public   class  SessionCounter  implements  HttpSessionListener {
     public   void  sessionCreated(HttpSessionEvent event) {
        ServletContext ctx = event.getSession( ).getServletContext( );
        Integer numSessions = (Integer) ctx.getAttribute("numSessions");
         if  (numSessions ==  null ) {
            numSessions =  new  Integer( 1 );
        }
         else  {
             int  count = numSessions.intValue( );
            numSessions =  new  Integer(count +  1 );
        }
        ctx.setAttribute("numSessions", numSessions);
    }
     public   void  sessionDestroyed(HttpSessionEvent event) {
        ServletContext ctx = event.getSession( ).getServletContext( );
        Integer numSessions = (Integer) ctx.getAttribute("numSessions");
         if  (numSessions ==  null ) {
            numSessions =  new  Integer( 0 );
        }
         else  {
             int  count = numSessions.intValue( );
            numSessions =  new  Integer(count -  1 );
        }
        ctx.setAttribute("numSessions", numSessions);
    }
}

  <listener>
      <listener- class ><путь к классу>.SessionCounter
      </listener- class >
  </listener