在Java中，實現用戶注冊和登錄需要使用數據庫和Web框架。以下是常用的方法：

Step 1: 創建數據庫

CREATE TABLE `user` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`username` varchar(50) NOT NULL,
`password` varchar(50) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Step 2: 創建Java Bean

public class User {
private int id;
private String username;
private String password;
// 省略getter和setter
}

Step 3: 創建DAO

public class UserDao {
public int addUser(User user) {
int result = 0;
try {
Connection connection = DBUtil.getConnection();
String sql = "INSERT INTO user (username, password) VALUES (?, ?)";
PreparedStatement preparedStatement = connection.prepareStatement(sql);
preparedStatement.setString(1, user.getUsername());
preparedStatement.setString(2, user.getPassword());
result = preparedStatement.executeUpdate();
} catch (SQLException e) {
e.printStackTrace();
} finally {
DBUtil.close();
}
return result;
}
public User getUser(String username, String password) {
User user = null;
try {
Connection connection = DBUtil.getConnection();
String sql = "SELECT * FROM user WHERE username=? AND password=?";
PreparedStatement preparedStatement = connection.prepareStatement(sql);
preparedStatement.setString(1, username);
preparedStatement.setString(2, password);
ResultSet resultSet = preparedStatement.executeQuery();
if (resultSet.next()) {
user = new User();
user.setId(resultSet.getInt("id"));
user.setUsername(resultSet.getString("username"));
user.setPassword(resultSet.getString("password"));
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
DBUtil.close();
}
return user;
}
}

Step 4: 創建Servlet

public class RegisterServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String username = request.getParameter("username");
String password = request.getParameter("password");
User user = new User();
user.setUsername(username);
user.setPassword(password);
UserDao userDao = new UserDao();
if (userDao.addUser(user) == 1) {
response.sendRedirect("/login.jsp");
} else {
request.setAttribute("message", "注冊失敗");
request.getRequestDispatcher("/register.jsp").forward(request, response);
}
}
}
public class LoginServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String username = request.getParameter("username");
String password = request.getParameter("password");
UserDao userDao = new UserDao();
User user = userDao.getUser(username, password);
if (user != null) {
HttpSession session = request.getSession();
session.setAttribute("user", user);
response.sendRedirect("/index.jsp");
} else {
request.setAttribute("message", "用戶名或密碼錯誤");
request.getRequestDispatcher("/login.jsp").forward(request, response);
}
}
}