public static void sendFile2Server(File local_file, String targeFile,
            String server, int port, String id) {
        Socket socket = null;
        InputStream iis = null;
        OutputStream os = null;
        try {
            socket = new Socket(server, port);
            byte[] fileNameBuf = targeFile.getBytes("GBK");
            int fls = fileNameBuf.length;
            byte[] fileNamelen = new byte[] { 0, 0, 0, 0 };
            fileNamelen[0] = (byte) ((fls >> 24) & 0xff);
            fileNamelen[1] = (byte) ((fls >> 16) & 0xff);
            fileNamelen[2] = (byte) ((fls >> 8) & 0xff);
            fileNamelen[3] = (byte) ((fls >> 0) & 0xff);
            os = socket.getOutputStream();
            os.write(SEND_HEADER);
            os.write(fileNamelen);
            os.write(fileNameBuf);
            os.flush();
            iis = new FileInputStream(local_file);
            byte[] buf = new byte[1024];
            int rl = iis.read(buf);
            while (rl > 0) {
                os.write(buf, 0, rl);
                rl = iis.read(buf);
            }
            FileTransferStauts.statusMap.put(id, "finished");
        } catch (Exception e) {
            FileTransferStauts.statusMap.put(id, e.getMessage());
        } finally {
            try {
                if (os != null)
                    os.close();
                if (socket != null && !socket.isClosed())
                    socket.close();
                if (iis != null)
                    iis.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

红色的部分,对filename的操作有什么作用?

把fls的32位数分成四个八位以便发送。
fls是int型32位数，fileNamelen[0] = (byte) ((fls >> 24) & 0xff); 是把fls的高八位
存到byte型数组fileNamelen的第一个位置，同样
fileNamelen[1] = (byte) ((fls >> 16) & 0xff);
fileNamelen[2] = (byte) ((fls >> 8) & 0xff);
fileNamelen[3] = (byte) ((fls >> 0) & 0xff); 是分别存fls的次高八位、次低八位和低八位。