我在多线程TCP服务器中遇到了互锁Monitor.Wait和Monitor.Pulse的问题.为了演示我的问题,这是我的服务器代码:
public class Server
{
TcpListener listener;
Object sync;
IHandler handler;
bool running;
public Server(IHandler handler, int port)
{
this.handler = handler;
IPAddress address = Dns.GetHostEntry(Dns.GetHostName()).AddressList[0];
listener = new TcpListener(address, port);
sync = new Object();
running = false;
}
public void Start()
{
Thread thread = new Thread(ThreadStart);
thread.Start();
}
public void Stop()
{
lock (sync)
{
listener.Stop();
running = false;
Monitor.Pulse(sync);
}
}
void ThreadStart()
{
if (!running)
{
listener.Start();
running = true;
lock (sync)
{
while (running)
{
try
{
listener.BeginAcceptTcpClient(new AsyncCallback(Accept), listener);
Monitor.Wait(sync); // Release lock and wait for a pulse
}
catch (Exception e)
{
Console.WriteLine(e.Message);
}
}
}
}
}
void Accept(IAsyncResult result)
{
// Let the server continue listening
lock (sync)
{
Monitor.Pulse(sync);
}
if (running)
{
TcpListener listener = (TcpListener)result.AsyncState;
using (TcpClient client = listener.EndAcceptTcpClient(result))
{
handler.Handle(client.GetStream());
}
}
}
}
这是我的客户端代码:
class Client
{
class EchoHandler : IHandler
{
public void Handle(Stream stream)
{
System.Console.Out.Write("Echo Handler: ");
StringBuilder sb = new StringBuilder();
byte[] buffer = new byte[1024];
int count = 0;
while ((count = stream.Read(buffer, 0, 1024)) > 0)
{
sb.Append(Encoding.ASCII.GetString(buffer, 0, count));
}
System.Console.Out.WriteLine(sb.ToString());
System.Console.Out.Flush();
}
}
static IPAddress localhost = Dns.GetHostEntry(Dns.GetHostName()).AddressList[0];
public static int Main()
{
Server server1 = new Server(new EchoHandler(), 1000);
Server server2 = new Server(new EchoHandler(), 1001);
server1.Start();
server2.Start();
Console.WriteLine("Press return to test...");
Console.ReadLine();
// Note interleaved ports
SendMsg("Test1", 1000);
SendMsg("Test2", 1001);
SendMsg("Test3", 1000);
SendMsg("Test4", 1001);
SendMsg("Test5", 1000);
SendMsg("Test6", 1001);
SendMsg("Test7", 1000);
Console.WriteLine("Press return to terminate...");
Console.ReadLine();
server1.Stop();
server2.Stop();
return 0;
}
public static void SendMsg(String msg, int port)
{
IPEndPoint endPoint = new IPEndPoint(localhost, port);
byte[] buffer = Encoding.ASCII.GetBytes(msg);
using (Socket s = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp))
{
s.Connect(endPoint);
s.Send(buffer);
}
}
}
客户端发送七条消息,但服务器只打印四条:
Press return to test... Press return to terminate... Echo Handler: Test1 Echo Handler: Test3 Echo Handler: Test2 Echo Handler: Test4
我怀疑显示器在Pulse发生Accept之前Wait(在ThreadStart方法中)允许发生(在服务器的方法中),即使ThreadStart在sync它调用之前仍然应该对对象进行锁定Monitor.Wait(),然后该Accept方法可以获取锁定并发送它Pulse.如果您在服务器的Stop()方法中注释掉这两行:
//listener.Stop(); //running = false;
Stop()调用服务器的方法时会显示剩余的消息(即唤醒服务器的sync对象会导致其分派剩余的传入消息).在我看来,这只能在ThreadStart和Accept方法之间的竞争条件下发生,但是sync对象周围的锁定应该阻止这种情况.
有任何想法吗？
非常感谢,西蒙.
PS.请注意,我知道输出显示无序等,我特别询问锁和监视器之间的竞争条件.干杯,SH.
问题是你使用Pulse/Wait作为信号.正确的信号(例如AutoResetEvent)具有一种状态,使其在线程调用WaitOne()之前保持信号状态.在没有任何线程等待的情况下调用Pulse将成为noop.
这与可以通过同一线程多次锁定的事实相结合.由于您使用的是Async编程,因此可以通过执行BeginAcceptTcpClient的同一线程调用Accept回调.
让我来说明一下.我注释掉了第二台服务器,并更改了服务器上的一些代码.
void ThreadStart()
{
if (!running)
{
listener.Start();
running = true;
lock (sync)
{
while (running)
{
try
{
Console.WriteLine("BeginAccept [{0}]",
Thread.CurrentThread.ManagedThreadId);
listener.BeginAcceptTcpClient(new AsyncCallback(Accept), listener);
Console.WriteLine("Wait [{0}]",
Thread.CurrentThread.ManagedThreadId);
Monitor.Wait(sync); // Release lock and wait for a pulse
}
catch (Exception e)
{
Console.WriteLine(e.Message);
}
}
}
}
}
void Accept(IAsyncResult result)
{
// Let the server continue listening
lock (sync)
{
Console.WriteLine("Pulse [{0}]",
Thread.CurrentThread.ManagedThreadId);
Monitor.Pulse(sync);
}
if (running)
{
TcpListener localListener = (TcpListener)result.AsyncState;
using (TcpClient client = localListener.EndAcceptTcpClient(result))
{
handler.Handle(client.GetStream());
}
}
}
我的运行输出如下所示.如果您自己运行此代码,则值会有所不同,但一般情况下它们会相同.
Press return to test... BeginAccept [3] Wait [3] Press return to terminate... Pulse [5] BeginAccept [3] Pulse [3] Echo Handler: Test1 Echo Handler: Test3 Wait [3]
正如您所看到的,有两个Pulse被调用,一个来自一个单独的线程(Pulse [5]),它唤醒了第一个Wait.然后,线程3执行另一个BeginAccept,但具有Pending传入连接,该线程决定立即调用Accept回调.由于Accept由同一个线程调用,因此Lock(sync)不会阻塞,而是立即在空线程队列上阻塞Pulse [3].
调用两个处理程序并处理这两个消息.
一切都很好,ThreadStart再次开始运行并无限期地进入等待状态.
现在,这里的根本问题是您正在尝试将监视器用作信号.因为它不记得第二个脉冲丢失的状态.
但是有一个简单的解决方案.使用AutoResetEvents,这是一个正确的信号,它将记住它的状态.
public Server(IHandler handler, int port)
{
this.handler = handler;
IPAddress address = Dns.GetHostEntry(Dns.GetHostName()).AddressList[0];
listener = new TcpListener(address, port);
running = false;
_event = new AutoResetEvent(false);
}
public void Start()
{
Thread thread = new Thread(ThreadStart);
thread.Start();
}
public void Stop()
{
listener.Stop();
running = false;
_event.Set();
}
void ThreadStart()
{
if (!running)
{
listener.Start();
running = true;
while (running)
{
try
{
listener.BeginAcceptTcpClient(new AsyncCallback(Accept), listener);
_event.WaitOne();
}
catch (Exception e)
{
Console.WriteLine(e.Message);
}
}
}
}
void Accept(IAsyncResult result)
{
// Let the server continue listening
_event.Set();
if (running)
{
TcpListener localListener = (TcpListener) result.AsyncState;
using (TcpClient client = localListener.EndAcceptTcpClient(result))
{
handler.Handle(client.GetStream());
}
}
}
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