在web.xml檔案中新增

<!-- 統計線上 監聽 -->
<listener>
    <listener-class>com.connxun.config.listener.OnLineListener</listener-class>
</listener>

然後新增對應路徑的監聽檔案

package com.connxun.config.listener;
import com.connxun.util.session.UserSession;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
 
import javax.servlet.ServletContextListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import java.util.ArrayList;
import java.util.HashMap;
import 
 
java.util.List;
/**
 * 統計線上人數
 *
 * @author gaoyf
 */
public class OnLineListener implements ServletContextListener, HttpSessionListener, HttpSessionAttributeListener {

    // 宣告一個ServletContext物件
private ServletContext application = null;
    private static HashMap<String, UserSession> hashMap = new 
 
HashMap<String, UserSession>();
    public void contextInitialized(ServletContextEvent sce) {
        // 容器初始化時，向application中存放一個空的容器
this.application = sce.getServletContext();
        this.application.setAttribute("allUser", new ArrayList<UserSession>());
}


    public void contextDestroyed(ServletContextEvent sce) {
    }

    public void sessionCreated(HttpSessionEvent se) {
        UserSession userSession = (UserSession) se.getSession().getAttribute("userSession");
        if (userSession != null) {
            hashMap.put(se.getSession().getId(), userSession);
System.out.println("使用者登入，當前使用者總數：" + hashMap.size());
}
        System.out.println(se.getSession().getId());
}

    public void sessionDestroyed(HttpSessionEvent se) {
        // 將使用者名稱稱從列表中刪除
List l = (List) this.application.getAttribute("allUser");
UserSession userSession = (UserSession) se.getSession().getAttribute("userSession");
        if (userSession != null)
            l.remove(userSession);
System.out.println("session過期，當前使用者數：" + l.size());
        this.application.setAttribute("allUser", l);
System.out.println(se.getSession().getId());
        if (hashMap.containsKey(se.getSession().getId())) {
            hashMap.remove(se.getSession().getId());
System.out.println("session自動銷燬：" + hashMap.size());
}
    }

    public void attributeAdded(HttpSessionBindingEvent se) {
        // 如果登入成功，則將使用者名稱儲存在列表之中
List<UserSession> l = (List<UserSession>) this.application.getAttribute("allUser");
l.add((UserSession) se.getValue());
        this.application.setAttribute("allUser", l);
hashMap.put(se.getSession().getId(), (UserSession) se.getValue());
System.out.println(se.getSession().getId());
System.out.println("系統登入，當前使用者數：" + hashMap.size());
UserSession userSession = (UserSession) se.getSession().getAttribute("userSession");
        if (userSession != null) {
            System.out.println(se.getSession().getId());
hashMap.put(se.getSession().getId(), userSession);
System.out.println("使用者登入，當前使用者總數：" + hashMap.size());
}
    }

    public void attributeRemoved(HttpSessionBindingEvent se) {
        List<UserSession> l = (List<UserSession>) this.application.getAttribute("allUser");
UserSession userSession = (UserSession) se.getValue();
        if (userSession != null)
            l.remove(userSession);
System.out.println(se.getSession().getId());
hashMap.remove(se.getSession().getId());
System.out.println("使用者登出，當前使用者數：" + hashMap.size());
        this.application.setAttribute("allUser", l);
}

    public void attributeReplaced(HttpSessionBindingEvent se) {
    }

}