JSP中iframe嵌套标签页无法填充表格，触发空指针异常

搭建了一个简单Web应用，包含login.jsp与LoginServlet实现用户校验，校验通过后跳转至Main.jsp。Main.jsp设有两个标签页，第二个标签页通过iframe加载DocTab.jsp，该页面的表格需由MainServlet提供的dataList数据填充。

直接访问http://localhost:8080/MyWebApp/MainServlet时，DocTab.jsp能正常显示填充后的表格；但通过登录流程进入Main.jsp后，第二个标签页抛出异常：

org.apache.jasper.JasperException: java.lang.NullPointerException: Cannot invoke "java.util.List.size()" because "dataList" is null

相关代码 #

login.jsp

<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="ISO-8859-1">
    <title>Login</title>
</head>
<body>
<form action="LoginServlet" method="post">
    Username: <input type="text" name="username"><br>
    Password: <input type="password" name="password"><br>
    <input type="submit" value="Login">
</form>
</body>
</html>

LoginServlet

package com.example.simplewebapp;
import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import java.io.IOException;
@WebServlet("/LoginServlet")
public class LoginServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        if(username.equals("1") && password.equals("1")) {
            response.sendRedirect("Main.jsp");
        } else {
            response.sendRedirect("login.jsp");
        }
    }
}

Main.jsp

<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="ISO-8859-1">
    <title>Main</title>

</head>
<body>
<h1>Welcome to the Main Page!</h1>
<ul>
    <li><a href="#tab1">Tab 1</a></li>
    <li><a href="#tab2">Document Tab</a></li>
</ul>

<div id="tab1">
    <p>Content for Tab 1.</p>
</div>

<div id="tab2">
    <iframe src="DocTab.jsp"></iframe>
</div>

</body>
</html>

MainServlet

package com.example.simplewebapp;

import jakarta.servlet.*;
import jakarta.servlet.http.*;
import jakarta.servlet.annotation.*;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

@WebServlet("/MainServlet")
public class MainServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        List<String> dataList = new ArrayList<String>();
        dataList.add("Data 1");
        dataList.add("Data 2");
        dataList.add("Data 3");
        dataList.add("Data 4");
        dataList.add("Data 5");

        request.setAttribute("dataList", dataList);
        request.getRequestDispatcher("DocTab.jsp").forward(request, response);
    }
}

DocTab.jsp

<%@ page import="java.util.List" %>
<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="ISO-8859-1">
    <title>Document Tab</title>
</head>
<body>

<table border=1 >
    <tr><th>Column 1</th><th>Column 2</th></tr>

    <% List<String> dataList = (List<String>) request.getAttribute("dataList");
        for(int i=0; i<dataList.size(); i++) { %>

    <tr><td><%=dataList.get(i)%></td><td>Row <%=i+1%>, Column 2</td></tr>

    <% } %>

</table>

</body>
</html>

通过登录流程进入Main.jsp后，iframe直接请求DocTab.jsp，该请求未经过MainServlet处理，因此request对象中不存在dataList属性，导致DocTab.jsp调用dataList.size()时触发空指针异常。

而直接访问MainServlet时，Servlet会先创建dataList并放入request，再转发到DocTab.jsp，此时request中存在数据，页面能正常渲染。

方案1：修改iframe请求路径指向MainServlet #

修改Main.jsp中iframe的src属性为MainServlet，让iframe请求先经过Servlet处理数据：

<div id="tab2">
    <iframe src="MainServlet"></iframe>
</div>

这样iframe加载时会触发MainServlet的doGet方法，生成dataList后转发到DocTab.jsp，页面即可正常获取数据。

方案2：给DocTab.jsp增加空值校验（可选，提升健壮性） #

在DocTab.jsp中增加dataList的空判断，避免空指针异常，同时提示无数据的情况：

<table border=1 >
    <tr><th>Column 1</th><th>Column 2</th></tr>

    <% 
    List<String> dataList = (List<String>) request.getAttribute("dataList");
    if (dataList != null) {
        for(int i=0; i<dataList.size(); i++) { 
    %>
    <tr><td><%=dataList.get(i)%></td><td>Row <%=i+1%>, Column 2</td></tr>
    <% 
        }
    } else {
    %>
    <tr><td colspan="2">暂无数据</td></tr>
    <% 
    }
    %>
</table>

内容的提问来源于stack exchange，提问作者user9753394