雙層HashMap嵌套遍歷記錄

要遍歷的雙層結構：Map<String, Map<String, WebSocketSession>>

遍歷代碼：

Set<String> mapSet = lanSession.keySet();
    for(String key:mapSet){
        Map<String,WebSocketSession> subMap = lanSession.get(key);
            for(Map.Entry<String,WebSocketSession> entry: subMap.entrySet()){
                session = entry.getValue();
                synchronized (session) {
                if (session.isOpen()) {
                //System.out.println(session);
                session.sendMessage(message);
                }
            }
        }
    }