1、监听器概述
监听器是javaweb中三大组件(Servlet、Listener、Filter)之一,整个监听事务中参与对象分为:
事件源(被监听者);
监听者;
监听器;
事件。
用生活中的事务举例,警察是监听者;小偷是事件源;小偷盗窃是事件;警察监听小偷,当小偷执行(触发)盗窃这个事件的时候,立即执行监听器中的代码(抓捕)。需要注意的是,只有当触发事件时,相应的监听器中的代码才能执行。
被监听的事件源为:ServletContext、HttpSession、ServletRequest,即三大域对象。监听器共有8种,分别为:
生命周期监听器:监听域对象“创建”与“销毁”,3种,即三大域对象各一种;
属性监听器:监听域对象操作域属性,3种,三大域对象各一种;
感知监听器:监听HttpSession,2种。
2、生命周期监听器
(1)生命周期监听器3种:
● ServletContextListener:Tomcat启动和关闭时调用下面两个方法
public void contextInitialized(ServletContextEvent evt):ServletContext对象被创建后调用;
public void contextDestroyed(ServletContextEvent evt):ServletContext对象被销毁前调用;
● HttpSessionListener:开始会话和结束会话时调用下面两个方法
public void sessionCreated(HttpSessionEvent evt):HttpSession对象被创建后调用;
public void sessionDestroyed(HttpSessionEvent evt):HttpSession对象被销毁前调用;
● ServletRequestListener:开始请求和结束请求时调用下面两个方法
public void requestInitiallized(ServletRequestEvent evt):ServletRequest对象被创建后调用;
public void requestDestroyed(ServletRequestEvent evt):ServletRequest对象被销毁前调用。
(2)事件对象
● ServletContextEvent:ServletContext getServletContext();
● HttpSeessionEvent:HttpSession getSession();
● ServletRequestEvent:
ServletRequest getServletRequest();
ServletContext getServletContext()。
(3)实例:
步骤:
编写MyServletContextListener类,实现ServletContextListener接口;
在web.xml文件中部署监听器;
为了看到session销毁的效果,在web.xml文件中设置session失效时间为1分钟;
- <span style="font-size:18px;">
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- public class MyServletContextListener implements ServletContextListener {
- public void contextDestroyed(ServletContextEvent evt) {
- System.out.println("销毁ServletContext对象");
- }
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- public void contextInitialized(ServletContextEvent evt) {
- System.out.println("创建ServletContext对象");
- }
- }</span>
- <span style="font-size:18px;">
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- public class MyHttpSessionListener implements HttpSessionListener {
- public void sessionCreated(HttpSessionEvent evt) {
- System.out.println("创建session对象");
- }
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- public void sessionDestroyed(HttpSessionEvent evt) {
- System.out.println("销毁session对象");
- }
- }</span>
- <span style="font-size:18px;">
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- public class MyServletRequestListener implements ServletRequestListener {
- public void requestDestroyed(ServletRequestEvent evt) {
- System.out.println("销毁request对象");
- }
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- public void requestInitialized(ServletRequestEvent evt) {
- System.out.println("创建request对象");
- }
- }</span>
XML配置:- <span style="font-size:18px;"><listener>
- <listener-class>cn.itcast.listener.MyServletContextListener</listener-class>
- </listener>
- <listener>
- <listener-class>cn.itcast.listener.MyHttpSessionListener</listener-class>
- </listener>
- <listener>
- <listener-class>cn.itcast.listener.MyServletRequestListener</listener-class>
- </listener>
- <session-config>
- <session-timeout>1</session-timeout>
- </session-config></span>
3、属性监听器
(1)属性监听器共3种:
● ServletContextAttributeListener:在ServletContext域进行增、删、改属性时调用下面方法:
public void attributeAdded(ServletContextAttributeEvent evt);
public void attributeRemoved(ServletContextAttributeEvent evt);
public void attributeReplaced(ServletContextAttributeEvent evt)。
● HttpSessionAttributeListener:在HttpSession域进行增、删、改属性时调用下面方法:
public void attributeAdded(HttpSessionBindingEvent evt);
public void attributeRemoved (HttpSessionBindingEvent evt);
public void attributeReplaced (HttpSessionBindingEvent evt) 。
● ServletRequestAttributeListener:在ServletRequest域进行增、删、改属性时调用下面方法:
public void attributeAdded(ServletRequestAttributeEvent evt);
public void attributeRemoved (ServletRequestAttributeEvent evt);
public void attributeReplaced (ServletRequestAttributeEvent evt)。
(2)事件对象介绍:
● ServletContextAttributeEvent:
String getName():获取当前操作的属性名;
Object getValue():获取当前操作的属性值;
ServletContext getServletContext():获取ServletContext对象。
● HttpSessionBindingEvent:
String getName():获取当前操作的属性名;
Object getValue():获取当前操作的属性值;
HttpSession getSession():获取当前操作的session对象。
● ServletRequestAttributeEvent:
String getName():获取当前操作的属性名;
Object getValue():获取当前操作的属性值;
ServletContext getServletContext():获取ServletContext对象;
ServletRequest getServletRequest():获取当前操作的ServletRequest对象。
- <span style="font-size:18px;">public class MyListener implements ServletContextAttributeListener,
- ServletRequestAttributeListener, HttpSessionAttributeListener {
- public void attributeAdded(HttpSessionBindingEvent evt) {
- System.out.println("向session中添加属性:" + evt.getName() + "=" + evt.getValue());
- }
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- public void attributeRemoved(HttpSessionBindingEvent evt) {
- System.out.println("从session中移除属性:" + evt.getName() + "=" + evt.getValue());
- }
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- public void attributeReplaced(HttpSessionBindingEvent evt) {
- System.out.println("修改session中的属性:" + evt.getName() + "=" + evt.getValue());
- }
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- public void attributeAdded(ServletRequestAttributeEvent evt) {
- System.out.println("向request中添加属性:" + evt.getName() + "=" + evt.getValue());
- }
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- public void attributeRemoved(ServletRequestAttributeEvent evt) {
- System.out.println("从request中移除属性:" + evt.getName() + "=" + evt.getValue());
- }
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- public void attributeReplaced(ServletRequestAttributeEvent evt) {
- System.out.println("修改request中的属性:" + evt.getName() + "=" + evt.getValue());
- }
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- public void attributeAdded(ServletContextAttributeEvent evt) {
- System.out.println("向context中添加属性:" + evt.getName() + "=" + evt.getValue());
- }
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- public void attributeRemoved(ServletContextAttributeEvent evt) {
- System.out.println("从context中移除属性:" + evt.getName() + "=" + evt.getValue());
- }
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- public void attributeReplaced(ServletContextAttributeEvent evt) {
- System.out.println("修改context中的属性:" + evt.getName() + "=" + evt.getValue());
- }
- }</span>
- <span style="font-size:18px;">public class ListenerServlet extends BaseServlet {
- public String contextOperation(HttpServletRequest request, HttpServletResponse response)
- throws ServletException, IOException {
- ServletContext context = this.getServletContext();
- context.setAttribute("a", "a");
- context.setAttribute("a", "A");
- context.removeAttribute("a");
- return "/index.jsp";
- }
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- public String sessionOperation(HttpServletRequest request, HttpServletResponse response)
- throws ServletException, IOException {
- HttpSession session = request.getSession();
- session.setAttribute("a", "a");
- session.setAttribute("a", "A");
- session.removeAttribute("a");
- return "/index.jsp";
- }
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- public String requestOperation(HttpServletRequest request, HttpServletResponse response)
- throws ServletException, IOException {
- request.setAttribute("a", "a");
- request.setAttribute("a", "A");
- request.removeAttribute("a");
- return "/index.jsp";
- }
- }</span>
- <span style="font-size:18px;"> <body>
- <a href="<c:url value='/ListenerServlet?method=contextOperation'/>">SevletContext操作属性</a>
- <br/>
- <a href="<c:url value='/ListenerServlet?method=sessionOperation'/>">HttpSession操作属性</a>
- <br/>
- <a href="<c:url value='/ListenerServlet?method=requestOperation'/>">ServletRequest操作属性</a> |
- </body></span>
4、感知监听器
感知监听器有HttpSessionBindingListener和,这是两个与HttpSession相关的特殊的监听器,特点如下:
● 不用在web.xml文件中部署;
● 这两个监听器不是给session添加,而是给Bean添加。即让Bean类实现监听器接口,然后再把Bean对象添加到session域中。
(1)HttpSessionBindingListener(绑定解绑监听器):
当某个类实现了该接口后,可以感知本类对象添加到session中,以及感知从session中移除。例如让Person类实现HttpSessionBindingListener接口,那么当把Person对象添加到session中,或者把Person对象从session中移除时会调用下面两个方法:
● public void valueBound(HttpSessionBindingEvent event):当把监听器对象添加到session中会调用监听器对象的本方法;
● public void valueUnbound(HttpSessionBindingEvent event):当把监听器对象从session中移除时会调用监听器对象的本方法;
这里要注意,HttpSessionBindingListener监听器的使用与前面介绍的都不相同,当该监听器对象添加到session中,或把该监听器对象从session移除时会调用监听器中的方法。并且无需在web.xml文件中部署这个监听器。
实例:
编写Person类,让其实现HttpSessionBindingListener监听器接口;
编写Servlet类,一个方法向session中添加Person对象,另一个从session中移除Person对象;
在index.jsp中给出两个超链接,分别访问Servlet中的两个方法。
- <span style="font-size:18px;">public class Person implements HttpSessionBindingListener {
- private String name;
- private int age;
- private String sex;
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- public Person(String name, int age, String sex) {
- super();
- this.name = name;
- this.age = age;
- this.sex = sex;
- }
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- public Person() {
- super();
- }
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- public String toString() {
- return "Person [name=" + name + ", age=" + age + ", sex=" + sex + "]";
- }
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- public String getName() {
- return name;
- }
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- public void setName(String name) {
- this.name = name;
- }
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- public int getAge() {
- return age;
- }
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- public void setAge(int age) {
- this.age = age;
- }
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- public String getSex() {
- return sex;
- }
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- public void setSex(String sex) {
- this.sex = sex;
- }
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- public void valueBound(HttpSessionBindingEvent evt) {
- System.out.println("把Person对象存放到session中:" + evt.getValue());
- }
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- public void valueUnbound(HttpSessionBindingEvent evt) {
- System.out.println("从session中移除Pseron对象:" + evt.getValue());
- }
- }
- </span>
ListenerServlet.Java:
- <span style="font-size:18px;">public class ListenerServlet extends BaseServlet {
- public String addPerson(HttpServletRequest request, HttpServletResponse response)
- throws ServletException, IOException {
- Person p = new Person("zhangSan", 23, "male");
- request.getSession().setAttribute("person", p);
- return "/index.jsp";
- }
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- public String removePerson(HttpServletRequest request, HttpServletResponse response)
- throws ServletException, IOException {
- request.getSession().removeAttribute("person");
- return "/index.jsp";
- }</span>
index.jsp:
- <span style="font-size:18px;"> <body>
- <a href="<c:url value='/ListenerServlet?method=addPerson'/>">addPerson</a>
- <br/>
- <a href="<c:url value='/ListenerServlet?method=removePerson'/>">removePerson</a>
- <br/>
- </body></span>
(2)HttpSessionActivationListener(钝化活化监听器):
Tomcat会在session从时间不被使用时钝化session对象,所谓钝化session,就是把session通过序列化的方式保存到硬盘文件中。当用户再使用session时,Tomcat还会把钝化的对象再活化session,所谓活化就是把硬盘文件中的session在反序列化回内存。
当session被Tomcat钝化时,session中存储的对象也被纯化,当session被活化时,也会把session中存储的对象活化。如果某个类实现了HttpSessionActiveationListener接口后,当对象随着session被钝化和活化时,下面两个方法就会被调用:
● public void sessionWillPassivate(HttpSessionEvent se):当对象感知被活化时调用本方法; ● public void sessionDidActivate(HttpSessionEvent se):当对象感知被钝化时调用本方法;
HttpSessionActivationListener监听器与HttpSessionBindingListener监听器相似,都是感知型的监听器,例如让Person类实现了HttpSessionActivationListener监听器接口,并把Person对象添加到了session中。
当Tomcat钝化session时,同时也会钝化session中的Person对象,这时Person对象就会感知到自己被钝化了,其实就是调用Person对象的sessionWillPassivate()方法。
当用户再次使用session时,Tomcat会活化session,这时Person会感知到自己被活化,其实就是调用Person对象的sessionDidActivate()方法。
注意:因为钝化和活化session,其实就是使用序列化和反序列化技术把session从内存保存到硬盘,和把session从硬盘加载到内存。这说明如果Person类没有实现Serializable接口,那么当session钝化时就不会钝化Person,而是把Person从session中移除再钝化!这也说明session活化后,session中就不在有Person对象了。
实例:
先不管HttpSessionActivationListener监听器接口,先来配置Tomcat钝化session的参数,把下面配置文件放到tomcat\conf\catalina\localhost目录下!文件名称为项目名称。
- <span style="font-size:18px;"><Context>
- <Manager className="org.apache.catalina.session.PersistentManager" maxIdleSwap="1">
- <Store className="org.apache.catalina.session.FileStore" directory="mysession"/>
- </Manager>
- </Context></span>
访问项目的index.jsp页面,这会使Tomcat创建Session对象,然后等待一分钟后,查看Tomcat\work\Catalina\localhost\listener\mysession目录下是否会产生文件,如果产生了,说明钝化session的配置成功了,可以开始下一步了。
创建Person类,让Person类实现HttpSessionActivationListener和Serializable接口:- <span style="font-size:18px;">public class Person implements HttpSessionActivationListener, Serializable {
- private String name;
- private int age;
- private String sex;
-
- public Person(String name, int age, String sex) {
- super();
- this.name = name;
- this.age = age;
- this.sex = sex;
- }
-
- public Person() {
- super();
- }
-
- public String toString() {
- return "Person [name=" + name + ", age=" + age + ", sex=" + sex + "]";
- }
-
- public String getName() {
- return name;
- }
-
- public void setName(String name) {
- this.name = name;
- }
-
- public int getAge() {
- return age;
- }
-
- public void setAge(int age) {
- this.age = age;
- }
-
- public String getSex() {
- return sex;
- }
-
- public void setSex(String sex) {
- this.sex = sex;
- }
-
- public void sessionDidActivate(HttpSessionEvent evt) {
- System.out.println("session已经活化");
- }
-
- public void sessionWillPassivate(HttpSessionEvent evt) {
- System.out.println("session被钝化了!");
- }
- }</span>
与上例一样,编写Servlet,提供两个方法:一个向session中添加Person对象,另一个从session中移除Person对象:
- <span style="font-size:18px;">public class ListenerServlet extends BaseServlet {
- public String addPerson(HttpServletRequest request, HttpServletResponse response)
- throws ServletException, IOException {
- Person p = new Person("zhangSan", 23, "male");
- request.getSession().setAttribute("person", p);
- return "/index.jsp";
- }
-
- public String removePerson(HttpServletRequest request, HttpServletResponse response)
- throws ServletException, IOException {
- request.getSession().removeAttribute("person");
- return "/index.jsp";
- }
- }</span>
在index.jsp页面中给出访问addPerson()和removePerson()的方法:
- <span style="font-size:18px;"> <body>
- <a href="<c:url value='/ListenerServlet?method=addPerson'/>">addPerson</a>
- <br/>
- <a href="<c:url value='/ListenerServlet?method=removePerson'/>">removePerson</a>
- <br/>
- </body></span>
打开index.jsp页面,这时Tomcat会创建session,必须在1分钟之前点击addPerson链接,这能保证在session被钝化之前把Person对象添加到session中;
等待一分钟,这时session会被钝化,也就会调用Person的sessionWillPassivate(); 刷新一下index.jsp页面,这会使session活化,会调用Person的sessionDidActivate()方法。
小结:监听器体现的就是一种监听机制,当事件源触发特定事件时,就会触发监听器中的代码从而执行。监听器共三种八类,分别监听ServletContext、HttpSession、ServletRequest三大域对象。