//通过路径得到一个输入流

String path =  "获取需要下载的文件路径";      //path.lastIndexOf(".")+1可以获取文件后缀名字 如:doc等//this.getServletContext().getRealPath("");可以访问当前服务器地址    

String fileName="保存文件的名称" ;       //设置保存文件的乱码问题
String encodedfileName = null;
String agent = request.getHeader("USER-AGENT");
if(null != agent && -1 != agent.indexOf("MSIE")){//IE
encodedfileName = java.net.URLEncoder.encode(fileName,"UTF-8");
}else if(null != agent && -1 != agent.indexOf("Mozilla")){
encodedfileName = new String (fileName.getBytes("UTF-8"),"iso-8859-1");
}else{
encodedfileName = java.net.URLEncoder.encode(fileName,"UTF-8");
}
//设置响应格式
response.setHeader("content-disposition", "attachment;filename=encodedfileName);//输入流，获得文件的字节流
InputStream is=new FileInputStream(path);//path是下载文件的路径
byte[] bytes=new byte[is.available()];
is.read(bytes);//将字节流写入response中
response.getOutputStream().write(bytes);
is.close();
response.flushBuffer();
response.getOutputStream().flush();

//设置上传的路径String savePath=this.getServletConfig().getServletContext().getRealPath("");
File file=new File(savePath);
//判断是否存在 不存在就创建
if(!file.exists()){
file.mkdirs();
}
DiskFileItemFactory fac=new DiskFileItemFactory();
ServletFileUpload upload=new ServletFileUpload(fac);
upload.setHeaderEncoding("utf-8");
List<FileItem> filelist=null;
try {
filelist=upload.parseRequest(request);
} catch (FileUploadException e) {
// TODO Auto-generated catch block
return;
}
Iterator<FileItem> it=filelist.iterator();
String name="";
String extName="";
while(it.hasNext()){
FileItem item=it.next();
if(!item.isFormField()){
name=item.getName();
long size=item.getSize();
String type=item.getContentType();
//判断是否为空
if(name==null||name.trim().equals("")){
continue;
}
if(name.lastIndexOf(".")>=0){
extName=name.substring(name.lastIndexOf("."));
}File files=null;
name=id;
files=new File(savePath+name+extName);
try {
item.write(files);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
} response.getWriter().print("上传成功");
}