使用java写一个ftp程序

好的，以下是一个简单的 Java FTP 程序：

import java.io.*;
import java.net.*;

public class FTPClient {
    public static void main(String[] args) {
        String server = "ftp.example.com";
        int port = 21;
        String user = "username";
        String pass = "password";
        String remoteFilePath = "/path/to/remote/file.txt";
        String localFilePath = "/path/to/local/file.txt";

        try {
            // 连接 FTP 服务器
            Socket socket = new Socket(server, port);
            BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));

            // 读取 FTP 服务器的欢迎信息
            String response = reader.readLine();
            System.out.println("FTP Server says: " + response);

            // 发送用户名和密码
            writer.write("USER " + user + "\r\n");
            writer.flush();
            response = reader.readLine();
            System.out.println("FTP Server says: " + response);

            writer.write("PASS " + pass + "\r\n");
            writer.flush();
            response = reader.readLine();
            System.out.println("FTP Server says: " + response);

            // 切换到被动模式
            writer.write("PASV\r\n");
            writer.flush();
            response = reader.readLine();
            System.out.println("FTP Server says: " + response);

            // 解析被动模式的响应信息，获取数据传输端口号
            String[] parts = response.split(",");
            int port1 = Integer.parseInt(parts[4]);
            int port2 = Integer.parseInt(parts[5].substring(0, parts[5].length() - 2));
            int dataPort = (port1 << 8) + port2;

            // 建立数据传输连接
            Socket dataSocket = new Socket(server, dataPort);
            DataOutputStream dataOutput = new DataOutputStream(dataSocket.getOutputStream());

            // 下载文件
            writer.write("RETR " + remoteFilePath + "\r\n");
            writer.flush();
            response = reader.readLine();
            System.out.println("FTP Server says: " + response);

            BufferedInputStream input = new BufferedInputStream(dataSocket.getInputStream());
            BufferedOutputStream output = new BufferedOutputStream(new FileOutputStream(localFilePath));

            byte[] buffer = new byte[1024];
            int bytesRead;
            while ((bytesRead = input.read(buffer)) != -1) {
                output.write(buffer, 0, bytesRead);
            }

            output.close();
            input.close();
            dataOutput.close();
            dataSocket.close();
            socket.close();

            System.out.println("File downloaded successfully!");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

上面的程序演示了如何连接 FTP 服务器、下载文件并保存到本地。你可以根据自己的需求进行修改和扩展。注意，这里只是一个简单的示例程序，实际使用中可能需要更多的错误处理和安全措施。