这里的线程是指下载的通道（和操作系统中的线程不一样），一个线程就是一个文件的下载通道，多线程也就是同时开起好几个下载通道.当服务器提供下载服务 时，使用下载者是共享带宽的，在优先级相同的情况下，总服务器会对总下载线程进行平均分配。不难理解，如果你线程多的话，那下载的越快。现流行的下载软件 都支持多线程。    <br />    <br />    <br /> 思路：    <br /> 1：用阻塞的方式获取目标地址的HTTP头部，得到目标文件的大小。    <br /> 2：算出每段文件的开始点，结尾点，并分别向目标地址发出请求。    <br /> 3：每次目标地址有数据返回，都将得到的数据写入文件。    <br />    <p>4：等待各段文件下载结果。</p>    <p> </p>    <p>运行截图：</p>    <p><img alt="在QT中采用多线程下载文件 " src="https://simg.open-open.com/show/fcf38b149825dfc1106a54f260e2d48d.jpg" width="674" height="443" /></p>    <p> </p>    <p>源代码：</p>    <pre class="brush:cpp; toolbar: true; auto-links: false;">#include <QtCore> #include <QtNetwork>  //多线程下载的线程数 const int PointCount = 5; //目标文件的地址（千千静听的下载地址，我用来做实验的） const QString strUrl = "http://ttplayer.qianqian.com/otherdown/alladin/ttpsetup_5713.exe";  //用于下载文件（或文件的一部分） class Download : public QObject {  Q_OBJECT private:  QNetworkAccessManager m_Qnam;  QNetworkReply *m_Reply;  QFile *m_File;   const int m_Index;  qint64 m_HaveDoneBytes;  qint64 m_StartPoint;  qint64 m_EndPoint;  public:  Download(int index, QObject *parent = 0);  void StartDownload(const QUrl &url, QFile *file,    qint64 startPoint=0, qint64 endPoint=-1);  signals:  void DownloadFinished();   public slots:   void FinishedSlot();   void HttpReadyRead(); };  //用于管理文件的下载 class DownloadControl : public QObject {  Q_OBJECT private:  int m_DownloadCount;  int m_FinishedNum;  int m_FileSize;  QUrl m_Url;  QFile *m_File; public:  DownloadControl(QObject *parent = 0);  void StartFileDownload(const QString &url, int count);  qint64 GetFileSize(QUrl url); signals:  void FileDownloadFinished();  private slots:   void SubPartFinished(); };  Download::Download(int index, QObject *parent) : QObject(parent), m_Index(index) {  m_HaveDoneBytes = 0;  m_StartPoint = 0;  m_EndPoint = 0;  m_File = NULL; }  void Download::StartDownload(const QUrl &url,           QFile *file,          qint64 startPoint/* =0 */,          qint64 endPoint/* =-1 */) {  if( NULL == file )   return;   m_HaveDoneBytes = 0;  m_StartPoint = startPoint;  m_EndPoint = endPoint;  m_File = file;   //根据HTTP协议，写入RANGE头部，说明请求文件的范围  QNetworkRequest qheader;  qheader.setUrl(url);  QString range;  range.sprintf("Bytes=%lld-%lld", m_StartPoint, m_EndPoint);  qheader.setRawHeader("Range", range.toAscii());   //开始下载  qDebug() << "Part " << m_Index << " start download";  m_Reply = m_Qnam.get(QNetworkRequest(qheader));  connect(m_Reply, SIGNAL(finished()),   this, SLOT(FinishedSlot()));  connect(m_Reply, SIGNAL(readyRead()),   this, SLOT(HttpReadyRead())); }  //下载结束 void Download::FinishedSlot() {  m_File->flush();  m_Reply->deleteLater();  m_Reply = 0;  m_File = 0;  qDebug() << "Part " << m_Index << " download finished";  emit DownloadFinished(); }  void Download::HttpReadyRead() {  if ( !m_File )   return;   //将读到的信息写入文件  QByteArray buffer = m_Reply->readAll();  m_File->seek( m_StartPoint + m_HaveDoneBytes );  m_File->write(buffer);  m_HaveDoneBytes += buffer.size(); }  //用阻塞的方式获取下载文件的长度 qint64 DownloadControl::GetFileSize(QUrl url) {  QNetworkAccessManager manager;  qDebug() << "Getting the file size...";  QEventLoop loop;  //发出请求，获取目标地址的头部信息  QNetworkReply *reply = manager.head(QNetworkRequest(url));  QObject::connect(reply, SIGNAL(finished()), &loop, SLOT(quit()), Qt::DirectConnection);  loop.exec();  QVariant var = reply->header(QNetworkRequest::ContentLengthHeader);  delete reply;  qint64 size = var.toLongLong();  qDebug() << "The file size is: " << size;  return size; }  DownloadControl::DownloadControl(QObject *parent) : QObject(parent) {  m_DownloadCount = 0;  m_FinishedNum = 0;  m_FileSize = 0;  m_File = new QFile; }  void DownloadControl::StartFileDownload(const QString &url, int count) {  m_DownloadCount = count;  m_FinishedNum = 0;  m_Url = QUrl(url);  m_FileSize = GetFileSize(m_Url);  //先获得文件的名字  QFileInfo fileInfo(m_Url.path());  QString fileName = fileInfo.fileName();  if (fileName.isEmpty())   fileName = "index.html";   m_File->setFileName(fileName);  //打开文件  m_File->open(QIODevice::WriteOnly);  Download *tempDownload;   //将文件分成PointCount段，用异步的方式下载  qDebug() << "Start download file from " << strUrl;  for(int i=0; i<m_DownloadCount; i++)  {   //先算出每段的开头和结尾（HTTP协议所需要的信息）   int start = m_FileSize * i / m_DownloadCount;   int end = m_FileSize * (i+1) / m_DownloadCount;   if( i != 0 )    start--;    //分段下载该文件   tempDownload = new Download(i+1, this);   connect(tempDownload, SIGNAL(DownloadFinished()),     this, SLOT(SubPartFinished()));   connect(tempDownload, SIGNAL(DownloadFinished()),     tempDownload, SLOT(deleteLater()));   tempDownload->StartDownload(m_Url, m_File, start, end);  } }  void DownloadControl::SubPartFinished() {  m_FinishedNum++;  //如果完成数等于文件段数，则说明文件下载完毕，关闭文件，发生信号  if( m_FinishedNum == m_DownloadCount )  {   m_File->close();   emit FileDownloadFinished();   qDebug() << "Download finished";  } }  #include "main.moc"  int main(int argc, char **argv) {  QCoreApplication app(argc, argv);  //用阻塞的方式下载文件，完成后退出  DownloadControl *control = new DownloadControl;  QEventLoop loop;  QObject::connect(control, SIGNAL(FileDownloadFinished()),    &loop, SLOT(quit()));  control->StartFileDownload(strUrl, PointCount);  loop.exec();  return 0; }</pre>    <p></p>