Java Programming

Hello again.

I have a JSP page, in witch I kept a session variable with a value, and later (in the same JSP page) I use a JAVA class to obtain the value stored in this session variable (I make this to verify that this works well and that this is possible)

When I run the file, I obtain the following error:

excepci�n 

org.apache.jasper.JasperException
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:372)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:802)


causa ra�z 

java.lang.NullPointerException
	ConfigAnlsEspect.Config.<init>(Config.java:18)
	org.apache.jsp.admin.PaleoPlot.configAnEsp_jsp._jspService(configAnEsp_jsp.java:103)
	org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

And my code is:
------------->The JSP file:<-----------------

.....
session.setAttribute("VarSession", "empty");
.....
<%@ page import="ConfigAnlsEspect.Config"%>
.....
<%
Config ObjConfigAE = new Config();
String valorDev = ObjConfigAE.getValueSession();
out.println("<br>Returned= -->"+valorDev+"<--<br>");
%>
.....

------------->The JAVA class:<-----------------

package ConfigAnlsEspect;

import java.io.*;
import java.net.*;
import java.util.*;
import java.text.*;

import javax.servlet.http.HttpSession;
import javax.servlet.*;
import javax.servlet.http.*;

public class Config extends HttpServlet{

	HttpSession session = null;
	String valor = null;
	
	public Config (){
		valor = (String)session.getAttribute("VarSession");
	}
	
	public String getValueSession(){	
		return valor;
	}
}

Why I obtain java.lang.NullPointerException ??

Thanks very much