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我是新的java，jsp an我试图使用index.jsp从Mysql数据库打印一些数据。正如标题所说我得到一个例外。我已经检查了这个例外很多。我在Stackoverflow看到了一些相同的帖子，但他们没有帮助我。这里是我的代码：HTTP状态500 - java.lang.NullPointerException
的index.jsp：
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
<form action ="New_printer.jsp" method="get">
<input type ="submit" value="Proceed">
</form>
</body>
</html>
New_printer.jsp：
<%@page import="java.util.ArrayList"%>
<%@page import="java.sql.ResultSet"%>
<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
<%@ page import="com.results.Printer" %>
<%
int i;
ArrayList Rows = new ArrayList();
Rows = (ArrayList) request.getAttribute("data");
for(i=1; i<Rows.size(); i++)
{
ArrayList row = new ArrayList();
row = (ArrayList) Rows.get(i);
for(int j=1; j<4; j++)
{
String word =(String) row.get(j);
out.print(word + " ");
}
out.println();
}
%>
</body>
</html>
Printer.java：
package com.results;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.ArrayList;
import java.io.IOException;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class Printer extends HttpServlet{
private static Connection con = null;
ResultSet rs;
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
resp.setContentType("text/jsp");
try
{
Class.forName("com.mysql.jdbc.Driver");
con = DriverManager.getConnection("jdbc:mysql://localhost:3306/tst","root","1234");
Statement stmt = con.createStatement();
rs = stmt.executeQuery("select * from test_db.person");
ArrayList Rows = new ArrayList();
while(rs.next())
{
ArrayList row =new ArrayList();
for(int i=1; i<4; i++)
{
row.add(rs.getString(i));
}
Rows.add(row);
}
req.setAttribute("data", Rows);
RequestDispatcher rqdp = getServletContext().getRequestDispatcher("/New_printer.jsp");
rqdp.forward(req, resp);
}
catch(Exception e)
{
e.printStackTrace();
}
}
}
web.xml中：
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">
<display-name>Simple_Print</display-name>
<!--
<servlet>
<servlet-name>printer</servlet-name>
<servlet-class>com.results.Printer</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>printer</servlet-name>
<url-pattern>/index.jsp</url-pattern>
</servlet-mapping> -->
<welcome-file-list>
<welcome-file>index.html</welcome-file>
<welcome-file>index.htm</welcome-file>
<welcome-file>default.html</welcome-file>
<welcome-file>default.htm</welcome-file>
<welcome-file>default.jsp</welcome-file>
</welcome-file-list>
</web-app>
EXCEPTION
HTTP Status 500 - java.lang.NullPointerException
type Exception report
message java.lang.NullPointerException
description The server encountered an internal error that prevented it from fulfilling this request.
exception
org.apache.jasper.JasperException: java.lang.NullPointerException
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:548)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:469)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
root cause
java.lang.NullPointerException
org.apache.jsp.New_005fprinter_jsp._jspService(New_005fprinter_jsp.java:123)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:431)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
note The full stack trace of the root cause is available in the Apache Tomcat/8.0.20 logs.
你能帮忙吗？请在这里堆栈好几天。
可能重复[什么是空指针异常，以及如何解决它？]（http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception-and-我怎么办 - 修复它） – Jens 2015-03-19 08:31:06
@Jens不，它不是dupli美食与您的链接。这是NPE的服务器代码错误。 – Cataclysm 2015-03-19 08:32:22
添加堆栈跟踪。 – Jens 2015-03-19 08:33:25