一个关于写U盘后没数据的问题

public static void writeSDFile_UTF8(Context ctx,String fileName, String write_str) throws IOException{



        File file = new File(fileName);

        if (!file.exists())     file.createNewFile();//建立文件



        FileOutputStream fos = new FileOutputStream(file);

        if (file.length() < 1) {



            final byte[] bom = new byte[] { (byte)0xEF, (byte)0xBB, (byte)0xBF };



            fos.write(bom);



        }

        OutputStreamWriter  osw = new OutputStreamWriter(fos, "UTF-8");

        osw.write(write_str);

        osw.flush();

        osw.close();

        fos.flush();

        fos.close();

        MediaScannerConnection.scanFile(ctx, new String[] { file.getAbsolutePath() }, null, null);

    }