用ftpclient在ftp服务器能上怎么才能拿到文件的length?

我用apache.commons.net的ftpclient做个从服务器上下载文件到本地.怎么才能拿到服务器上每个文件的length呢？程序要做个断点续传的功能。以下是代码段：
                           //创建文件
                            File file = new File(localpath + File.separator + filename);
                            //判断是否文件存在
                            if (file.exists()) {//我想在这里变成file.exists() && 本地文件的length==服务器上文件的长度
                                System.out.println(filename + ":文件存在");
                               
                            } else {
                                //断点续传
                                long lon = file.length();
                                RandomAccessFile raf = new RandomAccessFile(file, "rw");
                                raf.seek(lon);
                                ftpclient.setRestartOffset(lon);
                                InputStream is = ftpclient.retrieveFileStream(filename);
                                if (is == null) {
                                    System.out.println("没有:" + filename);
                                } else {
                                    System.out.println("开始下载文件:" + filename);                                    int b;
                                    while ((b = is.read()) != -1) {
                                    raf.write(b);
                                }
                                }