监听器

定义：

监听器主要是用来监听事件/对象的创建和销毁的过程，就像在用户的session创建或销毁、修改等变化的时候，我们可以给用户做什么；

监听流程：

首先定义一个类，这个类要实现Listener接口，然后在web.xml配置，在java中主要有三个域对象的监听：ServletContext、Session、Request

主要类别

生命周期事件（初始化、销毁时）的监听需要实现的接口
作用域中的值产生变化的时候进行监听（添加值、修改值、删除值）ServletContext监听，实现接口javax.servlet.ServletContextListener  ServletContext监听，实现接口     ...HttpSessionActivationListener                                         
HttpSession监听，实现的接口...HttpSessionListener                                                             ...HttpSessionAttributeListener
  ServletContext监听，实现接口     ...HttpSessionActivationListener                                         
                                                             ...HttpSessionBindingListenerServletRequest监听，实现接口...HttpRequestListenerServletRequest监听，实现接口     ...HttpRequestAttributeListener总结上面的表格：

生命周期事件（初始化、销毁时）的监听需要实现的接口：                                                                                                                  

ServletContext监听

实现接口：javax.servlet.ServletContextListener

       初始化监听：void contextInitialized(ServletContextEvent event)

       销毁监听：void contextDestroyed(ServletContextEvent event)

       访问ServletContext的方法：ServletContext servletContext=event.getServletContext()

实例：

/** * 初始化的时候会回调这个方法 */public void contextInitialized(ServletContextEvent event) {//通过event对象可以向application中添加初始化的数据ServletContext context = event.getServletContext();//查询 数据库把  把所有的都保存到applicationcontext.setAttribute("bookshop", "aaaaaaa");context.setAttribute("bookshop", "添加");//source是产生servletcontext的那个对象Object source = event.getSource();System.out.println(source + "**");}   /** * 销毁的时候会回调这个方法 */public void contextDestroyed(ServletContextEvent event) {System.out.println("contextDestroyed");}

HttpSession监听

实现接口：...HttpSessionListener

       初始化监听：void sessionCreated(HttpSessionEvent event)

       销毁监听：void sessionDestroyed(HttpSessionEvent event)

       访问Session的方法：HttpSession session=event.getSession()

ServletRequest监听

实现接口：...HttpRequestListener

       初始化监听：void requestInitialized(ServletRequestEvent event)

       销毁监听：void requestDestroyed(ServletRequestEvent event)

       访问ServletRequest的方法：ServletRequest servletRequest=event.getServletRequest()

作用域中的值产生变化的时候进行监听（添加值、修改值、删除值）                                                                                                

ServletContext监听

实现接口：...ServletContextAttributeListener

      增加监听：void attributeAdded(ServletContextAttributeEvent event)

      移除监听：void attributeRemoved(ServletContextAttributeEvent event)

      修改监听：void attributeReplaced(ServletContextAttributeEvent event)

实例：

//添加值的时候回调的方法public void attributeAdded(ServletContextAttributeEvent event) {//得到添加的值的名称 对应context.setAttribute("name", "value");中的nameString name = event.getName();event.getServletContext();//得到添加的值 对应context.setAttribute("name", "value");中的valueObject source = event.getSource();Object value = event.getValue();System.out.println(name + " source = " +  source + value + "******************************************");}/** * 删除值的时候回调的方法 * @param paramServletContextAttributeEvent */public void attributeRemoved(ServletContextAttributeEvent event) {System.out.println("removed");}/** * 替换值的时候回调的方法 * @param paramServletContextAttributeEvent */public void attributeReplaced{ServletContextAttributeEvent event) {//得到修改的值的名称 对应context.setAttribute("name", "value");中的nameString name = event.getName();//得到修改之后的值Object  obj = event.getServletContext().getAttribute(name);Object source = event.getSource();//注意这里得到的value是修改之前 的值Object value = event.getValue();System.out.println(name + " replaced method 888***** source = " +  source + value + " new Value = " + obj + "******************************************");}

HttpSession监听
实现接口：...HttpSessionAttributeListener

         增加监听：void attributeAdded(HttpSessionBindingEvent event)

         移除监听：void attributeRemoved(HttpSessionBindingEvent event)

         修改监听：void attributeReplaced(HttpSessionBindingEvent event)

实现接口：...HttpSessionActivationListener

         激活监听：void sessionDidActivate(HttpSessionEvent event)

         钝化监听：void sessionWillPassivate(HttpSessionEvent event)

实现接口：...HttpSessionBindingListener

         一个类想要知道什么时候绑定或者取消绑定到HttpSession上，那么这个类要实现HttpSessionBindingListener接口

ServletRequest监听

实现接口：...HttpRequestAttributeListener

         增加监听：void attributeAdded(ServletRequestAttributeEvent event)

         移除监听：void attributeRemoved(ServletRequestAttributeEvent event)

         修改监听：void attributeReplaced(ServletRequestAttributeEvent event)

通过getName()和getValue()来暴露相关的属性

实例                                                                                                                                                                                                        

网站人数在线统计

//计数器public static int count = 0;//新产生一个session的时候会调用这个方法public void sessionCreated(HttpSessionEvent event) {//计数器加1count ++;System.out.println("session listener ***" + source);}//session销毁的时候会调用这个方法public void sessionDestroyed(HttpSessionEvent event) {//计数器减1count--;System.out.println("session listener sessionDestroyed ***" + source);}