手机访问action下载apk文件 安装是出错:解析包时出现错误

手机访问action下载apk文件 安装是出错:解析包时出现错误。
在电脑上用浏览器直接下载安装在手机上是好的，但用手机访问action下载下来的文件与原文件大小不一致，且安装时出现错误提示：解析包时出现错误。
action代码如下:
public ActionForward execute(ActionMapping mapping, ActionForm form,
HttpServletRequest request, HttpServletResponse response) throws Exception {
URL downUrl = new URL("/APPDown/android/HGT.apk");
URLConnection conn = downUrl.openConnection();
InputStream is = conn.getInputStream();
BufferedInputStream in = new BufferedInputStream(is);
response.addHeader("Content-Disposition", "attachment;filename=" + "HGT.apk");
//response.setContentType("application/vnd.android.package-archive");
response.addHeader("Content-Type", "application/vnd.android.package-archive");
OutputStream outputStream = response.getOutputStream();
byte[] data = new byte[1024];
int size = 0;
int l = 0;
while ((l = in.read(data)) > 0) {
size += l;
outputStream.write(data, 0, l);
}
response.addHeader("Content-Length", size + "");
outputStream.flush();
outputStream.close();
in.close();
is.close();
return null;
}
文件可以正常下载到手机。但安装失败。