我遇到了多线程TCP服务器中的互锁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消息:
Press return to test... Press return to terminate... Echo Handler: Test1 Echo Handler: Test3 Echo Handler: Test2 Echo Handler: Test4
我怀疑在发生Pulse
之前(在Accept
方法中)允许Wait
发生(在服务器的ThreadStart
方法中),显示器会感到困惑,即使ThreadStart
仍然锁定sync
对象,直到它调用Monitor.Wait()
,然后Accept
方法可以获取锁并发送其Pulse
}。如果您在服务器的Stop()
方法中注释掉这两行:
//listener.Stop();
//running = false;
调用服务器的Stop()
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,但围绕sync
对象的锁定应该阻止这种情况。
有什么想法吗?
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。
答案 0 :(得分:5)
问题在于您使用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。
这与同一个线程可以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());
}
}
}
我的跑步输出如下所示。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]
正如你所看到的,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又开始运行,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());
}
}
}