import java.awt.Font;

 import java.awt.event.*;

 import java.sql.*;

 import javax.swing.*;

 import javax.swing.border.EmptyBorder;

 @SuppressWarnings("serial")

 public class Login extends JFrame {

     JLabel jla_tips;

     JLabel jla_username;

     JTextField jtf_username;

     JLabel jla_password;

     JPasswordField jpf_password;

     JButton jbt_login;

     JButton jbt_cancel;

     int count=0;

     public Login() {

         ImageIcon icon = new ImageIcon("images/favicon.png");

         setIconImage(icon.getImage());

         JPanel contentPane = new JPanel();

         contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));

         setContentPane(contentPane);

         contentPane.setLayout(null);

         jla_tips = new JLabel();

         jla_tips.setBounds(155, 40, 200, 25);

         contentPane.add(jla_tips);

         jla_username = new JLabel("用户名:");

         jla_username.setBounds(80, 70, 60, 25);

         contentPane.add(jla_username);

         jtf_username = new JTextField();

         jtf_username.setHorizontalAlignment(JTextField.LEFT);

         jtf_username.setBounds(150, 70, 150, 25);

         contentPane.add(jtf_username);

         jla_password = new JLabel("密  码:");

         jla_password.setBounds(80, 120, 60, 25);

         contentPane.add(jla_password);

         jpf_password = new JPasswordField();

         jpf_password.setBounds(150, 120, 150, 25);

         contentPane.add(jpf_password);

         // 登录按钮

         jbt_login = new JButton("登录");

         jbt_login.setFocusPainted(false);

         jbt_login.setBounds(150, 170, 70, 30);

         contentPane.add(jbt_login);

         jbt_login.addActionListener(new loginListener());

         jpf_password.addActionListener(new loginListener());

         // 置空按钮

         jbt_cancel = new JButton("取消");

         jbt_cancel.setFocusPainted(false);

         jbt_cancel.setBounds(235, 170, 70, 30);

         contentPane.add(jbt_cancel);

         jbt_cancel.addActionListener(new cancelListener());

         // 背景图

         Icon login3 = new ImageIcon("images/login3.jpg");

         JLabel jla_login3 = new JLabel(login3);

         jla_login3.setBounds(0, 0, 450, 300);

         contentPane.add(jla_login3);

         // 字体设置

         Font fontstr = new Font("宋体", Font.PLAIN, 14);

         jla_username.setFont(fontstr);

         jla_password.setFont(fontstr);

         jla_tips.setFont(fontstr);

         jbt_login.setFont(fontstr);

         jbt_cancel.setFont(fontstr);

         setTitle("登录");

         setSize(450, 300);

         setLocationRelativeTo(null);

         setResizable(false);

         setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

         setVisible(true);

     }

     public class cancelListener implements ActionListener{

         @Override

         public void actionPerformed(ActionEvent arg0) {

             jtf_username.setText(null);

             jpf_password.setText(null);

         }

     }

     public class loginListener implements ActionListener {

         @Override

         public void actionPerformed(ActionEvent e) {

             String username = jtf_username.getText().trim();

             char[] values = jpf_password.getPassword();

             String password = new String(values);

             if (username != null && username.length() != 0 && password != null

                     && password.length() != 0) {

                 try {

                     // 1.注册驱动

                     Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");

                     // 2.建立连接

                     // 2.1连接串

                     String DBurl = "jdbc:sqlserver://localhost:1433;DatabaseName=JavaDB";

                     String DBname="sa";

                     String DBpwd="940523";

                     Connection con = DriverManager.getConnection(DBurl,DBname,DBpwd);

                     // 创建会话

 //                    Statement st = con.createStatement();

 //                    String sql = "select count(*) as cnt from T_user where username='"

 //                            + username + "' and password='" + password + "'";

 //                    ResultSet rs = st.executeQuery(sql);

                     /*

                      * 防止SQL注入

                      */

                     String sql = "select count(*) as cnt from T_user where username=? and password=?";

                     PreparedStatement st = con.prepareStatement(sql);

                     st.setString(1, username);

                     st.setString(2, password);

                     String sql2 = "select count(*) as cnt from T_user where username=?";

                     PreparedStatement st2 = con.prepareStatement(sql2);

                     st2.setString(1, username);

                     // 结果集

                     ResultSet rs = st.executeQuery();

                     ResultSet rs2 = st2.executeQuery();

                     rs.next();

                     rs2.next();

                     if (rs.getInt("cnt") > 0) {

                         setVisible(false);

                         new Hello();

                     } else if (rs2.getInt("cnt") == 0) {

                         JOptionPane.showMessageDialog(null, "无该用户!");

                     }else {

                         jla_tips.setText("密码错误!");

                         count++;

                         if(count==3){

                             JOptionPane.showMessageDialog(null, "登录失败超过三次!");

                             System.exit(0);

                         }

                     }

                 } catch (Exception ex) {

                     ex.printStackTrace();

                 }

             } else {

                 if (username.length() == 0 && password.length() == 0)

                     jla_tips.setText("帐号与密码均不能为空!");

                 else if (username.length() == 0)

                     jla_tips.setText("帐号不能为空!");

                 else if (password.length() == 0)

                     jla_tips.setText("密码不能为空!");

             }

         }

     }

     public static void main(String[] args) {

         new Login();

     }

 }

 import java.awt.Font;

 import javax.swing.*;

 import javax.swing.border.EmptyBorder;

 @SuppressWarnings("serial")

 public class Hello extends JFrame {

     JLabel jla_title;

     public Hello() {

         ImageIcon icon = new ImageIcon("images/favicon.png");

         setIconImage(icon.getImage());

         JPanel contentPane = new JPanel();

         contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));

         setContentPane(contentPane);

         contentPane.setLayout(null);

         jla_title = new JLabel();

         jla_title.setText("欢迎使用本软件!");

         jla_title.setBounds(120, 100, 200, 25);

         // 字体设置

         Font fontstr = new Font("宋体", Font.PLAIN, 26);

         jla_title.setFont(fontstr);

         contentPane.add(jla_title);

         setTitle("欢迎界面");

         setSize(450, 300);

         setLocationRelativeTo(null);

         setResizable(false);

         setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

         setVisible(true);

     }

     // public static void main(String[] args) {

     // new Hello();

     // }

 }