httpsession中的SessionID是如何生成的?

//excel.java

package net.oicp.jing31;

import java.io.File;

import javax.servlet.http.HttpSessionListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSession;
public class excel implements HttpSessionListener
{
public void sessionCreated(HttpSessionEvent se)
{
}
public void sessionDestroyed(HttpSessionEvent se)
{
System.out.println("The session has been destoryed!!");
}
}
如何获得？

如何在JSP工程中填加自己的listener
我把web.xml文件里添加如下内容：
<listener>
<listener-class>net.oicp.jing31.excel</listener-class>
</listener>
public void sessionDestroyed(HttpSessionEvent se)
{
System.out.println("sessionid:\t"+se.getSession().getId());
}