Servlet监听器

监听器

Servlet规范中定义的一种特殊的组件，用来监听Servlet容器产生的事件并进行相应的处理

容器产生的两大类事件：
生命周期相关的事件
绑定数据相关的事件


绑定数据相关的事件

调用了request，session，ServletContext 的 setAttribute、removeAttribute方法时产生的事件
ServletRequestAttributeListener
attributeAdded(ServletRequestAttributeEvent srae)
attributeRemoved(ServletRequestAttributeEvent srae)
attributeReplaced(ServletRequestAttributeEvent srae)
HttpSessionAttributeListener
attributeAdded(HttpSessionBindingEvent event);
attributeRemoved(HttpSessionBindingEvent event);
attributeReplaced(HttpSessionBindingEvent event);
ServletContextAttributeListener
attributeAdded(ServletContextAttributeEvent event);
attributeRemoved(ServletContextAttributeEvent event);
attributeReplaced(ServletContextAttributeEvent event);


生命周期相关的事件

容器创建或者销毁request，session，ServletContext时产生的事件
ServletRequestListener
requestDestroyed(ServletRequestEvent sre)
requestInitialized (ServletRequestEvent sre)
HttpSessionListener
sessionCreated(HttpSessionEvent se)
sessionDestroyed(HttpSessionEvent se)
ServletContextListener
contextDestroyed(ServletContextEvent sce)

contextInitialized(ServletContextEvent sce)

绑定数据相关的事件案例：

1、web.xml

 <!--绑定数据相关的事件-->    <listener>        <listener-class>com.listener.MyServletContextAttributeListener</listener-class>    </listener>

2、MyServletContextAttributeListener.java

package com.listener;import javax.servlet.ServletContextAttributeEvent;import javax.servlet.ServletContextAttributeListener;public class MyServletContextAttributeListener implements ServletContextAttributeListener {    @Override    public void attributeAdded(ServletContextAttributeEvent servletContextAttributeEvent) {        System.out.println("attributeAdded...");    }    @Override    public void attributeRemoved(ServletContextAttributeEvent servletContextAttributeEvent) {        System.out.println("attributeRemoved...");    }    @Override    public void attributeReplaced(ServletContextAttributeEvent servletContextAttributeEvent) {        System.out.println("attributeReplaced...");    }}

3、截图

生命周期相关的事件案例：

统计网站在线人数

1、web.xml

 <!--监听器-->    <!--生命周期相关的事件-->    <listener>        <listener-class>com.listener.UserAccessCountListener</listener-class>    </listener>

2、index.jsp(使用el表达式)

在线人数: ${count}

3、UserAccessCountListener.java

package com.listener;import javax.servlet.ServletContext;import javax.servlet.http.HttpSession;import javax.servlet.http.HttpSessionEvent;import javax.servlet.http.HttpSessionListener;public class UserAccessCountListener implements HttpSessionListener {    private void add(HttpSession session, int add) {        ServletContext context = session.getServletContext();        Object attribute = context.getAttribute("count");        int count = attribute == null ? 0 : (int) attribute;        synchronized (this) {            count += add;        }        context.setAttribute("count", count);    }    @Override    public void sessionCreated(HttpSessionEvent se) {        HttpSession session = se.getSession();        session.setMaxInactiveInterval(30);//单位为秒，设置为-1则永不过期        System.out.println(session.getId() + "被创建...");        add(session, 1);    }    @Override    public void sessionDestroyed(HttpSessionEvent se) {        HttpSession session = se.getSession();        System.out.println(session.getId() + "被销毁...");        add(session, -1);    }}

4、截图