探秘System.Threading系列第二篇：还是Thread，Thread.Start 和 Thread.Join

最新推荐文章于 2025-06-17 17:38:26 发布

玉开

最新推荐文章于 2025-06-17 17:38:26 发布

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文章标签： thread

本文探讨了使用Thread和ThreadPool启动线程的方法，通过示例代码展示了Thread.Join方法在不同场景下的行为差异，包括线程阻塞及执行顺序的问题。

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1. 开始一个Thread
开始一个Thread很简单，声明一个Thread实例，然后调用Start方法即可

Thread.Start
Thread threadA = new Thread(new ThreadStart(WorkMethod));
threadA.Start();

Thread threadB = new Thread(new ParameterizedThreadStart(WorkMethodWithParam));
threadB.Start(obj);

2. Thread.Join阻塞调用线程，直到某个线程终止为止。
我按照msdn上的解释的理解是：
threadA.Join()方法是用来阻塞threadA线程，直到在threadA开始执行之后启动的线程执行完毕（或者Sleep了）之后，才开始执行threadA线程的方法

但是事实并非如我所想，下面是msdn上的例子，先声明了一个thread实例，然后将一个方法放入ThreadPool中执行，并调用thread的Join方法以阻塞thread的执行。显示结果在预料之中

MSDN Thread.Join例子
using System;
using System.Threading;

class IsThreadPool
{
 static void Main()
 {
 AutoResetEvent autoEvent = new AutoResetEvent(false);

 Thread regularThread =
 new Thread(new ThreadStart(ThreadMethod));
 regularThread.Start();
 ThreadPool.QueueUserWorkItem(new WaitCallback(WorkMethod),
 autoEvent);

 // Wait for foreground thread to end.
 regularThread.Join();

 // Wait for background thread to end.
 autoEvent.WaitOne();
 }

 static void ThreadMethod()
 {
 Console.WriteLine("ThreadOne, executing ThreadMethod, " +
 "is {0}from the thread pool.",
 Thread.CurrentThread.IsThreadPoolThread ? "" : "not ");
 }

 static void WorkMethod(object stateInfo)
 {
 Console.WriteLine("ThreadTwo, executing WorkMethod, " +
 "is {0}from the thread pool.",
 Thread.CurrentThread.IsThreadPoolThread ? "" : "not ");

 // Signal that this thread is finished.
 ((AutoResetEvent)stateInfo).Set();
 }
}

执行结果是：ThreadTwo, executing WorkMethod, is from the thread pool.
ThreadOne, executing ThreadMethod, is not from the thread pool.

但是当我对这个例子稍作修改之后就出现了不可理解的问题，我在WorkMethod中加上Thread.Sleep让线程池中的线程休眠2秒，这样操作之后执行结果是：

执行结果
ThreadTwo, executing WorkMethod, is from the thread pool.
Sleep
ThreadOne, executing ThreadMethod, is not from the thread pool.
Sleep end

可以看出当线程池中的线程Sleep之后，被Join的thread即获得执行权，开始执行，这个似乎和msdn的解释有出入

另外我还做了一个测试，把另一个线程用Thread启动，而不是放在线程池中执行，看是否可以正常的阻塞线程，但是结果却是出人意料的，不能按预期阻塞，请看下面的实例代码：

Code
 class IsThreadPool
 {
 static void Main()
 {
 AutoResetEvent autoEvent = new AutoResetEvent(false);

 Thread threadA =
 new Thread(new ThreadStart(ThreadMethod));

 Thread threadB = new Thread(new ParameterizedThreadStart(WorkMethod));

 threadA.Start();

 // Wait for foreground thread to end.
 threadA.Join();
 threadB.Start(autoEvent);

 // Wait for background thread to end.
 autoEvent.WaitOne();

 Console.Read();
 }

 static void ThreadMethod()
 {
 Console.WriteLine("ThreadOne, executing ThreadMethod, " +
 "is {0}from the thread pool.",
 Thread.CurrentThread.IsThreadPoolThread ? "" : "not ");
 }

 static void WorkMethod(object stateInfo)
 {
 Console.WriteLine("ThreadTwo, executing WorkMethod, " +
 "is {0}from the thread pool.",
 Thread.CurrentThread.IsThreadPoolThread ? "" : "not ");
 // Signal that this thread is finished.
 ((AutoResetEvent)stateInfo).Set();
 }
 }

不知道什么原因，上面的代码中threadA并没有被阻塞，难道是只能阻塞放在ThreadPool中执行的线程？没有道理，我反复做了实验都没有成功，把这个结果贴出来，请大家释疑。