated with this 
   // ServicePoint instance.
   Console::WriteLine( "CurrentConnections = {0}", sp->CurrentConnections );
   
   if ( sp->Certificate == nullptr )
      Console::WriteLine( "Certificate = (null)" );
   else
      Console::WriteLine( "Certificate = {0}", sp->Certificate );

   if ( sp->ClientCertificate == nullptr )
      Console::WriteLine( "Client Certificate = (null)" );
   else
      Console::WriteLine( "Client Certificate = {0}", sp->ClientCertificate );

   Console::WriteLine( "ProtocolVersion = {0}", sp->ProtocolVersion->ToString() );
   Console::WriteLine( "SupportsPipelining = {0}", sp->SupportsPipelining );
   
   Console::WriteLine( "UseNagleAlgorithm = {0} ", sp->UseNagleAlgorithm.ToString() );
   Console::WriteLine( "Expect 100-continue = {0}", sp->Expect100Continue.ToString() );
   
}


void makeWebRequest( int hashCode, String^ Uri )
{
   HttpWebResponse^ res = nullptr;
   
   // Make sure that the idle time has elapsed, so that a new 
   // ServicePoint instance is created.
   Console::WriteLine( "Sleeping for 2 sec." );
   Thread::Sleep( 2000 );
   try
   {
      
      // Create a request to the passed URI.
      HttpWebRequest^ req = dynamic_cast<HttpWebRequest^>(WebRequest::Create( Uri ));
      Console::WriteLine( "\nConnecting to {0} ............", Uri );
      
      // Get the response object.
      res = dynamic_cast<HttpWebResponse^>(req->GetResponse());
      Console::WriteLine( "Connected.\n" );
      ServicePoint^ currentServicePoint = req->ServicePoint;
      
      // Display new service point properties.
      int currentHashCode = currentServicePoint->GetHashCode();
      Console::WriteLine( "New service point hashcode: {0}", currentHashCode );
      Console::WriteLine( "New service point max idle time: {0}", currentServicePoint->MaxIdleTime );
      Console::WriteLine( "New service point is idle since {0}", currentServicePoint->IdleSince );
      
      // Check that a new ServicePoint instance has been created.
      if ( hashCode == currentHashCode )
            Console::WriteLine( "Service point reused." );
      else
            Console::WriteLine( "A new service point created." );
   }
   catch ( Exception^ e ) 
   {
      Console::WriteLine( "Source : {0}", e->Source );
      Console::WriteLine( "Message : {0}", e->Message );
   }
   finally
   {
      if ( res != nullptr )
            res->Close();
   }

}


// Show the user how to use this program when wrong inputs are entered.
void showUsage()
{
   Console::WriteLine( "Enter the proxy name as follows:" );
   Console::WriteLine( "\tcs_servicepoint proxyName" );
}


int main()
{
   array<String^>^args = Environment::GetCommandLineArgs();
   int port = 80;
   
   // Define a regular expression to parse the user's input.
   // This is a security check. It allows only
   // alphanumeric input strings between 2 to 40 characters long.
   Regex^ rex = gcnew Regex( "^[a-zA-Z]\\w{1,39}$" );
   if ( args->Length < 2 )
   {
      showUsage();
      return  -1;
   }

   String^ proxy = args[ 1 ];
   if ( (rex->Match(proxy))->Success != true )
   {
      Console::WriteLine( "Input string format not allowed." );
      return  -1;
   }

   String^ proxyAdd = String::Format( "http://{0}:{1}", proxy, port );
   
   // Create a proxy object.  
   WebProxy^ DefaultProxy = gcnew WebProxy( proxyAdd,true );
   
   // Set the proxy that all HttpWebRequest instances use.
   WebRequest::DefaultWebProxy = DefaultProxy;
   
   // Get the base interface for proxy access for the 
   // WebRequest-based classes.
   IWebProxy^ Iproxy = WebRequest::DefaultWebProxy;

   
   // Set the maximum number of ServicePoint instances to 
   // maintain. If a ServicePoint instance for that host already 
   // exists when your application requests a connection to
   // an Internet resource, the ServicePointManager object
   // returns this existing ServicePoint instance. If none exists 
   // for that host, it creates a new ServicePoint instance.
   ServicePointManager::MaxServicePoints = 4;
   
   // Set the maximum idle time of a ServicePoint instance to 10 seconds.
   // After the idle time expires, the ServicePoint object is eligible for
   // garbage collection and cannot be used by the ServicePointManager.
   ServicePointManager::MaxServicePointIdleTime = 10000;
   
   ServicePointManager::UseNagleAlgorithm = true;
   ServicePointManager::Expect100Continue = true;
   ServicePointManager::CheckCertificateRevocationList = true;
   ServicePointManager::DefaultConnectionLimit = ServicePointManager::DefaultPersistentConnectionLimit;
   
   // Create the Uri object for the resource you want to access.
   Uri^ MS = gcnew Uri( "http://msdn.microsoft.com/" );
   
   // Use the FindServicePoint method to find an existing 
   // ServicePoint object or to create a new one.   
   ServicePoint^ servicePoint = ServicePointManager::FindServicePoint( MS, Iproxy );
   ShowProperties( servicePoint );
   int hashCode = servicePoint->GetHashCode();
   Console::WriteLine( "Service point hashcode: {0}", hashCode );
   
   // Make a request with the same scheme identifier and host fragment
   // used to create the previous ServicePoint object.
   makeWebRequest( hashCode, "http://msdn.microsoft.com/library/" );
}

// This example shows how to use the ServicePoint and ServicePointManager classes.
// The ServicePointManager class uses the ServicePoint class to manage connections
// to a remote host. The networking classes reuse service points for all
// requests to a given URI. In fact, the same ServicePoint object
// is used to issue requests to Internet resources identified by the same
// scheme identifier (for example,  HTTP) and host fragment (for example,  www.contoso.com).
// This should improve your application performance.
// Reusing service points in this way can help improve application performance.
using System;
using System.Net;
using System.Threading;
using System.Text.RegularExpressions;

namespace Mssc.Services.ConnectionManagement
{
    class TestServicePoint
    {
        private static void ShowProperties(ServicePoint sp)
        {
            Console.WriteLine("Done calling FindServicePoint()...");

            // Display the ServicePoint Internet resource address.
            Console.WriteLine("Address = {0} ", sp.Address.ToString());

            // Display the date and time that the ServicePoint was last
            // connected to a host.
            Console.WriteLine("IdleSince = " + sp.IdleSince.ToString());

            // Display the maximum length of time that the ServicePoint instance
            // is allowed to maintain an idle connection to an Internet
            // resource before it is recycled for use in another connection.
            Console.WriteLine("MaxIdleTime = " + sp.MaxIdleTime);

            Console.WriteLine("ConnectionName = " + sp.ConnectionName);

            // Display the maximum number of connections allowed on this
            // ServicePoint instance.
            Console.WriteLine("ConnectionLimit = " + sp.ConnectionLimit);

            // Display the number of connections associated with this
            // ServicePoint instance.
            Console.WriteLine("CurrentConnections = " + sp.CurrentConnections);

            if (sp.Certificate == null)
                Console.WriteLine("Certificate = (null)");
            else
                Console.WriteLine("Certificate = " + sp.Certificate.ToString());

            if (sp.ClientCertificate == null)
                Console.WriteLine("ClientCertificate = (null)");
            else
                Console. WriteLine("ClientCertificate = " + sp.ClientCertificate.ToString());

            Console.WriteLine("ProtocolVersion = " + sp.ProtocolVersion.ToString());
            Console.WriteLine("SupportsPipelining = " + sp.SupportsPipelining);

            Console.WriteLine("UseNagleAlgorithm = " + sp.UseNagleAlgorithm.ToString());
            Console.WriteLine("Expect 100-continue = " + sp.Expect100Continue.ToString());
        }

        private static void makeWebRequest(int hashCode, string Uri)
        {
            HttpWebResponse res = null;

            // Make sure that the idle time has elapsed, so that a new
            // ServicePoint instance is created.
            Console.WriteLine("Sleeping for 2 sec.");
            Thread.Sleep(2000);
            try
            {
                // Create a request to the passed URI.
                HttpWebRequest req = (HttpWebRequest)WebRequest.Create(Uri);

                Console.WriteLine("\nConnecting to " + Uri + " ............");

                // Get the response object.
                res = (HttpWebResponse)req.GetResponse();
                Console.WriteLine("Connected.\n");

                ServicePoint currentServicePoint = req.ServicePoint;

                // Display new service point properties.
                int currentHashCode = currentServicePoint.GetHashCode();

                Console.WriteLine("New service point hashcode: " + currentHashCode);
                Console.WriteLine("New service point max idle time: " + currentServicePoint.MaxIdleTime);
                Console.WriteLine("New service point is idle since " + currentServicePoint.IdleSince );

                // Check that a new ServicePoint instance has been created.
                if (hashCode == currentHashCode)
                    Console.WriteLine("Service point reused.");
                else
                    Console.WriteLine("A new service point created.") ;
            }
            catch (Exception e)
            {
                Console.WriteLine("Source : " + e.Source);
                Console.WriteLine("Message : " + e.Message);
            }
            finally
            {
                if (res != null)
                    res.Close();
            }
        }

        // Show the user how to use this program when wrong inputs are entered.
        private static void showUsage()
        {
            Console.WriteLine("Enter the proxy name as follows:");
            Console.WriteLine("\tcs_servicepoint proxyName");
        }

        public static void Main(string[] args)
        {
            int port = 80;

            // Define a regular expression to parse the user's input.
            // This is a security check. It allows only
            // alphanumeric input strings between 2 to 40 characters long.
            Regex rex = new Regex(@"^[a-zA-Z]\w{1,39}$");

            if (args.Length < 1)
            {
                showUsage();
                return;
            }
            string proxy = args[0];

            if ((rex.Match(proxy)).Success != true)
            {
                Console.WriteLine("Input string format not allowed.");
                return;
            }
            string proxyAdd = "http://" + proxy + ":" + port;

            // Create a proxy object.
            WebProxy DefaultProxy = new WebProxy(proxyAdd, true);

            // Set the proxy that all HttpWebRequest instances use.
            WebRequest.DefaultWebProxy = DefaultProxy;

            // Get the base interface for proxy access for the
            // WebRequest-based classes.
            IWebProxy Iproxy = WebRequest.DefaultWebProxy;

            // Set the maximum number of ServicePoint instances to
            // maintain. If a ServicePoint instance for that host already
            // exists when your application requests a connection to
            // an Internet resource, the ServicePointManager object
            // returns this existing ServicePoint instance. If none exists
            // for that host, it creates a new ServicePoint instance.
            ServicePointManager.MaxServicePoints = 4;

            // Set the maximum idle time of a ServicePoint instance to 10 seconds.
            // After the idle time expires, the ServicePoint object is eligible for
            // garbage collection and cannot be used by the ServicePointManager object.
            ServicePointManager.MaxServicePointIdleTime = 10000;

            ServicePointManager.UseNagleAlgorithm = true;
            ServicePointManager.Expect100Continue = true;
            ServicePointManager.CheckCertificateRevocationList = true;
            ServicePointManager.DefaultConnectionLimit = ServicePointManager.DefaultPersistentConnectionLimit;
            // Create the Uri object for the resource you want to access.
            Uri MS = new Uri("http://msdn.microsoft.com/");

            // Use the FindServicePoint method to find an existing
            // ServicePoint object or to create a new one.
            ServicePoint servicePoint = ServicePointManager.FindServicePoint(MS, Iproxy);

            ShowProperties(servicePoint);

            int hashCode = servicePoint.GetHashCode();

            Console.WriteLine("Service point hashcode: " + hashCode);

            // Make a request with the same scheme identifier and host fragment
            // used to create the previous ServicePoint object.
            makeWebRequest(hashCode, "http://msdn.microsoft.com/library/");
        }
    }
}

' This example shows how to use the ServicePoint and ServicePointManager classes.
' The ServicePointManager class uses the ServicePoint class to manage connections
' to a remote host. The networking classes reuse service points for all 
' requests to a given URI. In fact, the same ServicePoint object 
' is used to issue requests to Internet resources identified by the same
' scheme identifier (for example,  HTTP) and host fragment (for example,  www.contoso.com).  
' Reusing service points in this way can help improve application  performance.

Imports System.Net
Imports System.Threading
Imports System.Text.RegularExpressions

Namespace Mssc.Services.ConnectionManagement
    Module M_TestServicePoint

        Class TestServicePoint
            Private Shared Sub ShowProperties(ByVal sp As ServicePoint)

                Console.WriteLine("Done calling FindServicePoint")

                ' Display the ServicePoint Internet resource address.
                Console.WriteLine(("Address = " + sp.Address.ToString()))

                ' Display the date and time that the ServicePoint was last 
                ' connected to a host.
                Console.WriteLine(("IdleSince = " + sp.IdleSince.ToString()))


                ' Display the maximum length of time that the ServicePoint instance 
                ' is allowed to maintain an idle connection to an Internet  
                ' resource before it is recycled for use in another connection.
                Console.WriteLine(("MaxIdleTime = " + sp.MaxIdleTime.ToString()))

                Console.WriteLine(("ConnectionName = " + sp.ConnectionName))

                ' Display the maximum number of connections allowed on this 
                ' ServicePoint instance.
                Console.WriteLine(("ConnectionLimit = " + sp.ConnectionLimit.ToString()))

                ' Display the number of connections associated with this 
                ' ServicePoint instance.
                Console.WriteLine(("CurrentConnections = " + sp.CurrentConnections.ToString()))

                If sp.Certificate Is Nothing Then
                    Console.WriteLine("Certificate = (null)")
                Else
                    Console.WriteLine(("Certificate = " + sp.Certificate.ToString()))
                End If

                If sp.ClientCertificate Is Nothing Then
                    Console.WriteLine("ClientCertificate = (null)")
                Else
                    Console.WriteLine(("ClientCertificate = " + sp.ClientCertificate.ToString()))
                End If

                Console.WriteLine("ProtocolVersion = " + sp.ProtocolVersion.ToString())
                Console.WriteLine(("SupportsPipelining = " + sp.SupportsPipelining.ToString()))


                Console.WriteLine("UseNagleAlgorithm = " + sp.UseNagleAlgorithm.ToString())
                Console.WriteLine("Expect 100-continue = " + sp.Expect100Continue.ToString())
            End Sub





            Private Shared Sub makeWebRequest(ByVal hashCode As Integer, ByVal Uri As String)
                Dim res As HttpWebResponse = Nothing

                ' Make sure that the idle time has elapsed, so that a new 
                ' ServicePoint instance is created.
                Console.WriteLine("Sleeping for 2 sec.")
                Thread.Sleep(2000)

                Try
                    ' Create a request to the passed URI.
                    Dim req As HttpWebRequest = CType(WebRequest.Create(Uri), HttpWebRequest)
                    Console.WriteLine((ControlChars.Lf + "Connecting to " + Uri + " ............"))

                    ' Get the response object.
                    res = CType(req.GetResponse(), HttpWebResponse)
                    Console.WriteLine("Connected." + ControlChars.Lf)
                    Dim currentServicePoint As ServicePoint = req.ServicePoint

                    ' Display new service point properties.
                    Dim currentHashCode As Integer = currentServicePoint.GetHashCode()
                    Console.WriteLine(("New service point hashcode: " + currentHashCode.ToString()))
                    Console.WriteLine(("New service point max idle time: " + currentServicePoint.MaxIdleTime.ToString()))
                    Console.WriteLine(("New service point is idle since " + currentServicePoint.IdleSince.ToString()))

                    ' Check that a new ServicePoint instance has been created.
                    If hashCode = currentHashCode Then
                        Console.WriteLine("Service point reused.")
                    Else
                        Console.WriteLine("A new service point created.")
                    End If
                Catch e As Exception
                    Console.WriteLine(("Source : " + e.Source))
                    Console.WriteLine(("Message : " + e.Message))
                Finally
                    If Not (res Is Nothing) Then
                        res.Close()
                    End If
                End Try
            End Sub


            ' Show the user how to use this program when wrong inputs are entered.
            Private Shared Sub showUsage()
                Console.WriteLine("Enter the proxy name as follows:")
                Console.WriteLine(ControlChars.Tab + "vb_servicepoint proxyName")
            End Sub

            ' This is the program entry point. It allows the user to enter 
            ' a server name that is used to locate its current homepage.
            Public Shared Sub Main(ByVal args() As String)
                Dim proxy As String = Nothing
                Dim port As Integer = 80

                ' Define a regular expression to parse the user's input.
                ' This is a security check. It allows only
                ' alphanumeric input strings between 2 to 40 characters long.
                Dim rex As New Regex("^[a-zA-Z]\w{1,39}$")

                If args.Length = 0 Then
                    ' Show how to use this program.
                    showUsage()
                    Return
                End If

                proxy = args(0)
                If (Not (rex.Match(proxy)).Success) Then
                    Console.WriteLine("Input string format not allowed.")
                    Return
                End If

                ' Create a proxy object.  
                Dim proxyAdd As String
                proxyAdd = "http://" + proxy + ":" + port.ToString()


                Dim DefaultProxy As New WebProxy(proxyAdd, True)

                ' Set the proxy that all HttpWebRequest instances use.
                WebRequest.DefaultWebProxy = DefaultProxy


                ' Get the base interface for proxy access for the 
                ' WebRequest-based classes.
                Dim Iproxy As IWebProxy = WebRequest.DefaultWebProxy

                ' Set the maximum number of ServicePoint instances to maintain.
                ' Note that, if a ServicePoint instance for that host already 
                ' exists when your application requests a connection to
                ' an Internet resource, the ServicePointManager object
                ' returns this existing ServicePoint. If none exists 
                ' for that host, it creates a new ServicePoint instance.
                ServicePointManager.MaxServicePoints = 4

                ' Set the maximum idle time of a ServicePoint instance to 10 seconds.
                ' After the idle time expires, the ServicePoint object is eligible for
                ' garbage collection and cannot be used by the ServicePointManager.
                ServicePointManager.MaxServicePointIdleTime = 10000


                ServicePointManager.UseNagleAlgorithm = True
                ServicePointManager.Expect100Continue = True
                ServicePointManager.CheckCertificateRevocationList = True
                ServicePointManager.DefaultConnectionLimit = _
                    ServicePointManager.DefaultPersistentConnectionLimit
                ' Create the Uri object for the resource you want to access.
                Dim MS As New Uri("http://msdn.microsoft.com/")

                ' Use the FindServicePoint method to find an existing 
                ' ServicePoint object or to create a new one.   
                Dim servicePoint As ServicePoint = ServicePointManager.FindServicePoint(MS, Iproxy)
                ShowProperties(servicePoint)
                Dim hashCode As Integer = servicePoint.GetHashCode()
                Console.WriteLine(("Service point hashcode: " + hashCode.ToString()))

                ' Make a request with the same scheme identifier and host fragment
                ' used to create the previous ServicePoint object.
                makeWebRequest(hashCode, "http://msdn.microsoft.com/library/")

            End Sub


        End Class

    End Module
End Namespace

備註

謹慎

WebRequest、HttpWebRequest、ServicePoint和 WebClient 已經過時，您不應該將它們用於新的開發。請改用 HttpClient。

ServicePoint 類別會根據資源統一資源標識碼（URI）中傳遞的主機資訊來處理因特網資源的連線。資源的初始連接會決定 ServicePoint 物件所維護的信息，然後由該資源的所有後續要求共用。

ServicePoint 物件是由 ServicePointManager 類別管理，並視需要由 ServicePointManager.FindServicePoint 方法建立。 ServicePoint 對象永遠不會直接建立，但一律由 ServicePointManager 類別建立和管理。 ServicePointManager.MaxServicePoints 屬性會設定可以建立 ServicePoint 物件的最大數目。

每個 ServicePoint 對象都會維護其與因特網資源的連線，直到其閑置時間超過 MaxIdleTime 屬性中指定的時間為止。當 ServicePoint 超過 MaxIdleTime 值時，它可以回收到另一個連線。 MaxIdleTime 的預設值是由 ServicePointManager.MaxServicePointIdleTime 屬性所設定。

當 ConnectionLeaseTimeout 屬性設定為 -1 以外的值，並在指定的時間經過之後，作用中 ServicePoint 連接會在服務下一個要求之後關閉。這適用於不需要無限期開啟作用中連線的應用程式，因為它們預設為。

注意

在高負載條件下，某些應用程式可能會用盡 ThreadPool 中的免費線程，這可能會導致系統效能不佳（例如高交易時間和可變交易時間）。

屬性

Address	取得這個 ServicePoint 物件所連線之伺服器的統一資源標識碼（URI）。
BindIPEndPointDelegate	指定要將本機 IPEndPoint 與 ServicePoint產生關聯的委派。
Certificate	取得針對這個 ServicePoint 物件接收的憑證。
ClientCertificate	取得傳送至伺服器的最後一個客戶端憑證。
ConnectionLeaseTimeout	取得或設定作用中 ServicePoint 連接關閉的毫秒數。
ConnectionLimit	取得或設定這個 ServicePoint 對象上允許的連線數目上限。
ConnectionName	取得連接名稱。
CurrentConnections	取得與這個 ServicePoint 對象相關聯的開啟連接數目。
Expect100Continue	取得或設定 Boolean 值，判斷是否使用 100-Continue 行為。
IdleSince	取得 ServicePoint 物件上次連接到主機的日期和時間。
MaxIdleTime	取得或設定與 ServicePoint 對象關聯的連接在關閉之前可以保持閑置的時間量。
ProtocolVersion	取得 ServicePoint 物件所使用的 HTTP 通訊協定版本。
ReceiveBufferSize	取得或設定這個 ServicePoint所使用的套接字接收緩衝區大小。
SupportsPipelining	指出 ServicePoint 物件是否支援管線連接。
UseNagleAlgorithm	取得或設定 Boolean 值，這個值會決定 Nagle 演算法是否用於這個 ServicePoint 物件所管理的連接。

方法

CloseConnectionGroup(String)	從這個 ServicePoint 物件中移除指定的連接群組。
Equals(Object)	判斷指定的物件是否等於目前的物件。 (繼承來源 Object)
GetHashCode()	傳回 ServicePoint 實例的哈希值。
GetHashCode()	做為預設哈希函式。 (繼承來源 Object)
GetType()	取得目前實例的 Type。 (繼承來源 Object)
MemberwiseClone()	建立目前 Object的淺層複本。 (繼承來源 Object)
SetTcpKeepAlive(Boolean, Int32, Int32)	啟用或停用 TCP 連線上的keep-alive 選項。
ToString()	傳回表示目前物件的字串。 (繼承來源 Object)

適用於

共用方式為

ServicePoint 類別

定義

範例

備註

屬性

方法

適用於

其他資源