Listener 监听对象的创建和销毁

HttpSessionListener、ServletContextListener、ServletRequestListener分别用于控制Session、context、request的创建和销毁。触发时机分别为：

• HttpSessionListener：监听Session的创建和销毁。创建session时执行sessionCreated（HttpSessionEvent se）方法。超时或者执行session.invalidate()时执行sessionDestroyed（HttpSessionEvent se）方法。该Listener用于收集在线者信息。
• ServletContextListener：监听context的创建和销毁。context代表当前的web应用程序。服务器启动或者热部署war包时执行contextInitialized(ServletContextEvent event)方法。服务器关闭或者只关闭该web时会执行contextDestoryed(ServletContextEvent event)方法。该Listener可用于获取web.xml里配置的初始化参数。
• ServletRequestListener：监听request的创建和销毁。用户每次请求request都会执行requestInitialized(ServletRequestEvent event)方法。request处理完毕自动销毁前执行requestDestoryed(ServletRequestEvent event)方法。如果一个HTML页面内含有多个图片，则请求一次HTML页面可能会出发多次request事件。

package linstener;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

public class LinstenerTest implements HttpSessionListener, ServletContextListener, ServletRequestListener {

    private Log log =LogFactory.getLog(this.getClass());
    @Override
    public void requestDestroyed(ServletRequestEvent arg0) {
        //销毁Request时调用
        HttpServletRequest request=(HttpServletRequest)arg0.getServletRequest();
        long time=System.currentTimeMillis()-(long)request.getAttribute("dateCreated");
        log.info(request.getRemoteAddr()+"请求处理结束，用时"+time+"毫秒。");
    }

    @Override
    public void requestInitialized(ServletRequestEvent arg0) {
        
        //创建Request时调用
        HttpServletRequest request=(HttpServletRequest) arg0.getServletRequest();
        String uri=request.getRequestURI();
        uri=request.getQueryString()==null?uri:(uri+"?"+request.getQueryString());
        request.setAttribute("dateCreated", System.currentTimeMillis());
        log.info("IP:"+request.getRemoteAddr()+" 请求 "+uri);
    }

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
        //销毁Servlet上下文时调用
        ServletContext servletContext=arg0.getServletContext();
        log.info("即将关闭"+servletContext.getContextPath());
    }

    @Override
    public void contextInitialized(ServletContextEvent arg0) {
        //加载Servlet上下文时调用
        ServletContext servletContext=arg0.getServletContext();
        log.info("即将启动"+servletContext.getContextPath());
    }

    @Override
    public void sessionCreated(HttpSessionEvent arg0) {
        //创建session时调用
        HttpSession session =arg0.getSession();
        log.info("新创建一个session，ID为："+session.getId());
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent arg0) {
        //销毁session前调用
        HttpSession session =arg0.getSession();
        log.info("销毁一个session，ID为："+session.getId());
    }

}

Listener类web.xml文件中的配置：

  <listener>
      <listener-class>
          linstener.LinstenerTest
      </listener-class>
  </listener>