在我登入成功之後，我會

session.setAttribute("user", userMap);//設定session

所以就寫了一個監聽器來控制登入的。

package org.system.utils;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;

 
public class OnlineUserListener implements HttpSessionAttributeListener {
    private final static ConcurrentHashMap<Integer, HttpSession> onlineMap = new ConcurrentHashMap<>();

    @Override
    public void attributeAdded(HttpSessionBindingEvent event) {
        /* 獲取session裡面的使用者資訊 
 
*/
        @SuppressWarnings("unchecked")
        Map<String, Object> userMap = (Map<String, Object>) event.getSession().getAttribute("user");
        int userId = (int) userMap.get("id");
        if (onlineMap.containsKey(userId)) {
            onlineMap.get(userId).invalidate();
        }
        onlineMap.put(userId, event.getSession());
        
        HttpSessionAttributeListener.
 
super.attributeAdded(event);
    }

}

在web.xml裡配置監聽器

 <listener>
        <listener-class>org.system.utils.OnlineUserListener</listener-class>
    </listener>