public class MyRequestListener implements ServletContextListener{
  public void contextInitialized(ServletContextEvent event){
    System.out.println("启动监听器");
  }
 
  public void contextDestroy(ServletContextEvent event){
    System.out.println("结束监听器");
  }
}

<session-config>
  <session-timeout> 30 </session-timeout>
</session-config>

public class MyRequestListener implements HttpSessionListener{
  public void sessionCreate(HttpSessionEvent event){
    System.out.println("event 创建:");
  }
 
  public void sessionDestroy(HttpSessionEvent event){
    System.out.println("event 销毁:");
  }
}

public class MyRequestListener implements HttpSessionAttributeListener{
  public void attributeAdded(HttpSessionBindEvent event){
     
  }
 
  public void attributeRemoved(HttpSessionBindEvent event){
     
  }
 
  public void attributeReplaced(HttpSessionBindEvent event){
     
  }
}

public class MyRequestListener implements HttpSessionListener{
  private int counts = 0;//用于统计在线人数
  public void sessionCreate(HttpSessionEvent event){
    counts++;
    event.getSession().getServletContext.setAttribute("name",counts); //用户人数+1
  }
 
  public void sessionDestroy(HttpSessionEvent event){
    counts--;
    event.getSession().getServletContext.setAttribute("name",counts);//用户人数-1
  }
}
 
@WebListener
public class MyRequestListener2 implements ServletRequestListener{
  private ArrayList<User>list; //创建一个在线用户的列表
  public void requestCreate(ServletRequestEvent event){
    HttpServletRequest request = (HttpServletRequest) request.getServletContext();
    String sessionId = request.getSession().getSessionId();
    if(sessionId ...){
      ....
      list.add(...);
    }
  }
 
  public void requestDestroy(ServletRequestEvent event){
 
  }
}
 
public class User{
  private int sessionId;
   
}