HttpSessionListener和HttpSessionAttributeListener

public interface HttpSessionListener
extends java.util.EventListener
Implementations of this interface are notified of changes to the list of active sessions in a web application. To receive notification events, the implementation class must be configured in the deployment descriptor for the web application

void sessionCreated(HttpSessionEvent se)
          Notification that a session was created.
void sessionDestroyed(HttpSessionEvent se)
          Notification that a session is about to be invalidated.


HttpSessionAttributeListener监听
HttpSession中的属性的操作。当在Session增加一个属性时，激发attributeAdded(HttpSessionBindingEvent se) 方法；当在Session删除一个属性时，激发attributeRemoved(HttpSessionBindingEvent se)方法；当在Session属性被重新设置时，激发attributeReplaced(HttpSessionBindingEvent se) 方法。


实际例子：

用户提出要求，需要知道哪些用户已经登录，但是在实际环境中，程序被部署到3台web上，前端用f5的负载均衡，那玩意没有玩过，幸好数据库是单一的，所以就简单建立了一个MySQL的内存表（Heap Table），来已经登录的用户信息。
Java代码
package com.nbrc.lddw.util;  
 
import javax.servlet.http.HttpSession;  
import javax.servlet.http.HttpSessionAttributeListener;  
import javax.servlet.http.HttpSessionBindingEvent;  
import javax.servlet.http.HttpSessionBindingListener;  
 
import org.apache.commons.logging.Log;  
import org.apache.commons.logging.LogFactory;  
import org.springframework.context.ApplicationContext;  
import org.springframework.web.context.support.WebApplicationContextUtils;  
 
import com.nbrc.lddw.interceptor.AuthorizeInterceptor;  
import com.nbrc.lddw.model.OnlineInfo;  
import com.nbrc.lddw.model.User;  
import com.nbrc.lddw.service.OnlineUserService;  
/** 
*  
* @author fox 
* @date 2009-02-09 
* @description 已登录用户的监听 
*/ 
public class OnlineUserListener implements HttpSessionAttributeListener {  
    private static Log log = LogFactory.getLog(OnlineUserListener.class);  
 
    public void attributeAdded(HttpSessionBindingEvent hse) {  
        log.info("value bound! make session info ...");  
        HttpSession session = hse.getSession();       
        ApplicationContext context =   
            WebApplicationContextUtils.getRequiredWebApplicationContext(session.getServletContext());  
        OnlineUserService svr = (OnlineUserService)context.getBean("onlineService");  
        User u = null;  
        if(session.getAttribute(AuthorizeInterceptor.USR_KEY)!=null)  
            u = (User) session.getAttribute(AuthorizeInterceptor.USR_KEY);  
        if(u!=null && svr.findByUserId(u.getId())==null){             
                OnlineInfo info = new OnlineInfo();  
                info.setSessionId(session.getId());  
                info.setUserId(u.getId());  
                svr.save(info);  
        }else{  
                log.error("can't get user in session");  
        }         
    }  
 
    public void attributeRemoved(HttpSessionBindingEvent hse) {  
        HttpSession session = hse.getSession();  
        ApplicationContext context = WebApplicationContextUtils.getRequiredWebApplicationContext(session.getServletContext());  
        OnlineUserService svr = (OnlineUserService)context.getBean("onlineService");  
        if(svr.findById(session.getId())!=null){  
            svr.removeById(session.getId());                  
        }  
    }  
 
    public void attributeReplaced(HttpSessionBindingEvent se) {  
        // TODO Auto-generated method stub  
          
    }  
      
 
}