用 C# 实现 HTTP 协议多线程下载文件

本文内容

环境
Internet 请求
演示
参考资料
修改记录

环境

开发工具：VS 2010/.NET Framework 4.0
系统环境：Microsoft Windows 7

Internet 请求

应用程序通过 WebRequest.Create 方法创建 WebRequest 实例。该方法是静态方法，基于传递的 URI 创建从 WebRequest 派生的类。

NET Framework 提供 HttpWebRequest 类，它派生自 WebRequest，来处理 HTTP 和 HTTPS 请求。在大多数情况下，WebRequest 类提供了你发出请求的所有属性，但是，如果需要的话，你可以把 WebRequest 对象强制类型转换成 HttpWebRequest，以访问请求的 HTTP 属性。类似地，HttpWebResponse 对象来处理 HTTP 和 HTTPS 请求的响应。若访问 HttpWebResponse 对象的属性，需要把 WebResponse 对象强制类型转换成 HttpWebResponse。

.NET Framework 还提供了 FileWebRequest 和 FileWebResponse 类，来处理使用 "file:" 资源的请求。类似地，FtpWebRequest 和 FtpWebResponse 类用户 "ftp:"。

若处理使用应用程序协议的请求，需要实现从 WebRequest 和 WebResponse 派生的特定协议类。

说明：不能混淆 HttpWebResponse 和 HttpResponse 类；后者用于 ASP.NET 应用程序，而且它的方法和属性是通过 ASP.NET 的内部 Response 对象公开的。

演示

下面演示利用 HTTP 协议编写一个多线程下载本文源代码的程序，源代码地址为 http://files.cnblogs.com/liuning8023/HttpDownload.rar。

1，新建项目 "HttpDownload"；

2，Form1.Desginer.cs 代码如下所示：

 
  namespace HttpDownload 
  { 
      partial class Form1 
      { 
          /// <summary> 
          /// 必需的设计器变量。 
          /// </summary> 
          private System.ComponentModel.IContainer components = null; 
    
          /// <summary> 
          /// 清理所有正在使用的资源。 
          /// </summary> 
          /// <param name="disposing">如果应释放托管资源，为 true；否则为 false。</param> 
          protected override void Dispose(bool disposing) 
          { 
              if (disposing && (components != null)) 
              { 
                  components.Dispose(); 
              } 
              base.Dispose(disposing); 
          } 
    
          #region Windows 窗体设计器生成的代码 
    
          /// <summary> 
          /// 设计器支持所需的方法 - 不要 
          /// 使用代码编辑器修改此方法的内容。 
          /// </summary> 
          private void InitializeComponent() 
          { 
              this.lst_processing = new System.Windows.Forms.ListBox(); 
              this.lbl_url = new System.Windows.Forms.Label(); 
              this.lbl_localFile = new System.Windows.Forms.Label(); 
              this.lbl_threadNum = new System.Windows.Forms.Label(); 
              this.txt_url = new System.Windows.Forms.TextBox(); 
              this.txt_localFile = new System.Windows.Forms.TextBox(); 
              this.txt_threadNum = new System.Windows.Forms.TextBox(); 
              this.btn_rec = new System.Windows.Forms.Button(); 
              this.txt_overTime = new System.Windows.Forms.TextBox(); 
              this.lbl_overTime = new System.Windows.Forms.Label(); 
              this.SuspendLayout(); 
              //  
              // lst_processing 
              //  
              this.lst_processing.FormattingEnabled = true; 
              this.lst_processing.ItemHeight = 12; 
              this.lst_processing.Location = new System.Drawing.Point(12, 12); 
              this.lst_processing.Name = "lst_processing"; 
              this.lst_processing.Size = new System.Drawing.Size(342, 364); 
              this.lst_processing.TabIndex = 0; 
              //  
              // lbl_url 
              //  
              this.lbl_url.AutoSize = true; 
              this.lbl_url.Location = new System.Drawing.Point(377, 17); 
              this.lbl_url.Name = "lbl_url"; 
              this.lbl_url.Size = new System.Drawing.Size(47, 12); 
              this.lbl_url.TabIndex = 1; 
              this.lbl_url.Text = "文件URL"; 
              //  
              // lbl_localFile 
              //  
              this.lbl_localFile.AutoSize = true; 
              this.lbl_localFile.Location = new System.Drawing.Point(377, 42); 
              this.lbl_localFile.Name = "lbl_localFile"; 
              this.lbl_localFile.Size = new System.Drawing.Size(53, 12); 
              this.lbl_localFile.TabIndex = 2; 
              this.lbl_localFile.Text = "本地地址"; 
              //  
              // lbl_threadNum 
              //  
              this.lbl_threadNum.AutoSize = true; 
              this.lbl_threadNum.Location = new System.Drawing.Point(377, 74); 
              this.lbl_threadNum.Name = "lbl_threadNum"; 
              this.lbl_threadNum.Size = new System.Drawing.Size(41, 12); 
              this.lbl_threadNum.TabIndex = 3; 
              this.lbl_threadNum.Text = "线程数"; 
              //  
              // txt_url 
              //  
              this.txt_url.Location = new System.Drawing.Point(450, 17); 
              this.txt_url.Name = "txt_url"; 
              this.txt_url.Size = new System.Drawing.Size(353, 21); 
              this.txt_url.TabIndex = 4; 
              this.txt_url.Text = "http://files.cnblogs.com/liuning8023/HttpDownload.rar"; 
              //  
              // txt_localFile 
              //  
              this.txt_localFile.Location = new System.Drawing.Point(450, 44); 
              this.txt_localFile.Name = "txt_localFile"; 
              this.txt_localFile.Size = new System.Drawing.Size(232, 21); 
              this.txt_localFile.TabIndex = 5; 
              this.txt_localFile.Text = "c:download.rar"; 
              //  
              // txt_threadNum 
              //  
              this.txt_threadNum.Location = new System.Drawing.Point(450, 71); 
              this.txt_threadNum.Name = "txt_threadNum"; 
              this.txt_threadNum.Size = new System.Drawing.Size(232, 21); 
              this.txt_threadNum.TabIndex = 6; 
              this.txt_threadNum.Text = "5"; 
              //  
              // btn_rec 
              //  
              this.btn_rec.Location = new System.Drawing.Point(607, 137); 
              this.btn_rec.Name = "btn_rec"; 
              this.btn_rec.Size = new System.Drawing.Size(75, 23); 
              this.btn_rec.TabIndex = 7; 
              this.btn_rec.Text = "接收"; 
              this.btn_rec.UseVisualStyleBackColor = true; 
              this.btn_rec.Click += new System.EventHandler(this.btn_rec_Click); 
              //  
              // txt_overTime 
              //  
              this.txt_overTime.Location = new System.Drawing.Point(450, 99); 
              this.txt_overTime.Name = "txt_overTime"; 
              this.txt_overTime.Size = new System.Drawing.Size(232, 21); 
              this.txt_overTime.TabIndex = 8; 
              //  
              // lbl_overTime 
              //  
              this.lbl_overTime.AutoSize = true; 
              this.lbl_overTime.Location = new System.Drawing.Point(377, 102); 
              this.lbl_overTime.Name = "lbl_overTime"; 
              this.lbl_overTime.Size = new System.Drawing.Size(53, 12); 
              this.lbl_overTime.TabIndex = 9; 
              this.lbl_overTime.Text = "结束时间"; 
              //  
              // Form1 
              //  
              this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 12F); 
              this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font; 
              this.ClientSize = new System.Drawing.Size(815, 384); 
              this.Controls.Add(this.lbl_overTime); 
              this.Controls.Add(this.txt_overTime); 
              this.Controls.Add(this.btn_rec); 
              this.Controls.Add(this.txt_threadNum); 
              this.Controls.Add(this.txt_localFile); 
              this.Controls.Add(this.txt_url); 
              this.Controls.Add(this.lbl_threadNum); 
              this.Controls.Add(this.lbl_localFile); 
              this.Controls.Add(this.lbl_url); 
              this.Controls.Add(this.lst_processing); 
              this.Name = "Form1"; 
              this.Text = "Form1"; 
              this.ResumeLayout(false); 
              this.PerformLayout(); 
    
          } 
    
          #endregion 
    
          public System.Windows.Forms.ListBox lst_processing; 
          private System.Windows.Forms.Label lbl_url; 
          private System.Windows.Forms.Label lbl_localFile; 
          private System.Windows.Forms.Label lbl_threadNum; 
          private System.Windows.Forms.TextBox txt_url; 
          private System.Windows.Forms.TextBox txt_localFile; 
          private System.Windows.Forms.TextBox txt_threadNum; 
          private System.Windows.Forms.Button btn_rec; 
          private System.Windows.Forms.TextBox txt_overTime; 
          private System.Windows.Forms.Label lbl_overTime; 
      } 
  } 
 

说明：

1）添加四个 Lable 控件和 TextBox 控件；一个 ListBox 控件；一个 Button 控件；

2） ListBox 控件需要将 private 属性改为 public，以便在外部使用。

3，新建 HttpMultiThreadDownload.cs 类，代码如下：

using System;

using System.Net;

using System.IO;

using System.Windows.Forms;

namespace HttpDownload

    /// <summary>

    /// 调用外部窗体

    /// </summary>

    /// <param name="text"></param>

    delegate void ProcessingCallback(string processing);

    public class HttpMultiThreadDownload

        const int _bufferSize = 512;

        public Form1 frm { get; set; }

        public int ThreadId { get; set; }             // 线程 ID

        public string Url { get; set; }               // 文件 Url

        public HttpMultiThreadDownload(Form1 form, int threadId)

            frm = form;

            ThreadId = threadId;

            Url = frm.Url;

        /// <summary>

        /// 析构方法

        /// </summary>

        ~HttpMultiThreadDownload()

            if (!frm.InvokeRequired)

                frm.Dispose();

        /// <summary>

        /// 接收

        /// </summary>

        public void receive()

            string filename = frm.fileNames[ThreadId];     // 线程临时文件

            byte[] buffer = new byte[_bufferSize];         // 接收缓冲区

            int readSize = 0;                              // 接收字节数

            FileStream fs = new FileStream(filename, System.IO.FileMode.Create);

            Stream ns = null;

            this.SetListBox("线程[" + ThreadId.ToString() + "] 开始接收......");

try

                HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);

                request.AddRange(frm.StartPos[ThreadId], frm.StartPos[ThreadId] + frm.fileSize[ThreadId]);

                ns = request.GetResponse().GetResponseStream();

                readSize = ns.Read(buffer, 0, _bufferSize);

                this.SetListBox("线程[" + ThreadId.ToString() + "] 正在接收 " + readSize);

                while (readSize > 0)

                    fs.Write(buffer, 0, readSize);

                    readSize = ns.Read(buffer, 0, _bufferSize);

                    this.SetListBox("线程[" + ThreadId.ToString() + "] 正在接收 " + readSize);

                fs.Close();

                ns.Close();

            catch (Exception er)

                MessageBox.Show(er.Message);

                fs.Close();

            this.SetListBox("进程[" + ThreadId.ToString() + "] 结束!");

            frm.threadStatus[ThreadId] = true;

        private void SetListBox(string processing)

            if (frm.lst_processing.InvokeRequired)

                ProcessingCallback d = new ProcessingCallback(SetListBox);

                frm.Invoke(d, new object[] { processing });

            else

                frm.lst_processing.Items.Add(processing);

说明：

1）该类使用了析构函数。通常，.NET Framework 垃圾回收器会隐式地管理对象的内存分配和释放。但是，当应用程序封装窗口、文件和网络连接这类非托管资源时，应当使用析构函数释放这些资源。当对象符合析构时，垃圾回收器将运行对象的 Finalize 方法。

参考：http://msdn.microsoft.com/zh-cn/library/66x5fx1b.aspx

2）另外，SetListBox 方法确保以线程安全方式访问 ListBox 控件。

参考：http://msdn.microsoft.com/zh-cn/library/ms171728(v=VS.90).aspx

3）多线程下载的关键在于能够定位下载文件中流的指定位置，例如 HttpWebRequest.AddRange 方法，从而让每个线程下载你指定的位置。该方式的下载类似于在浏览器中右键的“另存为”，不能用该方式下载类似 HTML 文件，因为这样的流是不能寻址，不能定位的。

4，Form1.cs 代码如下所示：

using System;

using System.Net;

using System.IO;

using System.Windows.Forms;

namespace HttpDownload

    public partial class Form1 : Form

        public int threadNum { get; set; }          // 进程

        public bool[] threadStatus { get; set; }    // 每个线程结束标志

        public string[] fileNames { get; set; }     // 每个线程接收文件的文件名

        public int[] StartPos { get; set; }     // 每个线程接收文件的起始位置

        public int[] fileSize { get; set; }         // 每个线程接收文件的大小

        public string Url { get; set; }             // 接受文件的URL

        public bool HasMerge { get; set; }           // 文件合并标志

        public Form1()

            InitializeComponent();

        /// <summary>

///

        /// </summary>

        /// <param name="sender"></param>

        /// <param name="e"></param>

        private void btn_rec_Click(object sender, EventArgs e)

            Url = this.txt_url.Text.Trim().ToString();

            long fileSizeAll = 0;

            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(this.Url);

            fileSizeAll = request.GetResponse().ContentLength;

            request.Abort();

            threadNum = int.Parse(this.txt_threadNum.Text.Trim().ToString());

            Init(fileSizeAll);

            for (int i = 0; i < threadNum; i++)

                this.lst_processing.Items.Add("线程[" + i + "]:" + (threadStatus[i] ? "结束" : "开始......"));

                this.lst_processing.Items.Add(("开始位置:" + StartPos[i].ToString()).PadLeft(20, ' ') +

                    ("总大小:" + fileSize[i].ToString()).PadLeft(20, ' '));

            this.lst_processing.Items.Add("------------------------------文件总大小：" + fileSizeAll);

            // 定义并启动线程数组

            System.Threading.Thread[] threads = new System.Threading.Thread[threadNum];

            HttpMultiThreadDownload[] httpDownloads = new HttpMultiThreadDownload[threadNum];

            for (int i = 0; i < threadNum; i++)

                httpDownloads[i] = new HttpMultiThreadDownload(this, i);

                threads[i] = new System.Threading.Thread(new System.Threading.ThreadStart(httpDownloads[i].receive));

                threads[i].Start();

            System.Threading.Thread merge = new System.Threading.Thread(new System.Threading.ThreadStart(MergeFile));

            merge.Start();

            this.txt_overTime.Text = DateTime.Now.ToString();

        /// <summary>

        /// 初始化

        /// </summary>

        /// <remarks>

        /// 每个线程平均分配文件大小，剩余部分由最后一个线程完成

        /// </remarks>

        /// <param name="filesize"></param>

        private void Init(long filesize)

            threadStatus = new bool[threadNum];

            fileNames = new string[threadNum];

            StartPos = new int[threadNum];

            fileSize = new int[threadNum];

            int filethread = (int)filesize / threadNum;

            int filethreade = filethread + (int)filesize % threadNum;

            for (int i = 0; i < threadNum; i++)

                threadStatus[i] = false;

                fileNames[i] = i.ToString() + ".dat";

                if (i < threadNum - 1)

                    StartPos[i] = filethread * i;

                    fileSize[i] = filethread - 1;

                else

                    StartPos[i] = filethread * i;

                    fileSize[i] = filethreade - 1;

        /// <summary>

        /// 合并文件

        /// </summary>

        public void MergeFile()

            while (true)

                HasMerge = true;

                for (int i = 0; i < threadNum; i++)

                    if (threadStatus[i] == false) // 若有未结束线程，则等待

                        HasMerge = false;

                        System.Threading.Thread.Sleep(100);

                        break;

                if (HasMerge == true) // 否则，停止等待

                    break;

            int bufferSize = 512;

            int readSize;

            string downFileNamePath = txt_localFile.Text.Trim().ToString();

            byte[] bytes = new byte[bufferSize];

            FileStream fs = new FileStream(downFileNamePath, FileMode.Create);

            FileStream fsTmp = null;

            for (int k = 0; k < threadNum; k++)

                fsTmp = new FileStream(fileNames[k], FileMode.Open);

                while (true)

                    readSize = fsTmp.Read(bytes, 0, bufferSize);

                    if (readSize > 0)

                        fs.Write(bytes, 0, readSize);

                    else

                        break;

                fsTmp.Close();

            fs.Close();

            MessageBox.Show("接收完毕!!!");