這里的線程是指下載的通道（和操作系統中的線程不一樣），一個線程就是一個文件的下載通道，多線程也就是同時開起好幾個下載通道.當服務器提供下載服務 時，使用下載者是共享帶寬的，在優先級相同的情況下，總服務器會對總下載線程進行平均分配。不難理解，如果你線程多的話，那下載的越快。現流行的下載軟件 都支持多線程。


思路：
1：用阻塞的方式獲取目標地址的HTTP頭部，得到目標文件的大小。
2：算出每段文件的開始點，結尾點，并分別向目標地址發出請求。
3：每次目標地址有數據返回，都將得到的數據寫入文件。

4：等待各段文件下載結果。

運行截圖：

源代碼：

#include <QtCore>

include <QtNetwork>
//多線程下載的線程數
const int PointCount = 5;
//目標文件的地址（千千靜聽的下載地址，我用來做實驗的）
const QString strUrl = "http://ttplayer.qianqian.com/otherdown/alladin/ttpsetup_5713.exe&quot;;
//用于下載文件（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();

};
//用于管理文件的下載
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;

//根據HTTP協議，寫入RANGE頭部，說明請求文件的范圍
QNetworkRequest qheader;
qheader.setUrl(url);
QString range;
range.sprintf("Bytes=%lld-%lld", m_StartPoint, m_EndPoint);
qheader.setRawHeader("Range", range.toAscii());

//開始下載
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()));

}
//下載結束
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;

//將讀到的信息寫入文件
QByteArray buffer = m_Reply->readAll();
m_File->seek( m_StartPoint + m_HaveDoneBytes );
m_File->write(buffer);
m_HaveDoneBytes += buffer.size();

}
//用阻塞的方式獲取下載文件的長度
qint64 DownloadControl::GetFileSize(QUrl url)
{
    QNetworkAccessManager manager;
    qDebug() << "Getting the file size...";
    QEventLoop loop;
    //發出請求，獲取目標地址的頭部信息
    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);
    //先獲得文件的名字
    QFileInfo fileInfo(m_Url.path());
    QString fileName = fileInfo.fileName();
    if (fileName.isEmpty())
        fileName = "index.html";

m_File->setFileName(fileName);
//打開文件
m_File->open(QIODevice::WriteOnly);
Download *tempDownload;

//將文件分成PointCount段，用異步的方式下載
qDebug() << "Start download file from " << strUrl;
for(int i=0; i<m_DownloadCount; i++)
{
    //先算出每段的開頭和結尾（HTTP協議所需要的信息）
    int start = m_FileSize * i / m_DownloadCount;
    int end = m_FileSize * (i+1) / m_DownloadCount;
    if( i != 0 )
        start--;

    //分段下載該文件
    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++;
    //如果完成數等于文件段數，則說明文件下載完畢，關閉文件，發生信號
    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);
    //用阻塞的方式下載文件，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>