创建用户登录程序，验证用户账号和密码信息是否在数据库student中的用户表tb_account中存在。用户登录界面如下图所示：

当单击“登录”按钮时，处理以下几种情况：

（1）用户名未输入，提示用户名不能为空；

（2）密码未输入，提示密码不能未空；

（3）用户名输入错误或者密码输入错误（在tb_account表中查询不到该用户id和密码），提示用户名或密码错误；

（4）用户名和密码输入都正确（在tb_account表中能查询到该用户id和密码），提示登录成功。

完整代码如下：

1.首先建立databaseConnection类，代码如下：

import java.sql.*;
import java.awt.*;
import javax.swing.*;
public class databaseconnection {
    static final String JDBC_DRIVER = "com.mysql.cj.jdbc.Driver";
    static final String DB_URL = "jdbc:mysql://localhost:3306/student";
    static final String USER = "root";
    static final String PASS = "123456";
    public static boolean checkCredentials(String userId, String password) {
        Connection conn = null;
        boolean isValid = false;
        try {
            // 加载 JDBC 驱动
            Class.forName(JDBC_DRIVER);
            // 建立连接
            conn = DriverManager.getConnection(DB_URL, USER, PASS);
            // SQL 查询
            String sql = "SELECT COUNT(*) FROM tb_account WHERE user_id = ? AND password = ?";
            try (PreparedStatement pstmt = conn.prepareStatement(sql)) {
                pstmt.setString(1, userId);
                pstmt.setString(2, password);
                ResultSet resultSet = pstmt.executeQuery();
                if (resultSet.next()) {
                    int count = resultSet.getInt(1);
                    isValid = count > 0;
                }
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } finally {
            // 关闭数据库连接
            if (conn != null) {
                try {
                    conn.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        }
        return isValid;
    }
}

2.然后建立LoginFrame类，代码如下： 

import javax.swing.*;

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class LoginFrame extends JFrame {
    private JTextField userIdField;
    private JPasswordField passwordField;
    private JButton loginButton;

    public LoginFrame() {
        super("用户登录");
        createUI();
        setSize(300, 200);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setVisible(true);
    }

    private void createUI() {
        setLayout(new FlowLayout());

        userIdField = new JTextField(20);
        passwordField = new JPasswordField(20);
        loginButton = new JButton("登录");

        add(new JLabel("账号:"));
        add(userIdField);
        add(new JLabel("密码:"));
        add(passwordField);
        add(loginButton);

        loginButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                performLogin();
            }
        });
    }

    private void performLogin() {
        String userId = userIdField.getText();
        String password = new String(passwordField.getPassword());

        if (userId.isEmpty()) {
            JOptionPane.showMessageDialog(this, "用户名不能为空", "错误", JOptionPane.ERROR_MESSAGE);
        } else if (password.isEmpty()) {
            JOptionPane.showMessageDialog(this, "密码不能为空", "错误", JOptionPane.ERROR_MESSAGE);
        } else {
            boolean loginSuccess = databaseconnection.checkCredentials(userId, password);
            if (loginSuccess) {
                JOptionPane.showMessageDialog(this, "登录成功", "成功", JOptionPane.INFORMATION_MESSAGE);
            } else {
                JOptionPane.showMessageDialog(this, "用户名或密码错误", "错误", JOptionPane.ERROR_MESSAGE);
            }
        }
    }

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