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编程是在 localhost 上模拟 FTP 服务器和客户端。服务器应该在 22222 端口监听客户端的命令，并在 22223 端口创建另一个连接来传输数据。我已经在同一个端口 22222 上实现了传输。问题是如何实际分离一个连接来接收命令：“get”，“put”并在不同的端口上创建另一个连接来传输数据？有人能帮帮我吗？

FTPServer.class

public class FTPServer {

public static void main(String args[]) throws Exception
{
    int PORT =22222;
    ServerSocket serverSocket = new ServerSocket(PORT); 
    System.out.println("FTP Server Listening on Port " + PORT);

    ExecutorService executor = Executors.newFixedThreadPool(10);

    while(true)
    {
        System.out.println("Server is waiting...");
        FileTransfer ft = new FileTransfer(serverSocket.accept());
        executor.execute(ft);
    }
}
}

class FileTransfer extends Thread
{
Socket clientSocket;

DataInputStream inFromClient;
DataOutputStream outToClient;

String filePath = "E:" +File.separator+"Studying"+
        File.separator+"WorkSpace"+File.separator+"FTPServer"+File.separator;


FileTransfer(Socket socket)
{
    try
    {
        clientSocket=socket;
        inFromClient = new DataInputStream(clientSocket.getInputStream());
        outToClient = new DataOutputStream(clientSocket.getOutputStream());

    }catch(Exception ex){}
}


public void ClientDownLoad() throws Exception
{
    String fileName = inFromClient.readUTF();

    String accessfile = filePath + fileName;

    File file = new File(accessfile);
    if(!file.exists())
    {
        outToClient.writeUTF("FILE NOT FOUND");
        return;
    }
    else
    {
        outToClient.writeUTF("Ready for download");

        FileInputStream f = new FileInputStream(file);
        int numOfByte;
        do
        {
            numOfByte = f.read();
            outToClient.writeUTF(String.valueOf(numOfByte));
        }while(numOfByte!=-1);

        f.close();
        outToClient.writeUTF("Download Completed");
    }   
}

public void ClientUpLoad() throws Exception
{
    String fileName = inFromClient.readUTF();
    String userChoose;
    String accessFile = filePath + fileName;
    File file=new File(accessFile);
    if(fileName.equalsIgnoreCase("No such file"))
    {
        return;
    }
    if(file.exists())
    {
        outToClient.writeUTF("File Exists in Server");
        userChoose=inFromClient.readUTF();
    }
    else
    {
        outToClient.writeUTF("OK...You can send file");
        userChoose="y";
    }

    if(userChoose.equalsIgnoreCase("y"))
    {
        FileOutputStream fileOut = new FileOutputStream(file);
        int numOfByte;
        String str;
        do
        {
            str=inFromClient.readUTF();
            numOfByte=Integer.parseInt(str);
            if(numOfByte!=-1)
            {
                fileOut.write(numOfByte);
            }
        }while(numOfByte!=-1);
        fileOut.close();
        outToClient.writeUTF("Uploading Completed");                
    }
    else
    {
        return;
    }


}


public void run()
{
    while(true)
    {
        try
        {
            String str = inFromClient.readUTF();
            if(str.equalsIgnoreCase("put"))
            {
                System.out.println("Accept Uploading...");
                ClientUpLoad();
                continue;
            }
            else if(str.equalsIgnoreCase("get"))
            {
                System.out.println("Accept Downloading");
                ClientDownLoad();
                continue;
            }
            else if(str.equalsIgnoreCase("exit"))
            {
                System.out.println("Accept Exit");
                System.exit(1);
            }
        }catch(Exception e){}


    }
}

}

FTPClient.class

public class FTPClient {

public static void main(String[] args) throws Exception
{
    final int PORT=22222;
    String serverHostname = new String("127.0.0.1");
    Socket clientSocket = new Socket(serverHostname, PORT);
    clientOperation co=new clientOperation(clientSocket);

    co.displaySelection();

}
}

class clientOperation
{
String filePath = "E:" +File.separator+"Studying"+
        File.separator+"WorkSpace"+File.separator+"FTPClient"+File.separator;
Socket socket;

DataInputStream inFromServer;
DataOutputStream outToServer;
BufferedReader br;

clientOperation(Socket s)
{
    try
    {
        socket=s;
        inFromServer=new DataInputStream(socket.getInputStream());
        outToServer=new DataOutputStream(socket.getOutputStream());
        br=new BufferedReader(new InputStreamReader(System.in));
    }catch(Exception e){}

}

public void ClientUpLoad() throws Exception
{
    String fileName;
    System.out.println("Enter File Name for uploading: ");
    fileName=br.readLine();

    String accessfile=filePath + fileName;
    System.out.println(accessfile);
    File file = new File(accessfile);
    if(!file.exists())
    {
        System.out.println("File Not Found on Client");
        outToServer.writeUTF("no such file");
        return;
    }

    outToServer.writeUTF(fileName);

    String existingMsg = inFromServer.readUTF();
    if(existingMsg.compareToIgnoreCase("File Exists in Server")==0)
    {
        System.out.println("File Exists in Server. Do you want to replace it? (Y/N)");
        String userChoose=br.readLine();
        if(userChoose.equalsIgnoreCase("y"))
        {
            outToServer.writeUTF("y");
        }
        else if(userChoose.equalsIgnoreCase("n"))
        {
            outToServer.writeUTF("n");
            return;
        }
        else{

        }
    }

    System.out.println("Uploading...");

    FileInputStream fi = new FileInputStream(file);
    int numOfByte;
    do
    {
        numOfByte=fi.read();
        outToServer.writeUTF(String.valueOf(numOfByte));
    }while(numOfByte!=-1);

    fi.close();
    System.out.println(inFromServer.readUTF());
}

public void ClientDownLoad()throws Exception
{
    String fileName;
    String userChoose;
    System.out.println("Enter File Name for download: ");
    fileName=br.readLine();

    outToServer.writeUTF(fileName);
    String str = inFromServer.readUTF();

    if(str.equalsIgnoreCase("FILE NOT FOUND"))
    {
        System.out.println("File doesn't exist on server...");
        return;
    }
    else if(str.equalsIgnoreCase("Ready for download"))
    {

        String accessfile=filePath + fileName;
        File file=new File(accessfile);
        if(file.exists())
        {
            System.out.println("File exists in Client. Do you want to replace it? (Y/N)");
            userChoose=br.readLine();
            if(userChoose.equalsIgnoreCase("n"))
            {
                outToServer.flush();
                return;
            }
        }

        FileOutputStream fileOut = new FileOutputStream(file);
        int numOfByte;
        do
        {
            numOfByte=Integer.parseInt(inFromServer.readUTF());
            if(numOfByte!=-1)
            {
                fileOut.write(numOfByte);
            }
        }while(numOfByte!=-1);

        fileOut.close();
        System.out.println(inFromServer.readUTF());
    }


}

public void displaySelection() throws Exception
{

    while(true)
    {
        Scanner scan = new Scanner(System.in);

        System.out.println("Please select the number for purpose!");
        System.out.println("1. Upload File to Server");
        System.out.println("2. Download File from Server");
        System.out.println("3. Exit");
        System.out.println("Enter your choice: ");

        int choice = scan.nextInt();
        if(choice ==1)
        {
            outToServer.writeUTF("put");
            ClientUpLoad();
        }
        else if(choice == 2)
        {
            outToServer.writeUTF("get");
            ClientDownLoad();
        }
        else
        {
            outToServer.writeUTF("exit");
            System.exit(1);
        }
    }
}

 }
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