关于java.lang.NumberFormatException: null的问题及解决

<c:forEach items="${listManager}" var="manager">
    <tr class="tr2">
       <td>${manager.id}</td>
       <td>${manager.name}</td>
       <td>${manager.price}</td>
       <td>${manager.introduce}</td>
       <td><a href="<%=path%>/cn/servlet/ManagerServlet?flag = update & m_id =${manager.id}" rel="external nofollow" >更新</a>&nbsp;&nbsp;&nbsp;
       <a href="<%=path%>/cn/servlet/ManagerServlet? flag =delete & m_id= ${manager.getId()}" rel="external nofollow" >删除</a>
      </td>
     </tr>
</c:forEach>

 if (flag.equals("delete")){
            System.out.println("=======delete"+flag);
            String m_id = req.getParameter("m_id");
            System.out.println(m_id);
            //将String类型的m_id转换成int型
            Integer id = Integer.parseInt(m_id.trim());
            System.out.println("---id--"+id);
            ManagerDao managerDao = new ManagerDao();
            int m = managerDao.delManager(id);
            String path = "";
            if (m==1){
                List<Manager>listManager ;
                listManager = managerDao.list();
               // HttpSession session = req.getSession();
                //session.setAttribute("listManager",listManager);
                req.setAttribute("listManager",listManager);
                path="/manager/menu.jsp";
            }else {
                path = "/error.jsp";
            }
            RequestDispatcher dispatcher = req.getRequestDispatcher(path);
            dispatcher.forward(req,resp);
        }else if (flag.equals("update")){
            System.out.println("=======update"+flag);
            String m_id = req.getParameter("m_id");
            System.out.println(m_id);
            int id = Integer.parseInt(m_id);
            ManagerDao managerDao = new ManagerDao();
            Manager manager = managerDao.getManager(id);
            req.setAttribute("manager",manager);
            RequestDispatcher dispatcher = req.getRequestDispatcher("/manager/update.jsp");
            dispatcher.forward(req,resp);

public int delManager(int id) {
        //数据库连接、
        Connection conn  = JDBC.getConnection();
        Statement state = null;
        // System.out.println(u);
        try {
            state  = conn.createStatement();
            String sql = "delete  from menus where  id="+id;
            System.out.println(sql);
            m = state.executeUpdate(sql);
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }
        JDBC.close(conn, state,null);
        return  m;
    }
    public Manager getManager(int id) {
        //数据库连接、
        Connection conn  = JDBC.getConnection();
        Statement state = null;
        //局部变量没有初始值
        Manager manager = null;
        // System.out.println(u);
        try {
            state  = conn.createStatement();
            String sql = "select *  from menus where  id="+id;
            ResultSet rs = state.executeQuery(sql);
            if(rs.next()){
                manager = new Manager();
                manager.setId(rs.getInt(1));
                manager.setName(rs.getString(2));
                manager.setPrice(rs.getString(3));
                //manager.setPrice(rs.getBigDecimal(3));
                manager.setIntroduce(rs.getString(4));
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }
        JDBC.close(conn, state,null);
        return  manager;
    }

int state = Integer.parseInt(request.getParameter(“state”))