个人理解：文件下载就是将服务器的文件以流的形式输出给客户端即输出到response的流中，但要设置输出的类型为下载类型

<%@page language="java"  contentType="text/html; charset=UTF-8"
import="java.io.*,java.net.* l" 
pageEncoding="UTF-8"%>
<%request.setCharacterEncoding("UTF-8"); %>
 
<%
//查询实体得到文件全路径
 
     String fileurl=FileUploadConstants.getPropValue("BASE_PATH")+UploadUtil.getSavePath("EDUCATION_PATH")+  File.separator+uploadInfo.getDocPath()  ;
     String filename=entity.getOldName() ;
 
    //System.out.print(fileurl+"   "+filename );
    
    OutputStream output = null;
    FileInputStream fis = null;
    File file = new File( fileurl );
    
    if(  !file.exists() ){
response.getWriter().print ("<script language='javascript'>alert('文件不存在');this.history.go(-1);</script>");
response.getWriter().close();
    }else{
        try
        {
            
            response.reset();//可以加也可以不加
            response.setContentType("application/x-download");//设置为下载application/x-download
            
            String userAgent = request.getHeader("User-Agent");  
            boolean isIE = (userAgent != null) && (userAgent.toLowerCase().indexOf("msie") != -1);  
         
            if (isIE) {
            filename = URLEncoder.encode(filename, "UTF-8");  
               response.setHeader("Content-Disposition", "attachment;filename=\"" + filename + "\"");  
            } else {  
           filename = new String(filename.getBytes("UTF-8"), "ISO8859-1");  
               response.setHeader("Content-Disposition", "attachment;filename=" + filename);  
            }  
            
            output = response.getOutputStream();
            fis = new FileInputStream(file);
            byte[] b = new byte[1024];
            int i = 0;
 
            while((i = fis.read(b)) > 0)
            {
                output.write(b, 0, i);
            }
 
            output.flush();
     output.close();
     
            out.clear();
            out=pageContext.pushBody();
            out.close();
            
        }
        catch(Exception e)
        {
        e.printStackTrace();
         //System.out.print(1);
        }finally{
        if(output!=null){
        output.close();
        output = null;
       
        }
       
        }
    }
 
   
%>