java连接ftp服务器失败

import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPClient;

public class FTPUploadExample {
    public static void main(String[] args) {
        String server = "ftp.example.com";
        int port = 21;
        String user = "username";
        String pass = "password";

        FTPClient ftpClient = new FTPClient();
        try {
            // 连接到FTP服务器
            ftpClient.connect(server, port);
            // 登录
            boolean login = ftpClient.login(user, pass);
            if (login) {
                System.out.println("Connected and logged in successfully.");

                // 设置文件类型为二进制，防止文件损坏
                ftpClient.setFileType(FTP.BINARY_FILE_TYPE);
                // 开启被动模式
                ftpClient.enterLocalPassiveMode();

                // 上传文件
                String localFilePath = "local-file.txt";
                String remoteFilePath = "remote-file.txt";
                boolean uploaded = ftpClient.storeFile(remoteFilePath, new FileInputStream(localFilePath));
                if (uploaded) {
                    System.out.println("File uploaded successfully.");
                } else {
                    System.out.println("File upload failed.");
                }

                // 登出并断开连接
                ftpClient.logout();
            } else {
                System.out.println("Could not login to the server.");
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ftpClient.isConnected()) {
                    ftpClient.disconnect();
                }
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
    }
}