使用 servlet 执行显示 JSP 页面时出现 NullPointerException
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【中文标题】使用 servlet 执行显示 JSP 页面时出现 NullPointerException【英文标题】：Getting NullPointerException while executing display JSP page using servlets 【发布时间】：2021-11-14 06:08:09 【问题描述】：我正在尝试从数据库中显示用户的详细信息。 获取 NullPointerException 下面是我的 Jsp 和 Servlet 页面
我也是新手，所以我猜肯定有一些愚蠢的错误。
JSP Page
<%@ page language="java" contentType="text/html; charset=ISO-8859-1" isELIgnored="false"
pageEncoding="ISO-8859-1"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>View Profile</title>
</head>
<html>
<body>
<form action="ProfileServlet?ac=view">
Enter your username:<input type=text name=username><br/>
<input type=submit value=Submit>
</form>
</body>
</html>
Servlet
package com.Webhtml.controller;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.sql.*;
public class ProfileServlet extends HttpServlet
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
String a=request.getParameter("ac");
response.setContentType("text/html");
PrintWriter out = response.getWriter();
if(a.equals("view"))
String username=request.getParameter("username");
try
Class.forName("com.mysql.jdbc.Driver");
Connection con=DriverManager.getConnection("jdbc:mysql://75.20.205.173:3306/,"contact","root","root");
PreparedStatement ps=con.prepareStatement("select * from user where username=?");
ps.setString(13,username);
out.print("<table width=50% border=1>");
out.print("<caption>Result:</caption>");
ResultSet rs=ps.executeQuery();
/* Printing column names */
ResultSetMetaData rsmd=rs.getMetaData();
int total=rsmd.getColumnCount();
out.print("<tr>");
for(int i=1;i<=total;i++)
out.print("<th>"+rsmd.getColumnName(i)+"</th>");
out.print("</tr>");
/* Printing result */
while(rs.next())
out.print("<tr><td>"+rs.getString(2)+ "</td><td>"+rs.getString(3)+"</td><td>"+rs.getLong(4)+"</td><td>"+rs.getString(5)+"</td><td>"+rs.getDate(6)+"</td><td>"+rs.getString(7)+"</td><td>"+rs.getString(8)+"</td><td>"+rs.getString(9)+"</td><td>"+rs.getString(10)+"</td><td>"+rs.getString(11)+"</td></tr>");
out.print("</table>");
catch (Exception e2) e2.printStackTrace();
finallyout.close();
Error
Sep 20, 2021 7:31:29 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [ProfileServlet] in context with path [/Webhtml] threw exception
java.lang.NullPointerException
at com.Webhtml.controller.ProfileServlet.doGet(ProfileServlet.java:20)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:475)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:651)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:500)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:754)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1376)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)
【问题讨论】：
【参考方案1】：您没有在变量“a”中执行空值检查。这个变量是一个参数，可以为空，所以你的代码崩溃了：
if(a.equals("view"))
因为在这种情况下 a 为空。
为什么“a”为空？也许您需要指定隐藏在表单中的输入类型中的参数，而不是直接将它们放在表单的操作中：
<form action="ProfileServlet">
Enter your username:<input type=text name=username><br/>
<input type="hidden" id="ac" name="ac" value="view">
<input type=submit value=Submit>
</form>
【讨论】：
你能告诉我这行的确切位置吗？我应该在表单操作中提到“ProfileServlet”吗？ 我编辑了答案以提供更好的示例。您可以尝试在 JSP 页面中更改它，看看是否有效 完成了，谢谢以上是关于使用 servlet 执行显示 JSP 页面时出现 NullPointerException的主要内容，如果未能解决你的问题，请参考以下文章
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