1：位元組流結構：
每次傳送12＋1024個位元組
其中，前七個位元組位元組表示命令
第七到第十二個位元組表示後面1024位元組中的有效位元組的長度
例如一幀中前12個位元組的內容為"UPFILEN00012"表示要上傳檔名，檔名的長度是12。那麼程式就在後面的1024個位元組中去12個位元組，在把它轉換為字串，作為要上傳的檔名。

2：命令結構
Server端：
DISCONN：斷開連線
LSFILES：傳送當前目錄檔案列表
ENDFILE：上傳一個檔案的結束標記
UPFILEN：表示要上傳一個新的檔案，並且此包中包含了檔名
UPDATAS：表示本包是要上傳的資料
DNFILEN：表示要下載的檔名，伺服器要執行向客戶端傳輸檔案的操作
Client端：
DISCONN：斷開連線
LSFILES：接收伺服器當前目錄檔案列表
ENDFILE：下載一個檔案的結束標記
DNDATAS：表示本包是要下載的資料

3：檔案結構
FtpServer:ftp軟體的伺服器端，目前在埠2121監聽，支援多執行緒，檔案的上傳，下載，列表。
FtpClient:ftp軟體的客戶端，預設連線本機的伺服器端，在埠2121，支援檔案的上傳，下載，列表。
FtpClientUI:ftp軟體的客戶端的使用者介面，完全採用Swing技術，手工編寫，沒有用JB自動生成。
PublicFunc:提供一些公共的靜態方法例如將給定的String物件分裝成要傳送的幀。將數字格式化成長度為五位的String型別物件。
package cn.edu.bit.software.ftptrans;

import java.io.*;
import java.net.*;
import java.util.Vector;
import java.util.logging.*;

public class FtpServer
{
  //客戶端socket物件
  private ServerSocket m_servSocket;

  //ftp伺服器的埠號
  private int SERVER_PORT;

  //ftp伺服器所允許的最大連線數
  private int MAX_CONN;

  //連入的客戶端處理物件管理器
  private Vector vecClient;

  //設定一個log日誌物件
  private Logger mylog;

  private ConsoleHandler handler;

  String strServHome;

  public FtpServer(int servPort, int maxConn)
  {
    SERVER_PORT = servPort;
    MAX_CONN = maxConn;
    strServHome = "c://";
    vecClient = new Vector();
    /*------------初始化log------------*/
    try
    {
      handler = new ConsoleHandler();
      handler.setLevel(Level.ALL);
      mylog = Logger.getLogger("FtpServer");
      mylog.addHandler(handler);
      mylog.setLevel(Level.ALL);
    }
    catch (SecurityException e)
    {
      mylog.warning("在設定程式日誌級別時出現異常");
      mylog.warning(e.getMessage());
    }

    /*--------------初始化伺服器，埠2121----------------------*/
    try
    {
      m_servSocket = new ServerSocket(SERVER_PORT, MAX_CONN);
      while (true)
      {
        mylog.finest("FtpServer開始在埠2121監聽");
        Socket clientSocket = m_servSocket.accept();
        vecClient.add(clientSocket);
        mylog.info("#" + vecClient.size() + "號客戶端連入");
        new TransHandler(this, clientSocket, vecClient.size()).start();
      }
    }
    catch (IOException e)
    {
      mylog.warning("在初始化FtpServ時出現錯誤");
      mylog.warning(e.getMessage());
    }
  }

  public void deleteClient(TransHandler handler)
  {
    try
    {
      vecClient.remove(handler);
      vecClient.setSize(vecClient.size() - 1);
      mylog.info("第#" + handler.iClientNum + "號客戶端斷開了與伺服器的連線！");
    }
    catch (Exception e)
    {
      mylog.warning("在刪除第#" + handler.iClientNum + "號客戶端時出現異常");
      mylog.warning(e.getMessage());
    }
  }

  public static void main(String[] args)
  {
    new FtpServer(2121, 50);
  }
}

/**
 * 當有客戶端連入時，處理客戶端請求的類
 * @author findfrog
 * @version 1.0
 */
class TransHandler extends Thread
{
  //伺服器控制代碼，用於最後銷燬TransHandler物件時用
  FtpServer main = null;
  //客戶端的socket
  private Socket m_clientSocket = null;

  //日誌物件
  private Logger mylog;

  //要上傳的檔案路徑
  private String strUpFilePath = null;

  //要下載的檔案路徑
  private String strDnFilePath = null;

  //本客戶端在的序號
  int iClientNum = -1;

  //緩衝位元組資料的大小
  private int ibufferlength;

  //緩衝位元組陣列
  byte[] inputBytes;

  //從客戶端傳來的指令
  String strClientOrder;
  //用於得到從socket端的輸入資訊
  InputStream m_inputStream;
  //用於向socket輸出的輸出流
  OutputStream m_outputStream;
  //用於上傳檔案的輸出流
  FileOutputStream m_fileOutputStream;
  //用於下載檔案的輸入流
  FileInputStream m_fileInputStream;

  //建構函式
  public TransHandler(FtpServer fserver, Socket s, int iNum)
  {
    try
    {
      main = fserver;
      //將客戶端socket控制代碼付給本地物件
      m_clientSocket = s;
      //初始化log物件
      mylog = Logger.getLogger("TransHandler");
      //初始化本客戶端序號
      iClientNum = iNum;
      //用於得到從socket端的輸入資訊
      m_inputStream = m_clientSocket.getInputStream();
      m_outputStream = m_clientSocket.getOutputStream();
      ibufferlength = 1024;
      inputBytes = new byte[ibufferlength + 12];

    }
    catch (Exception e)
    {
      mylog.warning("在初始化TransHandler時發生異常！");
      mylog.warning(e.getMessage());
    }
  }

  public void run()
  {
    try
    {
      int ilength;
      while ( (ilength = m_inputStream.read(inputBytes, 0, 12 + ibufferlength)) !=
             -1)
      {
        strClientOrder = new String(inputBytes, 0, 7);
        if (strClientOrder.equals("DISCONN"))
        { //斷開連線
          mylog.info("得到了DISCONN");
          exit();
        }
        else if (strClientOrder.equals("LSFILES"))
        { //傳送當前目錄檔案列表
          mylog.info("伺服器端接收到了LSFILES命令");
          File flHome = new File(main.strServHome);
          String[] strFileNames = flHome.list();
          strFileNames = AdjustStrings(strFileNames);
          for (int i = 0; i < strFileNames.length; i++)
          {
            String strFileNameLength = PublicFunc.formatLength(strFileNames[i].
                getBytes().length);
            byte[] fileNameBytes = strFileNames[i].getBytes();
            byte[] outBytes = PublicFunc.makepackage("LSFILES",
                strFileNameLength, fileNameBytes);
            m_outputStream.write(outBytes, 0, outBytes.length);
            m_outputStream.flush();
          }
        }
        else if (strClientOrder.equals("ENDFILE"))
        { //上傳一個檔案的結束標記
          mylog.info("收到檔案結束標誌符號");
          m_fileOutputStream.close();
        }
        else if (strClientOrder.equals("UPFILEN"))
        { //表示要上傳一個新的檔案，並且此包中包含了檔名
          int iFileNameLength = Integer.parseInt(new String(inputBytes, 7, 5));
          mylog.info("要上傳的檔名的長度為" + iFileNameLength);
          String strFileName = new String(inputBytes, 12, iFileNameLength);
          mylog.info("要上傳的檔名是：" + strFileName);
          //初始化上傳檔案路徑
          strUpFilePath = main.strServHome + strFileName;
          File upFile = new File(strUpFilePath);
          m_fileOutputStream = new FileOutputStream(upFile);
        }
        else if (strClientOrder.equals("UPDATAS"))
        { //表示本包是要上傳的資料
          //本次資料包的長度
          mylog.info("正在接收檔案...");
          int iDataLength = Integer.parseInt(new String(inputBytes, 7, 5));
          m_fileOutputStream.write(inputBytes, 12, iDataLength);
          m_fileOutputStream.flush();
        }
        else if (strClientOrder.equals("DNFILEN"))
        { //表示要下載的檔名，伺服器要執行向客戶端傳輸檔案的操作
          int iFileNameLength = Integer.parseInt(new String(inputBytes, 7, 5));
          mylog.info("要下載的檔名的長度為" + iFileNameLength);
          String strFileName = new String(inputBytes, 12, iFileNameLength);
          mylog.info("要下載的檔名是：" + strFileName);
          //初始化上傳檔案路徑
          strDnFilePath = main.strServHome + strFileName;
          File dnFile = new File(strDnFilePath);
          //初始化了檔案輸出流
          m_fileInputStream = new FileInputStream(dnFile);

          //開始向客戶端傳輸檔案
          mylog.info("開始向客戶端傳輸檔案" + strFileName + "...");
          int iInputLength = 0;
          String strInputLength;
          byte[] readBytes = new byte[ibufferlength];
          while ( (iInputLength = m_fileInputStream.read(readBytes, 0,
              ibufferlength)) !=
                 -1)
          {
            strInputLength = PublicFunc.formatLength(iInputLength);
            byte[] outBytes = PublicFunc.makepackage("DNDATAS", strInputLength,
                readBytes);
            m_outputStream.write(outBytes, 0, outBytes.length);
            m_outputStream.flush();
          }

          //最後傳送一個檔案結束標記
          m_outputStream.write(PublicFunc.makepackage("ENDFILE", "00001",
              new byte[1]));
          m_outputStream.flush();
        }
      }
    }
    catch (Exception e)
    {
      mylog.warning(e.getMessage());
    }
  }

  public void exit()
  {
    try
    {
      m_outputStream.write(PublicFunc.makepackage("DISCONN", "00001",
                                                  new byte[1]));
      m_inputStream.close();
      m_outputStream.close();
      main.deleteClient(this);
      main = null;
    }
    catch (Exception e)
    {
      mylog.warning("在斷開客戶端#" + this.iClientNum + "連線時出現異常！");
      mylog.warning(e.getMessage());
    }
  }

  public String[] AdjustStrings(String[] strFileNames)
  {
    String[] strItemNames = new String[strFileNames.length + 1];
    strItemNames[0] = "返回上一級";
    int j = 1;
    for (int i = 0; i < strFileNames.length; i++)
    {
      File upFile = new File(main.strServHome + strFileNames[i]);
      if (!upFile.isFile())
      {
        strItemNames[j++] = "[資料夾]" + strFileNames[i];
      }
    }
    for (int i = 0; i < strFileNames.length; i++)
    {
      File upFile = new File(main.strServHome + strFileNames[i]);
      if (upFile.isFile())
      {
        strItemNames[j++] = strFileNames[i];
      }
    }

    return strItemNames;
  }

}

package cn.edu.bit.software.ftptrans;

import java.io.*;
import java.net.*;
import java.util.logging.*;
import java.util.Vector;

public class FtpClient extends Thread
{
  Logger mylog = Logger.getLogger("FtpClient");
  Socket m_clientSocket;
  //緩衝位元組資料的大小
  private int ibufferlength;

  //緩衝位元組陣列
  byte[] inputBytes;

  Vector vecServFiles;

  //用於得到從socket端的輸入資訊
  InputStream m_inputStream;
  //用於向socket輸出的輸出流
  OutputStream m_outputStream;
  //向本地寫檔案的檔案輸出流
  FileOutputStream m_fileOutputStream;
  //從本地讀檔案的檔案輸入流
  FileInputStream m_fileInputStream;
  //從伺服器端傳來的指令
  String strServerOrder;
  //主機的ip地址
  String strServerIP;
  //伺服器的埠號
  int iServerPort;

  public FtpClient(String strServIP, int iServPort)
  {
    strServerIP = strServIP;
    iServerPort = iServPort;
  }

  public void run()
  {
    try
    {
      //建立連線
      m_clientSocket = new Socket(strServerIP, iServerPort);
      mylog.info("已經連到了主機" + strServerIP + "在埠" + iServerPort);
      m_inputStream = m_clientSocket.getInputStream();
      m_outputStream = m_clientSocket.getOutputStream();
      mylog.fine("客戶端得到了socket的輸入輸出流！");
      ibufferlength = 1024;
      inputBytes = new byte[ibufferlength + 12];
      vecServFiles = new Vector();
      doBusiness();
    }
    catch (UnknownHostException e)
    {
      mylog.warning("伺服器地址未知");
      mylog.warning(e.getMessage());
    }
    catch (IOException e)
    {
      mylog.warning(e.getMessage());
    }
    catch (Exception e)
    {
      mylog.warning(e.getMessage());
    }
  }

  public void doBusiness()
  {
    try
    {
      int iLength = 0;
      while ( (iLength = m_inputStream.read(inputBytes, 0, ibufferlength + 12)) !=
             -1)
      {
        strServerOrder = new String(inputBytes, 0, 7);
        if (strServerOrder.equals("DISCONN"))
        { //斷開連線
          mylog.info("在client端得到了DISCONN");
          int length = Integer.parseInt(new String(inputBytes, 7, 5));
          mylog.info("長度是" + length);
        }
        else if (strServerOrder.equals("LSFILES"))
        { //接收伺服器當前目錄檔案列表

          int iDataLength = Integer.parseInt(new String(inputBytes, 7, 5));
          mylog.info("在客戶端這個檔名的長度是：" + iDataLength);
          String strFileName = new String(inputBytes, 12, iDataLength);
          mylog.info("客戶端正在獲取伺服器目錄資訊....." + strFileName);
          vecServFiles.add(strFileName);
        }
        else if (strServerOrder.equals("ENDFILE"))
        { //下載一個檔案的結束標記
          mylog.info("收到下載檔案結束標誌符號");
          m_fileOutputStream.close();
        }
        else if (strServerOrder.equals("DNDATAS"))
        { //表示本包是要下載的資料
          int iDataLength = Integer.parseInt(new String(inputBytes, 7, 5));
          m_fileOutputStream.write(inputBytes, 12, iDataLength);
          m_fileOutputStream.flush();
        }
      }
    }
    catch (Exception e)
    {

    }
  }

  /**
   * 客戶端上傳檔名
   * @param strFileName 要上傳檔案的檔名
   */
  public void upFileName(String strFileName)
  {
    try
    {
      String strLength = PublicFunc.formatLength(strFileName.getBytes().length);
      byte[] outBytes = PublicFunc.makepackage("UPFILEN", strLength,
                                               strFileName.getBytes());
      m_outputStream.write(outBytes, 0, outBytes.length);
      m_outputStream.flush();
    }
    catch (Exception e)
    {
      mylog.warning("在客戶端在向伺服器寫要上傳的檔名時發生異常");
      mylog.warning(e.getMessage());
    }
  }

  /**
   * 講本地檔案strFilePath上傳到伺服器
   * @param strFilePath 本地檔案路徑
   */
  public void upFileData(String strFilePath)
  {
    try
    {
      File file = new File(strFilePath);
      m_fileInputStream = new FileInputStream(file);

      int iInputLength = 0;
      String strInputLength;
      byte[] readBytes = new byte[ibufferlength];
      while ( (iInputLength = m_fileInputStream.read(readBytes, 0,
          ibufferlength)) !=
             -1)
      {
        strInputLength = PublicFunc.formatLength(iInputLength);
        byte[] outBytes = PublicFunc.makepackage("UPDATAS", strInputLength,
                                                 readBytes);
        m_outputStream.write(outBytes, 0, outBytes.length);
        m_outputStream.flush();
      }
      //最後傳送一個檔案結束標記
      m_outputStream.write(PublicFunc.makepackage("ENDFILE", "00001",
                                                  new byte[1]));
      m_outputStream.flush();
    }
    catch (Exception e)
    {
      mylog.warning("從客戶端向伺服器傳輸檔案內容是發生異常");
      mylog.warning(e.getMessage());
    }
  }