【SessionListener】

@WebListenerpublic class SessionListener implements HttpSessionListener,HttpSessionIdListener{    @Override    public void sessionIdChanged(HttpSessionEvent e, String oldSessionId) {        SessionRegistry.updateSessionId(e.getSession(),oldSessionId);        System.out.println(this.date()+":Session ID:"+oldSessionId+" changed to "+e.getSession().getId());    }

    private String date() {        SimpleDateFormat f = new SimpleDateFormat("EEE dd,mm,yyyy HH:mm:ss");        return f.format(System.currentTimeMillis());    }

    @Override    public void sessionCreated(HttpSessionEvent e) {        SessionRegistry.addSession(e.getSession());        System.out.println(this.date() + ":Session "+e.getSession().getId()+"created.");    }

    @Override    public void sessionDestroyed(HttpSessionEvent e) {        SessionRegistry.removeSession(e.getSession());        System.out.println(this.date() + ":Session "+e.getSession().getId()+"destroyed.");    }}

【SessionRegistry】比较轻量，私有构造，一些静态方法

public final class SessionRegistry {    private static final Map<String,HttpSession> SESSIONS = new Hashtable<>();    public static void addSession(HttpSession session){        SESSIONS.put(session.getId(),session);    }    public static void updateSessionId(HttpSession session,String oldSessionId){        synchronized (SESSIONS){            SESSIONS.remove(oldSessionId);            addSession(session);        }    }

    public static void removeSession(HttpSession session){        SESSIONS.remove(session.getId());    }

    public static List<HttpSession> getAllSessions(){        return new ArrayList<>(SESSIONS.values());    }

    public static int getNumberOfSessions(){        return SESSIONS.size();    }    private SessionRegistry(){}}

【web.xml】

<listener>    <listener-class>net.mypla.SessionListener</listener-class></listener>