java实现ftp客户端

要实现一个 Java FTP 客户端，可以使用 Apache Commons Net 库。下面是一个简单的示例代码，可以连接到 FTP 服务器并下载文件：

import java.io.FileOutputStream;
import java.io.IOException;
import org.apache.commons.net.ftp.FTPClient;
import org.apache.commons.net.ftp.FTPReply;

public class FTPClientExample {
    public static void main(String[] args) {
        String server = "ftp.example.com";
        int port = 21;
        String user = "username";
        String password = "password";
        String remoteFile = "/remote/file.txt";
        String localFile = "local/file.txt";

        FTPClient ftpClient = new FTPClient();
        try {
            ftpClient.connect(server, port);
            ftpClient.login(user, password);

            int replyCode = ftpClient.getReplyCode();
            if (!FTPReply.isPositiveCompletion(replyCode)) {
                System.out.println("FTP server refused connection.");
                return;
            }

            ftpClient.enterLocalPassiveMode();
            ftpClient.setFileType(FTPClient.BINARY_FILE_TYPE);

            FileOutputStream outputStream = new FileOutputStream(localFile);
            boolean success = ftpClient.retrieveFile(remoteFile, outputStream);
            outputStream.close();

            if (success) {
                System.out.println("File downloaded successfully.");
            } else {
                System.out.println("Failed to download file.");
            }

            ftpClient.logout();
            ftpClient.disconnect();
        } catch (IOException ex) {
            System.out.println("Error: " + ex.getMessage());
            ex.printStackTrace();
        }
    }
}

这个示例代码连接到 FTP 服务器，登录，设置传输模式为二进制模式，下载文件并关闭连接。你可以根据你的需求修改代码。