btnShowProgress.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            try {
                download();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    });
}
public void download() throws IOException {
    URL url = new URL("URL");
        HttpURLConnection c = (HttpURLConnection) url.openConnection();
        c.setRequestMethod("GET");
        c.setDoOutput(true);
        c.connect();
        String PATH = Environment.getExternalStorageDirectory()
                        + "/download/";
        File file = new File(PATH);
        file.mkdirs();
        String fileName = "Dragonfly";
        File outputFile = new File(file, fileName);
        FileOutputStream fos = new FileOutputStream(outputFile);
        InputStream is = c.getInputStream();
        byte[] buffer = new byte[1024];
        int len1 = 0;
        while ((len1 = is.read(buffer)) > 0) {
            fos.write(buffer, 0, len1);
        }
        fos.flush();
        fos.close();
        is.close();

我使用该代码从自己的服务器下载一个文件，但文件总是以9,3kb下载（即使文件大小较小，如2kb），并且无法打开该文件。