My binding with datatable give me null ate the backing bean, do i miss some thing

Hi , what i'm trying to do is first to load arraylist of student objects in h:datatable in the jsp, than on submit i want to get those student collection back in backing bean.
The datatable will have student information in columns containing h:inputtext. So user can edit the collection of student .

For this i did binding with HTMLDatatable in backing bean. but this gives me null pointer exception. why?

This is as follows

my backing bean

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

package com.backingBean;

import com.VO.StudentVO;
import java.util.ArrayList;
import java.util.List;
import javax.faces.component.html.HtmlDataTable;
import javax.faces.context.FacesContext;
import javax.faces.event.ActionEvent;
import javax.servlet.http.HttpServletRequest;

public class StudentBackingBean {
        public StudentBackingBean(){
            System.out.println("  STUDENT BACKING BEAN ");
        }
        private List<StudentVO> studentList;
        private String flagS;
        private HtmlDataTable dataTable;
        
        public void getStudentList(ActionEvent evt){
            System.out.println("  getStudentList  ");
            FacesContext context = null;
            HttpServletRequest req = null;
            String param = null;
            try{
                    context = FacesContext.getCurrentInstance();
                    req = (HttpServletRequest) context.getExternalContext().getRequest();
                    param = req.getParameter("formstudent:flag");
                    if(param != null && param.equals("0")){
                        getStudent();
                    }
            }catch(Exception e){
                System.out.println("  "+e);
            }
        }

public List<StudentVO> getStudentList() {
        return studentList;
    }

public void setStudentList(List<StudentVO> studentList) {
        this.studentList = studentList;
    }

private void getStudent() throws Exception{
        if(studentList == null){
                studentList = new ArrayList<StudentVO>();
        }

studentList.add(new StudentVO("RAHUL","R28","CMP","SOLAPUR"));
        studentList.add(new StudentVO("HEMENDRA","H28","CMP","SAHARANPUR"));
        studentList.add(new StudentVO("SUNIL","S28","CMP","JAMSHEDPUR"));
    }

public String getFlagS() {
        return flagS;
    }

public void setFlagS(String flagS) {
        this.flagS = flagS;
    }

public HtmlDataTable getDataTable() {        
        return dataTable;
    }

public void setDataTable(HtmlDataTable dataTable) {
        this.dataTable = dataTable;
    }

public String doItRe(){
        System.out.println(" doItRe ");
        
        try{
            System.out.println(" dataTable.getRows "+dataTable.getRows());
        }catch(Exception e){
            System.out.println("  "+e);
        }
        return null;
    }
}

this is my jsp

<%-- 
    Document   : DatatablePractise
    Created on : Dec 19, 2010, 10:19:06 AM
    Author     : rahul
--%>

<%@page contentType="text/html"%>
<%@page pageEncoding="UTF-8"%>

<%@taglib prefix="f" uri="http://java.sun.com/jsf/core"%>
<%@taglib prefix="h" uri="http://java.sun.com/jsf/html"%>

<!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=UTF-8">
        <title>JSP Page</title>
        <script type="text/javascript">
            function    ss(){
                document.forms['formstudent']['formstudent:flag'].value = "0";
                document.forms['formstudent']['formstudent:sbButton'].click();
            }
        </script>
    </head>
    <body>

<f:view>
            <h4>Datatable Practise</h4>
            <div>
                <h:form id="formstudent">
                    <h:inputHidden id="flag" value="#{stBean.flagS}" />
                    <h:commandButton id="sbButton" style="display:none" type="submit"  actionListener="#{stBean.getStudentList}" value="ENTER" />
                    <input  type="button" value="ENTER-NO" onclick="ss()" />
                    <h:dataTable id="datatable" value="#{stBean.studentList}"  binding="#{stBean.dataTable}"  var="student">
                        <h:column id="col1">
                            <f:facet name="header">
                                <h:outputText id="nameheader" value="Name" />
                            </f:facet>
                            <h:inputText id="studentNameId" value="#{student.studentName}" />
                        </h:column>
                        <h:column id="col2">
                            <f:facet name="header">
                                <h:outputText id="idheader" value="Student Id" />
                            </f:facet>
                            <h:inputText id="studentIdID" value="#{student.studentId}" />
                        </h:column>
                        <h:column id="col3">
                            <f:facet name="header">
                                <h:outputText id="nativePlaceheader" value="Native Place" />
                            </f:facet>
                            <h:inputText id="nativePlaceID" value="#{student.nativePlace}" />
                        </h:column>
                        <h:column id="col4">
                            <f:facet name="header">
                                <h:outputText id="departmentNameheader" value="Department Name" />
                            </f:facet>
                            <h:inputText id="departmentNameID" value="#{student.departmentName}" />
                        </h:column>
                    </h:dataTable>

<h:commandButton type="submit" value="Enter-IT" action="#{stBean.doItRe}"  />

</h:form>
            </div>
        </f:view>
    </body>
</html>

i get null pointer exception in method doItRe() of backing bean why ?, did i missed out something

There are pointers here that could solve your problem --

http://balusc.blogspot.com/2006/06/using-datatables.html