案例一

使用监听器获得当前在线的人数。用变量count表示当前在线人数。在HttpSessionListener每次调用sessionCreated方法时更新count，并保存在ServletContext中。之后需要使用时从ServletContext中获取即可。

@WebListener
public class MySessionListener implements HttpSessionListener  {
    private int count = 0 ;

    @Override
    public void sessionCreated(HttpSessionEvent se) {
        count++;
        se.getSession().getServletContext().setAttribute("count",  count);
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        count--;
        se.getSession().getServletContext().setAttribute("count", count);
    }
}

案例二

下面这个例子，除了获得在线的人数，还可以需要获得一些其他信息

ServletRequestListener监听器，对于每一次请求，获得请求的相关信息，并保存在ArrayList中。

package listener;

import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;

import util.SessionUtil;
import entity.User;

@WebListener
public class MyServletRequestListener implements ServletRequestListener {
    ArrayList<User> userList = null ;

    @Override
    public void requestDestroyed(ServletRequestEvent arg0) {

    }

    @Override
    public void requestInitialized(ServletRequestEvent arg0) {
        HttpServletRequest request = (HttpServletRequest)arg0.getServletRequest();
        HttpSession session = request.getSession();

        userList = (ArrayList<User>)arg0.getServletContext().getAttribute("userList");
        if(userList == null){
            userList = new ArrayList();
        }


        String sessionId = session.getId();
        String Ip = request.getLocalAddr();
        String time = new SimpleDateFormat("yy-MM-dd hh:mm:ss").format(new Date());
        User user = new User();
        user.setFirstTime(time);
        user.setIp(Ip);
        user.setSessionID(sessionId);

        if(SessionUtil.getSessionByID(userList, sessionId)==null){
            userList.add(user);
        }

        arg0.getServletContext().setAttribute("userList", userList);
    }

}

HttpSessionListener监听器，更新count的值，并在用户退出时删除ArrayList中的用户数据

package listener;

import java.util.ArrayList;

import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

import util.SessionUtil;
import entity.User;

@WebListener
public class MyHttpSessionListener implements HttpSessionListener {
    private int count = 0 ;

    @Override
    public void sessionCreated(HttpSessionEvent arg0) {
        count++;
        arg0.getSession().getServletContext().setAttribute("count", count);
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent arg0) {
        count--;
        arg0.getSession().getServletContext().setAttribute("count", count);

        String id = arg0.getSession().getId();
        ArrayList<User> userList = (ArrayList<User>)arg0.getSession().getServletContext().getAttribute("userList");

        if(SessionUtil.getSessionByID(userList, id) != null){
            User user = SessionUtil.getSessionByID(userList,id);
            userList.remove(user);
        }
    }

}

效果如下：