多线程的同步机制

同步机制，实际上是事件驱动机制，意思是让线程平时处于“休眠”状态，除非发生某个事件才触发。

例如一个拷贝文件，拷贝线程完成一个程序块后，再唤醒进程条线程做一个格的填充。 研究多线程的同步机制的必要性在于，多线程同步工作时，如果同时调用相同的资源就可能会出现问题，一般读出是不会有问题的，但是，如果写入（全局变量、数据库），就会发生冲突，甚至产生死锁和竞争问题。

在.NET中，使用ReaderWriterLock()类来实现同步，它有两个重要的方法： AcquireWriterLock(时间) 在该时间内锁定线程； ReleaseWriterLock()释放锁定。 合理的设计，就可以实现同步：

下面是一个例子：

' 新添加的一个命名空间。 Imports System.Threading

Public Class Form1

Inherits System.Windows.Forms.Form

Dim rwl As New ReaderWriterLock() Dim thdProduce As Thread Dim thdConsume As Thread Dim m_iCounter As Integer

' 生产者线程函数 Sub ThreadProduce()

While True

'使用超时值获取阅读锁,Timeout.Infini为无限长等待超时时间 rwl.AcquireWriterLock(Timeout.Infinite) While m_iCounter < 300

m_iCounter += 1

labProduce.Text = m_iCounter.ToString() Thread.Sleep(10) End While '释放阅读锁

rwl.ReleaseWriterLock() End While End Sub

' 消费者线程函数 Sub ThreadConsume()

While True

rwl.AcquireReaderLock(Timeout.Infinite) While m_iCounter > 0

m_iCounter -= 1

labConsume.Text = m_iCounter.ToString() Thread.Sleep(10) End While

rwl.ReleaseReaderLock() End While

End Sub

' 启动线程的运行。

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

Button1.Enabled = False Button2.Enabled = True

thdProduce = New Thread(New ThreadStart(AddressOf ThreadProduce)) thdConsume = New Thread(New ThreadStart(AddressOf ThreadConsume)) thdProduce.Start() thdConsume.Start()

End Sub

Private Sub Form1_Closing(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles MyBase.Closing

'释放锁，而不管线程取锁的次数如何 rwl.ReleaseLock() Try

'终止线程

thdProduce.Abort()

Catch End Try Try

thdConsume.Abort() Catch End Try End Sub

Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click

rwl.ReleaseLock() Try

thdProduce.Abort() Catch End Try Try

thdConsume.Abort() Catch End Try End Sub End Class

同样的例子，也可以用C#写出来：

using System;

using System.Drawing;

using System.Collections; using System.ComponentModel; using System.Windows.Forms; using System.Data; // 新添加的一个命名空间。 using System.Threading;

namespace ThreadSync {

///

protected override void Dispose( bool disposing ) {

if( disposing ) { }

if (components != null) { }

components.Dispose();

public Form1() { }

// Windows 窗体设计器支持所必需的 InitializeComponent();

// TODO: 在 InitializeComponent 调用后添加任何构造函数代码

///

public class Form1 : System.Windows.Forms.Form {

private System.Windows.Forms.Label label1; private System.Windows.Forms.Label label2; private System.Windows.Forms.Button button1; private System.Windows.Forms.Button button2; private System.Windows.Forms.Label labProduce; private System.Windows.Forms.Label labConsume; ///

private System.ComponentModel.Container components = null;

}

base.Dispose( disposing );

///

private ReaderWriterLock rwl = new ReaderWriterLock(); private Thread thdProduce = null; private Thread thdConsume = null; private int m_iCounter = 0; // 生产者线程函数

private void ThreadProduce() { }

// 消费者线程函数

private void ThreadConsume() {

while(true) { }

rwl.AcquireReaderLock(Timeout.Infinite); while(m_iCounter > 0) { }

rwl.ReleaseReaderLock();

m_iCounter--;

labConsume.Text = m_iCounter.ToString(); Thread.Sleep(10);

while(true) { }

rwl.AcquireWriterLock(Timeout.Infinite); while(m_iCounter < 300) { }

rwl.ReleaseWriterLock();

m_iCounter++;

labProduce.Text = m_iCounter.ToString(); Thread.Sleep(10);

Application.Run(new Form1());

}

}

}

// 启动线程的运行。

private void button1_Click(object sender, System.EventArgs e) { }

// 停止线程的运行。

private void button2_Click(object sender, System.EventArgs e) { }

private void Form1_Closing(object sender, System.ComponentModel.CancelEventArgs e) { }

rwl.ReleaseLock(); if(thdProduce != null)

thdProduce.Abort(); thdConsume.Abort(); if(thdConsume != null) rwl.ReleaseLock(); if(thdProduce != null)

thdProduce.Abort(); thdProduce = null; if(thdConsume != null)

thdConsume.Abort(); thdConsume = null; button1.Enabled = true; button2.Enabled = false; button1.Enabled = false; button2.Enabled = true;

thdProduce = new Thread(new ThreadStart(this.ThreadProduce)); thdConsume = new Thread(new ThreadStart(this.ThreadConsume)); if(thdProduce != null)

thdProduce.Start(); thdConsume.Start(); if(thdConsume != null)

效果是完全一样的。

C#代码：

ReaderWriterLock rwl=new ReaderWriterLock(); Thread thdProduce; Thread thdConsume; int m_iCounter;

//生产者线程函数 void ThreadProduce() { }

while (true) { }

//使用超时值获取阅读锁,Timeout.Infini为无限长等待超时时间 while (m_iCounter < 300) { }

//释放阅读锁

rwl.ReleaseWriterLock();

m_iCounter += 1;

labProduce.Text = m_iCounter.ToString(); Thread.Sleep(10);

rwl.AcquireWriterLock(Timeout.Infinite);

//消费者线程函数 void ThreadConsume() {

while (true) {

rwl.AcquireReaderLock(Timeout.Infinite); while (m_iCounter > 0) { }

m_iCounter -= 1;

labConsume.Text = m_iCounter.ToString(); Thread.Sleep(10);

}

}

rwl.ReleaseReaderLock();

private void button1_Click(object sender, System.EventArgs e) {

thdProduce = new Thread(new ThreadStart(ThreadProduce)); thdConsume = new Thread(new ThreadStart(ThreadConsume)); thdProduce.Start(); thdConsume.Start();

private void Form1_Closed(object sender, System.EventArgs e) { }

try { } catch{} try { } catch{}

thdConsume.Abort(); thdProduce.Abort();

}