一、多线程入的好处
在应用程序中使用多个线程的好处是每个线程都可以异步执行。对于Windows应用程序,耗时的任务可以在后台执行,而使应用程序的窗口和控件保持响应。对于服务器应用程序,多线程处理提供了用不同线程处理每个传入请求的能力。否则,在完全满足前一个请求之前,将无法处理每个新请求。
二、多线程带来的并发问题
然而,线程的异步特性意味着必须协调对资源(如文件句柄、网络连接和内存)的访问、即必须确保任何共享数据都处于被保护状态。否则,两个或更多的线程可能在同一时间访问相同的资源,而每个线程都不知道其他线程的操作。结果将产生不可预知的数据损坏。
案例测试代码:
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namespace MultiThreadPrint
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{
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public class CPrinter
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{
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public void PrintNumbers()
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{
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Console.WriteLine("{0} is excuting PrintNumbers()", Thread.CurrentThread.Name);
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for (int i = 0; i < 10; i++)
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{
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Random r = new Random();
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Thread.Sleep(1000 * r.Next(5));
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Console.Write("{0}, ", i);
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}
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Console.WriteLine();
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}
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};
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class Program
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{
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static void Main(string[] args)
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{
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CPrinter p = new CPrinter();
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// Thread threads[] = new Thread[10]; /* Note that this is the wrong way. */
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Thread[] threads = new Thread[10]; // This is create array,not concrete thread.
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for (int i = 0; i < 10; i++)
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{
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threads[i] = new Thread(new ThreadStart(p.PrintNumbers));
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threads[i].Name = "Worker thread #" + i.ToString();
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}
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foreach (Thread t in threads)
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{
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t.Start();
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}
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Console.ReadLine();
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}
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}
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}
图1
由图1可以看出,十个线程共享一个CPrinter对象时,输出的数据并不是“0,1,2,3,4,5,6,7,8,9”这样有规律的输出,而是出现了各个数据的穿插。说明有某一个时间片线程A调用了CPrinter的PrintNumbers(),别一个时间片线程B也调用了CPrinter的PrintNumbers(),很明显线程A的循环并没有完全执行。而CPrinter对象在各个不同线程的相互竞争中所以打印出了这图1的没规律数据。
三、实现数据同步的相关方法
对于整数数据类型简单操作,可以用Interlocked类的成员来实现线程同步。对于其他所有数据类型和非线程安全的资源,需要使用lock\mutex等机制来处理,详情后解。下面给出Interlocked的案例。
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namespace InterlockedHandle
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{
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class Program
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{
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//0 for false, 1 for true.
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private static int usingResource = 0;
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private static Object currentMso;
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private static Object globalMso = new Object();
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private const int numThreadIterations = 5;
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private const int numThreads = 10;
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static void Main()
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{
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Thread myThread;
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Random rnd = new Random();
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for (int i = 0; i < numThreads; i++)
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{
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myThread = new Thread(new ThreadStart(MyThreadProc));
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myThread.Name = String.Format("Thread{0}", i + 1);
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//Wait a random amount of time before starting next thread.
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Thread.Sleep(rnd.Next(0, 1000));
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myThread.Start();
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}
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Console.ReadLine();
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}
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private static void MyThreadProc()
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{
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for (int i = 0; i < numThreadIterations; i++)
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{
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UseResource();
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//Wait 1 second before next attempt.
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Thread.Sleep(1000);
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}
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}
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//A simple method that denies reentrancy.
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static bool UseResource()
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{
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//0 indicates that the method is not in use.
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if (0 == Interlocked.Exchange(ref usingResource, 1))
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{
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Console.WriteLine("{0} acquired the lock", Thread.CurrentThread.Name);
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//Code to access a resource that is not thread safe would go here.
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//Simulate some work
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Thread.Sleep(500);
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Console.WriteLine("{0} exiting lock", Thread.CurrentThread.Name);
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//Release the lock
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Interlocked.Exchange(ref usingResource, 0);
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return true;
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}
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else
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{
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Console.WriteLine(" {0} was denied the lock", Thread.CurrentThread.Name);
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return false;
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}
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}
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}
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}
图2
图2可以看到usingResource 作为一个整型数据类型,当多个线程想对异步其进行访问时,因为这存在数据的同步问题。这里使用了Interlocked来使用每个线程要想访问usingResource,必须等待别一个线程使用完退出后才行!这样保证了数据的安全。
参考文献
MSDN--Interlocked 类
《C#与.NET4高级程序设计(第5版)》
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