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() {@Overridepublic 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();}
}