文件批量下载

这两天在公司做的一个需求，由于是新手做任务，所以花了一点时间。

这里简要说明一下需求需要处理的步骤：1、前台页面选择需要批量下载的文件；2、点击“一键下载”按钮，执行一键下载；3、如果选择了多个文件，保存到本地文件之后应该是一个压缩包。

思路：1、前台选择文件时要能得到文件在服务器上的地址；2、点击“一键下载“之后，应该将这些地址传到后台；3、打包。这是难点，不过现在一般都有sharpziplib可以使用，但是要考虑大文件下载的问题。如果只是几个小文件下载的话可以直接在内存中打包，但是如果是大文件就不能这么做了，这时就需要在物理硬盘上打包，并且如果文件过大，打包的时间也会有点稍长，这对用户体验是很不好的，如果想最速度地打包成功，那就setlever为0吧，sharpziplib封装了这个类。dll下载官网是http://www.icsharpcode.net/OpenSource/SharpZipLib/Download.aspx

下面看代码

''' <summary>
''' 批量下载函数
''' </summary>
''' <param name="ArrFileName">文件路径</param>
''' <param name="strDocName">文件名</param>
''' <param name="context"></param>
''' <remarks>2014/03/12工作流批量下载增加此函数</remarks>
Private Sub PLDownLoad(ByVal ArrFilePath() As String, ByVal context As System.Web.HttpContext)
'将服务器端文件的相对路径转换为绝对路径
For i As Integer = 0 To ArrFilePath.Length - 1
ArrFilePath(i) = context.Server.MapPath(ArrFilePath(i))
Next

HttpContext.Current.Response.ContentType = "application/octet-stream"
HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment; filename=" + Date.Now.ToString() + ".zip")

'在TempFile文件夹下新建一个.zip的临时文件
Dim tempFileName As String = context.Server.MapPath("/TempFiles") + "\\" + Guid.NewGuid().ToString() + ".zip"
Dim fs As FileStream = New FileStream(tempFileName, FileMode.Create)
fs.Close()
'将要批量下载的文件压缩到临时文件目录，为避免压缩浪费效率，设置压缩比为0
Dim tempFileStream As FileStream = New FileStream(tempFileName, FileMode.Open, FileAccess.ReadWrite, FileShare.Read)
Dim zos As ZipOutputStream = New ZipOutputStream(tempFileStream)
zos.SetLevel(0)

'获取web.config中配置的分段下载下限
Dim splitDownloadSizeConfig As String = System.Configuration.ConfigurationSettings.AppSettings("SplitDownloadSize")
Dim splitDownloadSize As Long = If(String.IsNullOrEmpty(splitDownloadSizeConfig), 40960 * 1024, CInt(splitDownloadSizeConfig) * 1024)

'要压缩的文件总大小
Dim totalFileSize As Long = 0

'读取需要批量压缩的文件
For Each itemPath As String In ArrFilePath

If File.Exists(itemPath) Then
'获取单个文件实际大小
Dim fileSize As Long = 0
Dim response As FileWebResponse
Try
response = HttpWebRequest.Create(itemPath).GetResponse()
fileSize = response.ContentLength
Catch ex As Exception
MyDB.LogException(ex)
Finally
If response IsNot Nothing Then
response.Close()
End If
End Try
totalFileSize += fileSize

'压缩
Dim fileInfo As FileInfo = New FileInfo(itemPath)
Dim stream As FileStream = New FileStream(itemPath, FileMode.Open, FileAccess.Read, FileShare.Read)

Dim buffer() As Byte = New [Byte](fileSize) {}
Dim length As Integer = stream.Read(buffer, 0, fileSize)

Dim entry As ZipEntry = New ZipEntry(fileInfo.Name)

zos.PutNextEntry(entry)
zos.Write(buffer, 0, length)

stream.Close()
End If

Next
zos.Finish()
zos.Close()

If totalFileSize < splitDownloadSize Then '文件实际大小小于分段下载下限直接下载
context.Response.WriteFile(tempFileName)
context.Response.End()
Else '文件实际大小大于等于分段下载下限分段下载
Dim stream As Stream = Nothing
Dim buffer As Byte() = New [Byte](10000) {}
Dim length As Integer
Dim dataToRead As Long
Try
stream = New FileStream(tempFileName, FileMode.Open, FileAccess.Read, FileShare.Read)
dataToRead = stream.Length
While dataToRead > 0
If context.Response.IsClientConnected Then
'分段下载
length = stream.Read(buffer, 0, 10000)
context.Response.OutputStream.Write(buffer, 0, length)
context.Response.Flush()
buffer = New [Byte](10000) {}
dataToRead = dataToRead - length
Else
'客户端在没有下载完前，关闭连接，直接设置为-1,避免死循环
dataToRead = -1
End If
End While
Catch ex As Exception
MyDB.LogException(ex)
context.Response.Write("下载过程中发生异常:" & ex.Message)
Finally
If stream IsNot Nothing Then
stream.Close()
End If
End Try
End If
tempFileStream.Close()

HttpContext.Current.Response.Clear()
HttpContext.Current.Response.End()

End Sub