java - 更新后只下载文件？

Question

我正在使用下面的代码下载文件。但是，如果远程文件比本地存储的文件更新(如果有的话)，我只想下载。我能以某种方式使用 if-modified-since http header 吗？如何更新我的代码以实现我的目标？

private class DownloadFile extends AsyncTask<String, Integer, String> {
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        mProgressDialog.show();
    }

    @Override
    protected String doInBackground(String... sUrl) {
        try {
            URL url = new URL(sUrl[0]);
            URLConnection connection = url.openConnection();
            connection.connect();
            // this will be useful so that you can show a typical 0-100% progress bar
            int fileLength = connection.getContentLength();

            // download the file
            InputStream input = new BufferedInputStream(url.openStream());
            OutputStream output = new FileOutputStream(path);

            byte data[] = new byte[1024];
            long total = 0;
            int count;
            while ((count = input.read(data)) != -1) {
                total += count;
                publishProgress((int) (total * 100 / fileLength));
                output.write(data, 0, count);
            }

            output.flush();
            output.close();
            input.close();
        }
        catch(MalformedURLException e)
        {
            e.printStackTrace();
        }
        catch(FileNotFoundException e)
        {
            e.printStackTrace();
        }
        catch(IOException e)
        {
            e.printStackTrace();
        }
        return null;
    }

    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);
        mProgressDialog.dismiss();

        // TODO: here file is downloaded and we are ready to process it.
    }

    @Override
    protected void onProgressUpdate(Integer... progress) {
        super.onProgressUpdate(progress);
        mProgressDialog.setProgress(progress[0]);
    }
}

我已经更新了我的代码，看起来像这样......

    @Override
    protected String doInBackground(String... sUrl) {
        long lastModified = new File(path).lastModified();
        try
        {
            URL url = new URL(sUrl[0]);
            URLConnection connection = url.openConnection();
            connection.connect();
            if(lastModified != 0)
            {
                connection.setIfModifiedSince(lastModified);
            }
            // this will be useful so that you can show a typical 0-100% progress bar
            int fileLength = connection.getContentLength();
...

关于如何实际测试它有什么好的想法吗？如果文件不是更新的，while 循环不应该运行，对吗？

Answer 1

这正是该标题的目的。您需要发出 HTTP HEAD 请求以仅获取 header ，然后将 header 中的时间戳与文件上的最后修改时间戳进行比较。如果服务器的副本较新，则发出 GET 以下载新副本。