在QT中采用多线程下载文件

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。

思路：

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。

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。

3：每次目标地址有数据返回，都将得到的数据写入文件。

4：等待各段文件下载结果。

 

运行截图：

 

源代码：

#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 &amp;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 &amp;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 &amp;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()), &amp;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 &amp;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()), &amp;loop, SLOT(quit()));control->StartFileDownload(strUrl, PointCount);loop.exec();return 0;
}