Java Applet 實作 FTP 用戶端功能

示範在 Applet 中使用 sun.net.ftp.FtpClient 實作 FTP 連線、登入、列出檔案、上傳檔案功能。介面包含主機、帳號、密碼輸入欄、連線/斷開按鈕、檔案列表顯示區。注意：sun.* 套件為內部 API，不保證跨 JDK 版本相容，生產環境建議改用 Apache Commons Net 等標準庫。

import sun.net.ftp.*;
import sun.net.*;
import java.awt.*;
import java.awt.event.*;
import java.applet.*;
import java.io.*;

public class FtpApplet extends Applet {
    FtpClient ftp;
    DataOutputStream outputs;
    TelnetInputStream ins;
    TelnetOutputStream outs;
    TextArea lsArea;
    Label lblPrompt;
    Button btnConn, btnClose;
    TextField txtHost, txtUser, txtPass;
    String hostname = "";
    String msg = "未連線";

    public void init() {
        setBackground(Color.white);
        setLayout(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();

        lblPrompt = new Label(msg);
        btnConn = new Button("連接");
        btnClose = new Button("斷開"); btnClose.setEnabled(false);
        txtHost = new TextField(20);
        txtUser = new TextField(15);
        txtPass = new TextField(15); txtPass.setEchoChar('*');
        lsArea = new TextArea(30, 80); lsArea.setEditable(false);

        // 佈局程式碼省略，請參考原始碼
    }

    public boolean connect(String host, String user, String pass) {
        this.hostname = host;
        lblPrompt.setText("連線中...");
        try {
            ftp = new FtpClient(host);
            ftp.login(user, pass);
            ftp.binary();
            showFileContents();
            lblPrompt.setText("連線成功：" + host);
            return true;
        } catch (FtpLoginException e) { msg = "登入失敗：" + host; }
        catch (IOException e) { msg = "連線失敗：" + host; }
        catch (SecurityException e) { msg = "安全限制：" + host; }
        lblPrompt.setText(msg);
        return false;
    }

    public void stop() {
        try { if (ftp != null) ftp.closeServer(); } catch (IOException ignored) {}
    }

    public boolean action(Event evt, Object obj) {
        if (evt.target == btnConn) {
            if (connect(txtHost.getText(), txtUser.getText(), txtPass.getText())) {
                btnConn.setEnabled(false); btnClose.setEnabled(true);
            }
            return true;
        }
        if (evt.target == btnClose) {
            stop();
            btnConn.setEnabled(true); btnClose.setEnabled(false);
            lblPrompt.setText("已斷開：" + hostname);
            return true;
        }
        return super.action(evt, obj);
    }

    public boolean sendFile(String path) {
        if (ftp == null) return false;
        lblPrompt.setText("上傳中...");
        try {
            String fileName = path.substring(path.lastIndexOf("\\") + 1);
            RandomAccessFile raf = new RandomAccessFile(path, "r");
            outs = ftp.put(fileName);
            outputs = new DataOutputStream(outs);
            int ch;
            while ((ch = raf.read()) != -1) outputs.write(ch);
            outs.close(); raf.close();
            lblPrompt.setText("上傳成功");
            showFileContents();
            return true;
        } catch (IOException e) {
            lblPrompt.setText("上傳失敗");
            return false;
        }
    }

    void showFileContents() {
        try {
            ins = ftp.list();
            StringBuilder sb = new StringBuilder();
            int ch;
            while ((ch = ins.read()) >= 0) sb.append((char)ch);
            lsArea.setText(sb.toString());
            ins.close();
        } catch (IOException ignored) {}
    }

    public static void main(String[] args) {
        Frame f = new Frame("FTP Client");
        f.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) { System.exit(0); }
        });
        FtpApplet applet = new FtpApplet();
        applet.init(); applet.start();
        f.add(applet); f.pack(); f.setVisible(true);
    }
}

安全性提醒：

• Applet 需簽名才能突破沙箱限制存取本地檔案與網路連線。
• 現代瀏覽器已停用 Java Applet 支援，建議改為獨立應用程式或 Web 應用實作 FTP 功能。