预览功能

File file = new File(System.getProperty("webapp.root") + valleyFile.getPath());
FileInputStream inputStream = new FileInputStream(file);
String download_filename = URLEncoder.encode(valleyFile.getFilename(), "UTF-8");//对文件名记性加密
response.reset();
response.setHeader("Content-Disposition", "attachment;filename=" + download_filename);
response.setContentType("application/octet-stream"); byte[] buffer = new byte[1024];
ServletOutputStream stream = response.getOutputStream(); 

boolean len = false; int len1;

 while((len1 = inputStream.read(buffer)) != -1) {
    stream.write(buffer, 0, len1);
}
inputStream.close();
stream.flush();
stream.close();