import javax.servlet.http.HttpSessionEvent;
  
import javax.servlet.http.HttpSessionListener;
  
public class SessionListener implements HttpSessionListener {
  private static int count = 0;
  
  public void sessionCreated(HttpSessionEvent se) {
    count++;
    System.out.println("session创建：" + new java.util.Date());
  }
  
  public void sessionDestroyed(HttpSessionEvent se) {
    count--;
    System.out.println("session销毁：" + new java.util.Date());
  }
  
  public static int getCount() {
    return count;
  }
}

<listener>
  <description>session监听器</description> 
  <listener-class>com.xxx.SessionListener</listener-class>
</listener>

<%
  int count=com.xxx.SessionListener.getCount();
  out.println("在线人数："+count);
%>