程序报错java.lang.NumberFormatException: null

SumServlet.java：

package com.imoc.servleet;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/sum")
public class SumServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String num=request.getParameter("num");
        int number=Integer.parseInt(num);
        if(number<100){
            int sum=0;
            for(int i=0;i<=number;i++){
                sum+=i;
            }
            request.setAttribute("sum",num);
            request.getRequestDispatcher("/result").forward(request,response);
        }else {
            response.getWriter().println("<h1>数字超出范围</h1>");
        }
    }
}

ResultServlet.java：

package com.imoc.servleet;

import javax.jws.WebService;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/result")
public class Result extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String sum=(String)request.getAttribute("sum");
        response.getWriter().println("<h1>"+sum+"</h1>");
    }
}

sum.html：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<form action="/sum" method="get">
    <input type="text" name="num" placeholder="请输入100以内的数字"><button type="submit" value="求累加和"></button>
</form>
</body>
</html>

报错：

java.lang.NumberFormatException: null
		at java.lang.Integer.parseInt(Integer.java:542)
		at java.lang.Integer.parseInt(Integer.java:615)
		at com.imoc.servleet.SumServlet.doGet(SumServlet.java:16)

慕村4629685 2021-10-13

源自：Servlet进阶 1-14 自由编程