微信js上传的图片只能保存3天 下载到自己的服务器

prePath 为下载到服务器的地址
serverId为调用微信js上传图片成功后返回的id

/**
 * download photo by serverId
 * 
 * @param access_token
 * @param serverId
 */
public static String downloadPhoto(String access_token, String serverId) {
        String requestUrl = "http://file.api.weixin.qq.com/cgi-bin/media/get?access_token=" + MyTask.ACCESS_TOKEN
                + "&media_id=" + serverId + "";
        URL url;
try {
            url = new URL(requestUrl);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setDoInput(true);
            conn.setRequestMethod("GET");

            BufferedInputStream bis = new BufferedInputStream(conn.getInputStream());
            FileOutputStream fos = new FileOutputStream(new File(prePath+serverId+".jpg"));
byte[] buf = new byte[8096];
int size = 0;
while ((size = bis.read(buf)) != -1)
                fos.write(buf, 0, size);
            fos.close();
            bis.close();

            conn.disconnect();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (ProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
return serverId+".jpg";
    }