javaweb实现增加修改的核心代码包括实体类、Dao和servlet、service的

这里是一个简单的示例代码，实现了一个用户管理系统的增删改查功能，包括实体类User、Dao层UserDao、Service层UserService和Servlet层UserServlet。

User实体类：

public class User {
    private int id;
    private String name;
    private int age;
    private String gender;
    // 省略getter和setter方法
}

UserDao层：

public class UserDao {
    // 添加用户
    public boolean addUser(User user) {
        // 数据库操作
    }
    
    // 根据id查询用户
    public User getUserById(int id) {
        // 数据库操作
    }
    
    // 更新用户信息
    public boolean updateUser(User user) {
        // 数据库操作
    }
    
    // 删除用户
    public boolean deleteUser(int id) {
        // 数据库操作
    }
}

UserService层：

public class UserService {
    private UserDao userDao = new UserDao();
    
    // 添加用户
    public boolean addUser(User user) {
        return userDao.addUser(user);
    }
    
    // 根据id查询用户
    public User getUserById(int id) {
        return userDao.getUserById(id);
    }
    
    // 更新用户信息
    public boolean updateUser(User user) {
        return userDao.updateUser(user);
    }
    
    // 删除用户
    public boolean deleteUser(int id) {
        return userDao.deleteUser(id);
    }
}

UserServlet层：

public class UserServlet extends HttpServlet {
    private UserService userService = new UserService();
    
    // 处理添加用户请求
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String name = request.getParameter("name");
        int age = Integer.parseInt(request.getParameter("age"));
        String gender = request.getParameter("gender");
        User user = new User();
        user.setName(name);
        user.setAge(age);
        user.setGender(gender);
        boolean result = userService.addUser(user);
        if (result) {
            response.sendRedirect("success.jsp");
        } else {
            response.sendRedirect("error.jsp");
        }
    }
    
    // 处理更新用户请求
    protected void doPut(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        int id = Integer.parseInt(request.getParameter("id"));
        String name = request.getParameter("name");
        int age = Integer.parseInt(request.getParameter("age"));
        String gender = request.getParameter("gender");
        User user = new User();
        user.setId(id);
        user.setName(name);
        user.setAge(age);
        user.setGender(gender);
        boolean result = userService.updateUser(user);
        if (result) {
            response.sendRedirect("success.jsp");
        } else {
            response.sendRedirect("error.jsp");
        }
    }
    
    // 处理删除用户请求
    protected void doDelete(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        int id = Integer.parseInt(request.getParameter("id"));
        boolean result = userService.deleteUser(id);
        if (result) {
            response.sendRedirect("success.jsp");
        } else {
            response.sendRedirect("error.jsp");
        }
    }
    
    // 处理查询用户请求
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        int id = Integer.parseInt(request.getParameter("id"));
        User user = userService.getUserById(id);
        // 将user对象传递给jsp页面进行显示
        request.setAttribute("user", user);
        request.getRequestDispatcher("user.jsp").forward(request, response);
    }
}

以上代码仅供参考，实际开发中需要根据具体需求进行修改和完善。