HttpSessionListener 接口用于监视在应用程序服务器上创建和销毁会话的时间。 HttpSessionEvent.getSession（）返回一个新创建或销毁的会话（取决于它是否被 sessionCreated /调用）分别 sessionDestroyed 。

The HttpSessionListener interface is used to monitor when sessions are created and destroyed on the application server. The HttpSessionEvent.getSession() returns you a session that is newly created or destroyed (depending if it's called by sessionCreated/sessionDestroyed respectively).

如果您想要现有会话，则必须从请求中获取会话。

If you want an existing session, you will have to get the session from the request.

HttpSession session = request.getSession(true).
String username = (String)session.getAttribute("username");