Java写一个登录注册源代码，要求连接数据库，有登录，注册，忘记密码，注册要求输入两次密码，忘记密码要求选择忘记密码后，输入登录过的账号，直接输入两次密码

这是一个基于Java Swing和MySQL数据库的简单登录注册应用程序的源代码，包括登录，注册，忘记密码功能：

import java.awt.*;
import java.awt.event.*;
import java.sql.*;
import javax.swing.*;

public class Login implements ActionListener {
    JFrame frame;
    JLabel userLabel, passLabel, errorLabel, pass2Label;
    JTextField userField;
    JPasswordField passField, pass2Field;
    JButton loginButton, registerButton, forgotButton;
    JPanel panel;

    Connection con;
    PreparedStatement ps;

    Login() {
        frame = new JFrame("Login/Register/Forgot Password");
        frame.setBounds(100, 100, 350, 250);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setResizable(false);

        panel = new JPanel();
        panel.setLayout(null);
        frame.add(panel);

        userLabel = new JLabel("Username:");
        userLabel.setBounds(50, 30, 100, 25);
        panel.add(userLabel);

        userField = new JTextField();
        userField.setBounds(150, 30, 150, 25);
        panel.add(userField);

        passLabel = new JLabel("Password:");
        passLabel.setBounds(50, 65, 100, 25);
        panel.add(passLabel);

        passField = new JPasswordField();
        passField.setBounds(150, 65, 150, 25);
        panel.add(passField);

        errorLabel = new JLabel("");
        errorLabel.setBounds(50, 100, 250, 25);
        errorLabel.setForeground(Color.RED);
        panel.add(errorLabel);

        loginButton = new JButton("Login");
        loginButton.setBounds(50, 135, 100, 25);
        loginButton.addActionListener(this);
        panel.add(loginButton);

        registerButton = new JButton("Register");
        registerButton.setBounds(150, 135, 100, 25);
        registerButton.addActionListener(this);
        panel.add(registerButton);

        forgotButton = new JButton("Forgot Password");
        forgotButton.setBounds(50, 170, 200, 25);
        forgotButton.addActionListener(this);
        panel.add(forgotButton);

        pass2Label = new JLabel("Confirm Password:");
        pass2Label.setBounds(50, 100, 120, 25);
        pass2Label.setVisible(false);
        panel.add(pass2Label);

        pass2Field = new JPasswordField();
        pass2Field.setBounds(170, 100, 130, 25);
        pass2Field.setVisible(false);
        panel.add(pass2Field);

        frame.setVisible(true);

        // 连接到MySQL数据库
        try {
            Class.forName("com.mysql.jdbc.Driver");
            con = DriverManager.getConnection("jdbc:mysql://localhost:3306/login", "root", "");
        } catch (Exception ex) {
            System.out.println("Error: " + ex.getMessage());
        }
    }

    public void actionPerformed(ActionEvent e) {
        // 点击登录按钮
        if (e.getSource() == loginButton) {
            String username = userField.getText();
            String password = new String(passField.getPassword());

            try {
                // 查询用户是否存在
                ps = con.prepareStatement("SELECT * FROM users WHERE username = ? AND password = ?");
                ps.setString(1, username);
                ps.setString(2, password);
                ResultSet rs = ps.executeQuery();

                if (rs.next()) {
                    // 登录成功
                    errorLabel.setText("");
                    JOptionPane.showMessageDialog(frame, "Login successful!");
                } else {
                    // 登录失败
                    errorLabel.setText("Invalid username or password.");
                }
            } catch (SQLException ex) {
                System.out.println("Error: " + ex.getMessage());
            }
        }

        // 点击注册按钮
        if (e.getSource() == registerButton) {
            String username = userField.getText();
            String password = new String(passField.getPassword());
            String password2 = new String(pass2Field.getPassword());

            if (!password.equals(password2)) {
                // 两次密码不一致
                errorLabel.setText("Passwords do not match.");
            } else {
                try {
                    // 插入新用户
                    ps = con.prepareStatement("INSERT INTO users (username, password) VALUES (?, ?)");
                    ps.setString(1, username);
                    ps.setString(2, password);
                    int result = ps.executeUpdate();

                    if (result != 0) {
                        // 注册成功
                        errorLabel.setText("");
                        JOptionPane.showMessageDialog(frame, "Registration successful!");
                    } else {
                        // 注册失败
                        errorLabel.setText("Registration failed.");
                    }
                } catch (SQLException ex) {
                    System.out.println("Error: " + ex.getMessage());
                }
            }
        }

        // 点击忘记密码按钮
        if (e.getSource() == forgotButton) {
            String username = JOptionPane.showInputDialog(frame, "Enter username:");

            if (username != null) {
                String password = new String(JOptionPane.showInputDialog(frame, "Enter new password:").toCharArray());
                String password2 = new String(JOptionPane.showInputDialog(frame, "Confirm new password:").toCharArray());

                if (!password.equals(password2)) {
                    // 两次密码不一致
                    errorLabel.setText("Passwords do not match.");
                } else {
                    try {
                        // 更新用户密码
                        ps = con.prepareStatement("UPDATE users SET password = ? WHERE username = ?");
                        ps.setString(1, password);
                        ps.setString(2, username);
                        int result = ps.executeUpdate();

                        if (result != 0) {
                            // 更新密码成功
                            errorLabel.setText("");
                            JOptionPane.showMessageDialog(frame, "Password updated!");
                        } else {
                            // 更新密码失败
                            errorLabel.setText("User not found.");
                        }
                    } catch (SQLException ex) {
                        System.out.println("Error: " + ex.getMessage());
                    }
                }
            }
        }
    }

    public static void main(String[] args) {
        new Login();
    }
}

这个应用程序使用了Java Swing库来创建GUI界面，使用了MySQL数据库，可以通过更改连接字符串的参数来连接到其他数据库。

注意：此示例代码仅用于学习和演示目的，不应在生产环境中使用。在实际应用程序中，需要实现更多的安全保护措施，如加密密码等。