session object = null?

[Mellihoney Michael]

I use a session obj in the servlet class BookServlet:

public class BookServlet extends HttpServlet

in the following method:

private boolean insertBookToSession(HttpServletRequest request)

I use a session obj to store all info pre-setted in an array of java beans

//create a new booklist with 10 elements
BookBean [] newbooklist  = new BookBean [10];
newbooklist[0] = newBook;
//insert the booklist into session object
session.setAttribute("BOOKLIST", newbooklist);
return true;

I attempted to retrieve the obj in this session in another method in the same servlet class:

private Document generateDOM(HttpServletRequest request)

but it will generate a NullPointerException in runtime:

HttpSession session= request.getSession(false);  
Object books = session.getAttribute("BOOKLIST");//NullPointerException
BookBean [] booklist  = (BookBean []) books;

can anyone help to solve this problem?
many thanks....
[ January 17, 2004: Message edited by: Mellihoney Michael ]

[Moorthi Ramasamy]

Originally posted by Mellihoney Michael:

//create a new booklist with 10 elements
BookBean [] newbooklist  = new BookBean [10];
newbooklist[0] = newBook;
//insert the booklist into session object
session.setAttribute("BOOKLIST", newbooklist);
return true;

Mellihoney,
Where did you get this session variable?. To get the current session use
HttpSession session = req.getSession(true);
then set your book list in this session using
session.setAttribute("BOOKLIST", newbooklist);
In this case, your session object won't be NULL in insertBookToSession() method.
Thanks,
Moorthi

[Mellihoney Michael]

import beans.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
import javax.xml.parsers.*;
import org.w3c.dom.*;
import  org.apache.xml.serialize.OutputFormat;
import  org.apache.xml.serialize.Serializer;
import  org.apache.xml.serialize.SerializerFactory;
import  org.apache.xml.serialize.XMLSerializer;
public class BookServlet extends HttpServlet {
  //add new book into session object
  private boolean insertBookToSession(HttpServletRequest request) throws ServletException, IOException {
    String title = request.getParameter("title");
    String author = request.getParameter("author");
    String isbn = request.getParameter("isbn");
    String publisher = request.getParameter("publisher");
    String publisherCountry = request.getParameter("publisherCountry");
BookBean newBook = new BookBean(title, author, isbn, publisher, publisherCountry);
        /*-------------------------------------------
         I have already used the statement here
        --------------------------------------------*/
HttpSession session= request.getSession(true);
Object books = session.getAttribute("BOOKLIST");

if (books == null) {
BookBean [] newbooklist  = new BookBean [10];
newbooklist[0] = newBook;
session.setAttribute("BOOKLIST", newbooklist);
return true;
}
else {
BookBean [] booklist = (BookBean []) books;
int newbookindex = 10;
for (int i=0; i<4; i++) {
if (booklist[i] == null) {
newbookindex = i;
break;
}
}
if (newbookindex == 10) {
return false;
}
else {
booklist[newbookindex] = newBook;
session.setAttribute("BOOKLIST", booklist);
return true;
}
}
  }
  private Document generateDOM(HttpServletRequest request) throws ServletException, IOException {
DocumentBuilderFactory factory;
DocumentBuilder builder;
try {
factory = DocumentBuilderFactory.newInstance();
builder = factory.newDocumentBuilder();
}
catch (ParserConfigurationException pe) {
throw new IOException("ParserConfiguration problem");
}
Document document = builder.newDocument();
Element root = document.createElement("booklist");
document.appendChild(root);

HttpSession session= request.getSession(false);
Object books = session.getAttribute("BOOKLIST");
BookBean [] booklist  = (BookBean []) books;
int bookcount = 0;
for (int i=0; i<booklist.length; i++) {
if (booklist[i] != null)
bookcount++;
else
continue;
}
for (int j=0; j<bookcount; j++) {
Element bookElement = document.createElement("book");
Element bookTitleElement = document.createElement("title");
Text textnode1 = document.createTextNode(booklist[j].getTitle());
bookTitleElement.appendChild(textnode1);
bookElement.appendChild(bookTitleElement);
Element bookAuthorElement = document.createElement("author");
Text textnode2 = document.createTextNode(booklist[j].getAuthor());
bookAuthorElement.appendChild(textnode2);
bookElement.appendChild(bookAuthorElement);
Element bookISBNElement = document.createElement("isbn");
Text textnode3 = document.createTextNode(booklist[j].getIsbn());
bookISBNElement.appendChild(textnode3);
bookElement.appendChild(bookISBNElement);
Element bookPublisherElement = document.createElement("publisher");
Element publisherNameElement = document.createElement("name");
Text textnode4 = document.createTextNode(booklist[j].getPublisher());
publisherNameElement.appendChild(textnode4);
bookPublisherElement.appendChild(publisherNameElement);
Element publisherCountryElement = document.createElement("country");
Text textnode5 = document.createTextNode(booklist[j].getPublisherCountry());
publisherCountryElement.appendChild(textnode5);
bookPublisherElement.appendChild(publisherCountryElement);
bookElement.appendChild(bookPublisherElement);
root.appendChild(bookElement);
}
return document;
  }
  public void generateXMLDocument (Document document) throws IOException {
File newXMLFile = new File("..//webapps//book//xml//booklist.xml");
FileWriter xmlwriter = new FileWriter(newXMLFile);
    StringWriter  stringOut = new StringWriter();
OutputFormat xmlformat = new OutputFormat(document);
xmlformat.setIndenting(true);
XMLSerializer serializer1 = new XMLSerializer(xmlwriter, xmlformat);
XMLSerializer serializer2 = new XMLSerializer(stringOut, xmlformat);
serializer1.asDOMSerializer();
serializer2.asDOMSerializer();
serializer1.serialize(document.getDocumentElement());
serializer2.serialize(document.getDocumentElement());
System.out.println( "" + stringOut.toString() );
System.out.println("Output completed");
  }
  public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doPost(request, response);
  }
  public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String action = request.getParameter("action");
    if (action != null && action.equals("SAVE")) {
if (insertBookToSession(request)) {
response.sendRedirect("../jsp/Book.html");
}
else {
response.sendRedirect("../jsp/ListFullError.html");
}
}
else if (action != null && action.equals("CREATE XML")) {
Document newDOMTree = generateDOM(request);
generateXMLDocument(newDOMTree);
HttpSession session= request.getSession(true);
session.removeAttribute("BOOKLIST");
response.sendRedirect("../jsp/Book.html");
}
  }
}

Why?

[Moorthi Ramasamy]

Is you action parameter of the request comes with a value of "SAVE" in the first request and "CREATE XML" in the second request??
I guess insertBookToSession(request) & generateDOM(request) methods are invoked in two different request based on the action parameter.
generateDOM(request)
HttpSession session= request.getSession(false);//this is the one throwing NULL POINTER
Object books = session.getAttribute("BOOKLIST");
I doubt whether cookie is enabled in your browser. Could you please add the following line and let me know what's the value you are getting.

if (insertBookToSession(request)) {
/******** LINES TO BE ADDED ******/
HttpSession session= request.getSession();
out.println(session.isNew());
/******** LINES TO BE ADDED ******/
response.sendRedirect("../jsp/Book.html");}else {response.sendRedirect("../jsp/ListFullError.html");}

Thanks,
Moorthi