java 写一个ftp

要实现一个FTP，需要使用Java中的FTP客户端库。常用的FTP客户端库有Apache Commons Net和Java FTP Client。

下面是使用Apache Commons Net实现FTP上传文件的示例代码：

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import org.apache.commons.net.ftp.FTP;
import org.apache.commons.net.ftp.FTPClient;

public class FtpClientExample {
    public static void main(String[] args) {
        String server = "ftp.example.com";
        int port = 21;
        String user = "username";
        String password = "password";
        FTPClient ftpClient = new FTPClient();
        try {
            ftpClient.connect(server, port);
            ftpClient.login(user, password);
            ftpClient.enterLocalPassiveMode();
            ftpClient.setFileType(FTP.BINARY_FILE_TYPE);
            File localFile = new File("localfile.txt");
            String remoteFile = "remotefile.txt";
            FileInputStream inputStream = new FileInputStream(localFile);
            System.out.println("Start uploading file");
            boolean done = ftpClient.storeFile(remoteFile, inputStream);
            inputStream.close();
            if (done) {
                System.out.println("The file is uploaded successfully.");
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (ftpClient.isConnected()) {
                    ftpClient.logout();
                    ftpClient.disconnect();
                }
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
    }
}

这段代码通过FTP连接到服务器，设置传输模式为二进制模式，然后上传一个本地文件到FTP服务器。可以根据需要修改代码实现其他FTP操作。