Getting Error 500 when pressing the backbutton of the browser after log out

Question

I am working on a web project , and am using AJAX in my application. I need to get list of templates and events stored in the database when my page is being loaded so im using AJAX. the problem is, when i press the logout in my page and after logging out if i press the browsers back button then im getting the Http Error 500.
Regarding this issue i previously also posted but i havent get solved my problem. (may be i havent posted my source code , rather i just posted error message.).
Here is the error message of the browser,
HTTP Status 500 -
type Exception report
message
description The server encountered an internal error () that prevented it from fulfilling this request.
exception
java.lang.NullPointerException
     com.koffee.eon.subscriber.action.TemplateNames.doPost(TemplateNames.java:66)
     javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
     javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
     org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:416)
Here is my source code of the TemplateNames class,
package com.koffee.eon.subscriber.action;
import com.koffee.eon.subscriber.persistence.*;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.http.HttpServlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.text.SimpleDateFormat;
import java.util.*;
public class TemplateNames extends HttpServlet {
public void doPost(HttpServletRequest request, HttpServletResponse response)
     throws ServletException, IOException {
     response.setContentType("text/html");
     PrintWriter out = response.getWriter();
     String html="",html1="";
     HttpSession session=request.getSession();
     UserCompleteDetails sessionDetails=(UserCompleteDetails)session.getAttribute("userdetails");
     System.out.println("checking session after logout"+sessionDetails);
     if(sessionDetails==null)
     System.out.println("session details in if condition is:"+sessionDetails);
     response.sendRedirect("/login.jsp");
     System.out.println("after checking "+sessionDetails);
     String type=null;
     type=request.getParameter("type");
String subscriberUserEONId=sessionDetails.getUserEonId();
System.out.println("subscriberUserEONId from session is: "+subscriberUserEONId);
     SimpleDateFormat sdfDate=new SimpleDateFormat("yyyy-MM-dd");
     SimpleDateFormat sdfTime=new SimpleDateFormat("HH:mm:ss");
     TemplateManagement dbObj=new TemplateManagement();          
EventManager dbObj1=new EventManager(); // for events
     List templateNames=dbObj.getTemplates(subscriberUserEONId);
     List eventIds=dbObj1.getEventsCompleteList(subscriberUserEONId); // for events
     System.out.println("size of list of templates is:" + templateNames.size());
     System.out.println("size of list of events is: "+eventIds.size()); // for events
     if(type==null)
     html+="<table><tr><td>Select Your Invitation Card: ";
     html+="<select name=\"templateName\" class=\"cell\" onchange=getImage(this.value) onblur=checkTemplate() tabindex=\"1\">";
     html+= "<option value=\"0\">Select</option>";
     HashMap hmap=new HashMap();
     try {
     for(int i=0;i<templateNames.size();i++) {
     TemplateMaster bean=(TemplateMaster)templateNames.get(i);
     Integer templateid=bean.getTemplateId();
     String templatepath=bean.getTemplateRelevantpath();
     hmap.put(templateid,templatepath);
     System.out.println("path recevied from db for template is: "+templatepath);
System.out.println("template id received from db is : "+templateid);
     html+= "<option value=\"" + templateid + "\">" + templateid + "</option>";
     } catch(Exception ee) {ee.printStackTrace();}
     session.setAttribute("hmap", hmap);
     html+="</select></td></tr><tr><td>";
     System.out.println("path new is : "+html);
     html+="</td>";
     System.out.println(html);
html+="</td></tr><tr><td>Select an Event: ";
          html+="<select name=\"eventId\" class=\"cell\">";
          try {
     for(int j=0;j<eventIds.size();j++) {
     EventMaster bean=(EventMaster)eventIds.get(j);
     String eventName=bean.getEventName();
     Integer eventId1=bean.getEventId();
     String eventStartDate=sdfDate.format(bean.getEventStartdate());
     String eventStartTime=sdfTime.format(bean.getEventStarttime());
     String eventId=eventName+" on "+eventStartDate+" at "+eventStartTime;
     System.out.println("event id from database is : "+eventId1);
     System.out.println("event list from database is : "+eventId);
     html+= "<option value=\"" + eventId1 + "\">" + eventId + "</option>";
     } catch(Exception eee) {eee.printStackTrace();}
     html+="</select></td></tr></table>";
     System.out.println(html);
     out.println(html);
     out.flush();
     out.close();     
     if(type!=null)
     html1+="</td></tr><tr><td>Select an Event: ";
     html1+="<select name=\"eventId\" class=\"cell\">";
     try {
     for(int j=0;j<eventIds.size();j++) {
     EventMaster bean=(EventMaster)eventIds.get(j);
     Integer eventId1=bean.getEventId();
String eventId2=eventId1.toString();
     String eventName=bean.getEventName();
     String eventStartDate=sdfDate.format(bean.getEventStartdate());
     String eventStartTime=sdfTime.format(bean.getEventStarttime());
     String eventId=eventName+" on "+eventStartDate+" at "+eventStartTime;
     System.out.println(eventId);
     html1+= "<option value=\""
+ eventId2 + "\">" + eventId + "</option>";
     } catch(Exception eee) {eee.printStackTrace();}
     html1+="</select></td></tr></table>";
     out.println(html1);
     out.flush();
     out.close();     
}

Answer 1

chinni wrote:
java.lang.NullPointerException
     com.koffee.eon.subscriber.action.TemplateNames.doPost(TemplateNames.java:66)Do you understand when a NPE will be thrown? Some object reference at line 66 of TemplateNames.java is null while you didn't expect and you still invoke/access it. To fix this, add a nullcheck or just instantiate the reference.