NullPointerException in con.createStatement() in servlet - eclipse

Question

I am using eclipse kepler IDE.
I am getting java.lang.nullpointerexception and i know the reason because i am getting null in con object and con is used to call a method createStatement() which is throwing this exception.
I jst want to know why this con object accessing null from MyListener.java through getAttribute() method.
I have created msaccess DB and datasource named servletTest. But i don't know
why con is accessing null that's why it is giving exception.
Exception is given below:
Feb 05, 2014 11:48:12 AM org.apache.catalina.core.AprLifecycleListener init
INFO: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: C:\Program Files\Java\jdk1.8.0\bin;C:\Windows\Sun\Java\bin;C:\Windows\system32;C:\Windows;C:/Program Files/Java/jre8/bin/client;C:/Program Files/Java/jre8/bin;C:/Program Files/Java/jre8/lib/i386;C:\Windows\System32;C:\Program Files\Java\jdk1.8.0\bin;C:\Users\praveen\Desktop\eclipse;;.
Feb 05, 2014 11:48:12 AM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]{Server/Service/Engine/Host/Context} Setting property 'source' to 'org.eclipse.jst.jee.server:advanced java' did not find a matching property.
Feb 05, 2014 11:48:12 AM org.apache.coyote.AbstractProtocol init
INFO: Initializing ProtocolHandler ["http-bio-8080"]
Feb 05, 2014 11:48:12 AM org.apache.coyote.AbstractProtocol init
INFO: Initializing ProtocolHandler ["ajp-bio-8009"]
Feb 05, 2014 11:48:12 AM org.apache.catalina.startup.Catalina load
INFO: Initialization processed in 1175 ms
Feb 05, 2014 11:48:12 AM org.apache.catalina.core.StandardService startInternal
INFO: Starting service Catalina
Feb 05, 2014 11:48:12 AM org.apache.catalina.core.StandardEngine startInternal
INFO: Starting Servlet Engine: Apache Tomcat/7.0.50
Feb 05, 2014 11:48:14 AM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["http-bio-8080"]
Feb 05, 2014 11:48:14 AM org.apache.coyote.AbstractProtocol start
INFO: Starting ProtocolHandler ["ajp-bio-8009"]
Feb 05, 2014 11:48:14 AM org.apache.catalina.startup.Catalina start
INFO: Server startup in 1189 ms
java.lang.NullPointerException
at com.student.servletcontextevent.FetchData.doGet(FetchData.java:47)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:409)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1044)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:607)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:315)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:744)
**inde.html**
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
fetch records
</body>
</html>
**FetchData**
package com.student.servletcontextevent;
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.Statement;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* Servlet implementation class FetchData
*/
#WebServlet("/FetchData")
public class FetchData extends HttpServlet {
private static final long serialVersionUID = 1L;
/**
* #see HttpServlet#HttpServlet()
*/
public FetchData() {
super();
// TODO Auto-generated constructor stub
}
/**
* #see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
* response)
*/
protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
response.setContentType("text/html");
PrintWriter out = response.getWriter();
try {
// Retrieving connection object from ServletContext object
ServletContext ctx = getServletContext();
Connection con = (Connection) ctx.getAttribute("mycon");
// retieving data from emp table
Statement ps = con.createStatement();
ResultSet rs = ps.executeQuery("Select * from emp");
while (rs.next()) {
out.print("<br>" + rs.getString(1) + " " + rs.getString(2));
}
con.close();
} catch (Exception e) {
e.printStackTrace();
}
out.close();
}
/**
* #see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse
* response)
*/
protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
}
}
**MyListener**
package com.student.servletcontextevent;
import java.io.IOException;
import java.sql.Connection;
import java.sql.DriverManager;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* Servlet implementation class MyListener
*/
#WebServlet("/MyListener")
public class MyListener extends HttpServlet implements ServletContextListener {
private static final long serialVersionUID = 1L;
/**
* #see HttpServlet#HttpServlet()
*/
public MyListener() {
super();
// TODO Auto-generated constructor stub
}
/**
* #see ServletContextListener#contextDestroyed(ServletContextEvent)
*/
public void contextDestroyed(ServletContextEvent arg0) {
// TODO Auto-generated method stub
}
/**
* #see ServletContextListener#contextInitialized(ServletContextEvent)
*/
public void contextInitialized(ServletContextEvent event) {
// TODO Auto-generated method stub
try{
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String dataSourceName = "servletTest";
String dbUrl = "jdbc:odbc:" + dataSourceName;
Connection con=DriverManager.getConnection(dbUrl);
//storing connection object as an attribute in ServletContext
ServletContext ctx=event.getServletContext();
ctx.setAttribute("mycon", con);
}catch(Exception e){e.printStackTrace();}
}
/**
* #see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
}
/**
* #see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
}
}

Answer 1

Your listener class shouldn't extend HttpServlet, since it's not a servlet, but only a ServletContextListener. And as such, it should be annotated with #WebListener, and not with #WebServlet.
That said, you shouldn't use a single database connection in your webapp since, obviously, if two requests are handled concurrently, both will the use the same connection and will thus be unable to do their transactional work correctly.
Read the documentation of your servlet container to understand how to properly create a DataSource and access it from the servlets (see http://tomcat.apache.org/tomcat-7.0-doc/jndi-datasource-examples-howto.html for Tomcat). Each request handling method should get a connection from the DataSource, use it, then close the connection. Also, don't use the JDBC/ODBC bridge, which is buggy, slow and completely obsolete. Use the JDBC driver for your database. Every database on earth has a JDBC driver these days.
A side note to finish: whether you're using Eclipse, NetBeans or Notepad to write your code has nothing to do with the exception you're getting, at runtime, once your code is executed inside your web container. The IDE is completely irrelevant to your question.

Answer 2

Are you sure you can connect to your database?
My guess is 'con' is null in FetchData servlet because an exception is thrown in MyListener when connecting to database or MyListener is never used at all.
Indeed database connection should not be managed programmatically in your application, if you google for 'tomc1t database jndi' you should find a great tutorial.

Answer 3

Just to add more to #JB Nizet's correct answer..
Reasons for null pointer exception on database connection:
1) Not able to access database.
2) database access might require username and password which is not supplied or incorrect.
3) Wrong database name/url.
And just some suggestions :
1) Why are you creating connection object in listener? You can create them in seperate java file and access those in your servlets and not listeners.
2)Why are you saving connection object(con) in servlet context? Makes no sense to me.. Since you are sure that your application uses single database, you can make connection object static, and you can access it just by classname.