JSP页面展示数据时出现NullPointerException问题排查与解决

基于NetBeans IDE、Tomcat服务器和MySQL数据库开发的Web应用，在JSP页面展示数据列表时出现异常：直接访问JSP页面报错，但通过Servlet访问则功能正常。

相关代码 #

DA层数据查询方法 #

public List<Automoveis> consultarTodosAutomoveis() throws ClassNotFoundException, SQLException {
    Connection con = Conexao.getConexao();
    PreparedStatement comando = con.prepareStatement("select * from automoveis;");
    ResultSet rs = comando.executeQuery();
    List<Automoveis> lauto  = new ArrayList<Automoveis>();
    int cont = 0;
    while(rs.next()) {
        Automoveis auto = new Automoveis();
        auto.setIdAuto(rs.getInt("idAuto"));
        auto.setMarca(rs.getString("marca"));
        auto.setModelo(rs.getString("modelo"));
        auto.setCor(rs.getString("cor"));
        auto.setImportado(rs.getString("importado"));
        auto.setNumeroRodas(rs.getInt("numeroRodas"));
        auto.setPreco(rs.getDouble("preco"));
        lauto.add(auto);
        cont++;
    }
    System.out.println("Cont..: " + cont);
    con.close();
    return lauto;
}

Servlet数据转发代码 #

try (PrintWriter out = response.getWriter()) {
    MetodosDAO mdao = new MetodosDAO();
    try {
        List<Automoveis> lauto = mdao.consultarTodosAutomoveis();
        request.setAttribute("lauto", lauto);
        request.getRequestDispatcher("resultadoconsultartodsautomoveis.jsp").forward(request, response);
    } catch (ClassNotFoundException | SQLException ex) {
        System.out.println("Error: " + ex.getMessage());
    }
}

JSP展示页面（resultadoconsultartodsautomoveis.jsp） #

<%@page import="java.util.List"%>
<%@page import="model.Automoveis"%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>JSP Page</title>
</head>
<body>
    <h1>Result of Consulting All Automoveis</h1>
    <%
        List<Automoveis> lauto = (List<Automoveis>) request.getAttribute("lauto");
    %>
    <% for (Automoveis auto : lauto) { %>
        Id: <%= auto.getIdAuto() %><br>
        Brand: <%= auto.getMarca() %><br><br>
        Model: <%= auto.getIdAuto() %><br><br>
        Color: <%= auto.getCor() %><br><br>
        Imported: <%= auto.getImportado() %><br><br>
        Number of Wheels: <%= auto.getNumeroRodas() %><br><br>
        Price: <%= auto.getPreco() %><br><br>
    <% } %>
</body>
</html>

错误信息 #

直接访问resultadoconsultarautomoveis.jsp时触发以下异常：

HTTP Status 500 – Internal Server Error
Type Exception Report

Message An exception occurred processing [/resultadoconsultartodsautomoveis.jsp] at line [21]

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception
org.apache.jasper.JasperException: An exception occurred processing [/resultadoconsultartodsautomoveis.jsp] at line [21]
...
java.lang.NullPointerException: Cannot invoke "java.util.List.iterator()" because "lauto" is null
...

错误原因 #

直接访问JSP时，请求未经过Servlet处理流程，request对象中没有被设置lauto属性，导致request.getAttribute("lauto")返回null。当JSP尝试遍历这个空列表时，就会触发空指针异常。而通过Servlet访问时，Servlet已经完成数据查询并将列表存入请求属性，因此JSP能正常获取数据。

修复方案 #

方案1：JSP添加空值判断 #

在遍历列表前先检查lauto是否为空，避免空指针，同时修正原JSP中Model行的错误（误用了getIdAuto()，应改为getModelo()）：

<%@page import="java.util.List"%>
<%@page import="model.Automoveis"%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>JSP Page</title>
</head>
<body>
    <h1>Result of Consulting All Automoveis</h1>
    <%
        List<Automoveis> lauto = (List<Automoveis>) request.getAttribute("lauto");
    %>
    <% if (lauto != null && !lauto.isEmpty()) { %>
        <% for (Automoveis auto : lauto) { %>
            Id: <%= auto.getIdAuto() %><br>
            Brand: <%= auto.getMarca() %><br><br>
            Model: <%= auto.getModelo() %><br><br>
            Color: <%= auto.getCor() %><br><br>
            Imported: <%= auto.getImportado() %><br><br>
            Number of Wheels: <%= auto.getNumeroRodas() %><br><br>
            Price: <%= auto.getPreco() %><br><br>
        <% } %>
    <% } else { %>
        <p>无数据或页面被直接访问</p>
    <% } %>
</body>
</html>

方案2：强制通过Servlet访问JSP #

在JSP开头添加逻辑，若lauto不存在则重定向到Servlet，确保请求经过数据查询流程：

<%@page import="java.util.List"%>
<%@page import="model.Automoveis"%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<%
    List<Automoveis> lauto = (List<Automoveis>) request.getAttribute("lauto");
    if (lauto == null) {
        // 替换为实际的Servlet访问路径
        response.sendRedirect("你的Servlet映射路径");
        return;
    }
%>
<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>JSP Page</title>
</head>
<body>
    <h1>Result of Consulting All Automoveis</h1>
    <% for (Automoveis auto : lauto) { %>
        Id: <%= auto.getIdAuto() %><br>
        Brand: <%= auto.getMarca() %><br><br>
        Model: <%= auto.getModelo() %><br><br>
        Color: <%= auto.getCor() %><br><br>
        Imported: <%= auto.getImportado() %><br><br>
        Number of Wheels: <%= auto.getNumeroRodas() %><br><br>
        Price: <%= auto.getPreco() %><br><br>
    <% } %>
</body>
</html>

额外优化建议 #

1. 资源自动关闭：DAO层使用try-with-resources自动关闭数据库连接、Statement和ResultSet，避免资源泄漏：

public List<Automoveis> consultarTodosAutomoveis() throws ClassNotFoundException, SQLException {
    try (Connection con = Conexao.getConexao();
         PreparedStatement comando = con.prepareStatement("select * from automoveis;");
         ResultSet rs = comando.executeQuery()) {
        List<Automoveis> lauto = new ArrayList<>();
        int cont = 0;
        while (rs.next()) {
            Automoveis auto = new Automoveis();
            auto.setIdAuto(rs.getInt("idAuto"));
            auto.setMarca(rs.getString("marca"));
            auto.setModelo(rs.getString("modelo"));
            auto.setCor(rs.getString("cor"));
            auto.setImportado(rs.getString("importado"));
            auto.setNumeroRodas(rs.getInt("numeroRodas"));
            auto.setPreco(rs.getDouble("preco"));
            lauto.add(auto);
            cont++;
        }
        System.out.println("Cont..: " + cont);
        return lauto;
    }
}

2. 使用JSTL+EL替代脚本：让JSP页面更简洁易维护：

<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>JSP Page</title>
</head>
<body>
    <h1>Result of Consulting All Automoveis</h1>
    <c:if test="${not empty lauto}">
        <c:forEach items="${lauto}" var="auto">
            Id: ${auto.idAuto}<br>
            Brand: ${auto.marca}<br><br>
            Model: ${auto.modelo}<br><br>
            Color: ${auto.cor}<br><br>
            Imported: ${auto.importado}<br><br>
            Number of Wheels: ${auto.numeroRodas}<br><br>
            Price: ${auto.preco}<br><br>
        </c:forEach>
    </c:if>
    <c:if test="${empty lauto}">
        <p>无数据或页面被直接访问</p>
    </c:if>
</body>
</html>

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