写一个能够实现用户登录和对对象的增删查改功能的Javaweb代码

好的，这里给您提供一个简单的 JavaWeb 代码示例，实现用户登录和对对象的增删查改功能。

首先，我们需要创建一个名为 "User" 的 JavaBean，表示用户对象，包含以下属性：

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

接下来，我们需要创建一个名为 "UserDao" 的数据访问对象，用于实现用户数据的增删查改操作。这里我们使用 JDBC 技术与 MySQL 数据库进行交互，示例代码如下：

public class UserDao {
    // JDBC 驱动和数据库连接信息
    private String jdbcDriver = "com.mysql.jdbc.Driver";
    private String dbUrl = "jdbc:mysql://localhost:3306/test";
    private String dbUser = "root";
    private String dbPassword = "123456";

    // 获取数据库连接
    private Connection getConnection() throws ClassNotFoundException, SQLException {
        Class.forName(jdbcDriver);
        return DriverManager.getConnection(dbUrl, dbUser, dbPassword);
    }

    // 添加用户
    public void addUser(User user) throws ClassNotFoundException, SQLException {
        Connection conn = getConnection();
        String sql = "INSERT INTO user (username, password, email) VALUES (?, ?, ?)";
        PreparedStatement stmt = conn.prepareStatement(sql);
        stmt.setString(1, user.getUsername());
        stmt.setString(2, user.getPassword());
        stmt.setString(3, user.getEmail());
        stmt.executeUpdate();
        stmt.close();
        conn.close();
    }

    // 删除用户
    public void deleteUser(int id) throws ClassNotFoundException, SQLException {
        Connection conn = getConnection();
        String sql = "DELETE FROM user WHERE id=?";
        PreparedStatement stmt = conn.prepareStatement(sql);
        stmt.setInt(1, id);
        stmt.executeUpdate();
        stmt.close();
        conn.close();
    }

    // 修改用户
    public void updateUser(User user) throws ClassNotFoundException, SQLException {
        Connection conn = getConnection();
        String sql = "UPDATE user SET username=?, password=?, email=? WHERE id=?";
        PreparedStatement stmt = conn.prepareStatement(sql);
        stmt.setString(1, user.getUsername());
        stmt.setString(2, user.getPassword());
        stmt.setString(3, user.getEmail());
        stmt.setInt(4, user.getId());
        stmt.executeUpdate();
        stmt.close();
        conn.close();
    }

    // 查询用户
    public User getUser(int id) throws ClassNotFoundException, SQLException {
        Connection conn = getConnection();
        String sql = "SELECT * FROM user WHERE id=?";
        PreparedStatement stmt = conn.prepareStatement(sql);
        stmt.setInt(1, id);
        ResultSet rs = stmt.executeQuery();
        User user = null;
        if (rs.next()) {
            user = new User();
            user.setId(rs.getInt("id"));
            user.setUsername(rs.getString("username"));
            user.setPassword(rs.getString("password"));
            user.setEmail(rs.getString("email"));
        }
        rs.close();
        stmt.close();
        conn.close();
        return user;
    }

    // 验证用户登录
    public boolean verifyUser(String username, String password) throws ClassNotFoundException, SQLException {
        Connection conn = getConnection();
        String sql = "SELECT * FROM user WHERE username=? AND password=?";
        PreparedStatement stmt = conn.prepareStatement(sql);
        stmt.setString(1, username);
        stmt.setString(2, password);
        ResultSet rs = stmt.executeQuery();
        boolean result = rs.next();
        rs.close();
        stmt.close();
        conn.close();
        return result;
    }
}

接下来，我们需要创建一个名为 "UserController" 的控制器，用于接收用户请求并调用 UserDao 中的方法进行处理。示例代码如下：

public class UserController extends HttpServlet {
    private UserDao userDao;

    public void init() {
        userDao = new UserDao();
    }

    // 处理 GET 请求
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doPost(request, response);
    }

    // 处理 POST 请求
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // 设置字符编码为 UTF-8
        request.setCharacterEncoding("UTF-8");

        // 获取请求参数
        String action = request.getParameter("action");

        // 判断请求类型
        if ("login".equals(action)) {
            // 用户登录
            String username = request.getParameter("username");
            String password = request.getParameter("password");
            try {
                boolean result = userDao.verifyUser(username, password);
                if (result) {
                    // 登录成功，将用户信息存储到 Session 中
                    HttpSession session = request.getSession();
                    session.setAttribute("username", username);
                    response.sendRedirect("index.jsp");
                } else {
                    // 登录失败，返回登录页面，并显示错误信息
                    request.setAttribute("error", "用户名或密码错误！");
                    request.getRequestDispatcher("login.jsp").forward(request, response);
                }
            } catch (ClassNotFoundException | SQLException e) {
                e.printStackTrace();
                response.getWriter().println("系统错误！");
            }
        } else if ("add".equals(action)) {
            // 添加用户
            String username = request.getParameter("username");
            String password = request.getParameter("password");
            String email = request.getParameter("email");
            User user = new User();
            user.setUsername(username);
            user.setPassword(password);
            user.setEmail(email);
            try {
                userDao.addUser(user);
                response.sendRedirect("index.jsp");
            } catch (ClassNotFoundException | SQLException e) {
                e.printStackTrace();
                response.getWriter().println("系统错误！");
            }
        } else if ("delete".equals(action)) {
            // 删除用户
            int id = Integer.parseInt(request.getParameter("id"));
            try {
                userDao.deleteUser(id);
                response.sendRedirect("index.jsp");
            } catch (ClassNotFoundException | SQLException e) {
                e.printStackTrace();
                response.getWriter().println("系统错误！");
            }
        } else if ("update".equals(action)) {
            // 修改用户
            int id = Integer.parseInt(request.getParameter("id"));
            String username = request.getParameter("username");
            String password = request.getParameter("password");
            String email = request.getParameter("email");
            User user = new User();
            user.setId(id);
            user.setUsername(username);
            user.setPassword(password);
            user.setEmail(email);
            try {
                userDao.updateUser(user);
                response.sendRedirect("index.jsp");
            } catch (ClassNotFoundException | SQLException e) {
                e.printStackTrace();
                response.getWriter().println("系统错误！");
            }
        } else if ("get".equals(action)) {
            // 查询用户
            int id = Integer.parseInt(request.getParameter("id"));
            try {
                User user = userDao.getUser(id);
                request.setAttribute("user", user);
                request.getRequestDispatcher("edit.jsp").forward(request, response);
            } catch (ClassNotFoundException | SQLException e) {
                e.printStackTrace();
                response.getWriter().println("系统错误！");
            }
        } else {
            // 显示用户列表
            try {
                List<User> userList = new ArrayList<>();
                userList.add(new User(1, "张三", "123456", "zhangsan@example.com"));
                userList.add(new User(2, "李四", "123456", "lisi@example.com"));
                userList.add(new User(3, "王五", "123456", "wangwu@example.com"));
                request.setAttribute("userList", userList);
                request.getRequestDispatcher("index.jsp").forward(request, response);
            } catch (ServletException | IOException e) {
                e.printStackTrace();
                response.getWriter().println("系统错误！");
            }
        }
    }

    public void destroy() {
        userDao = null;
    }
}

最后，我们需要创建相应的 JSP 页面，包括登录页面、用户列表页面、编辑页面等。这里不再赘述，您可以根据具体需求自行编写。

以上是一个简单的 JavaWeb 代码示例，实现了用户登录和对对象的增删查改功能。需要注意的是，这只是一个基础示例，实际应用中还需要考虑更多的安全性、性能等方面的问题。