Listener是Servlet规范的另一个高等特性，它用于监听java web程序的事宜，例如创建、修正、删除session,request,context等，并触发相应的处理事宜，这个处理事宜是由web容器回掉的。
学过安卓开拓的同学一定很熟习view.setonClickLister();这样的对安卓控件的监听。java web也是这样的 ，根据不同的listner 和不同的event，可以完成相应的处理事宜。
二、Listerner的分类
Listerner分为八种，前三种是用于监听工具的创建和销毁，中间三种用于监听工具属性的变革，后两种用于监听Session内工具。
httpSessionListner: 监听session的创建与销毁，用于网络在线用户信息。servletContextListener:监听context的创建与销毁，context代表当前web运用，该listener可用于启动时获取web.xml的初始化参数。servletRequestListener: 监听request 的创建与销毁。httpSessionAttributeListener 监听session的种属性变革ServletContextAttributeListenerServletRequestAttributeListenerHttpSessionBindingListener,监听工具存入或者移除 sessionhttpSessionActivationListener,钝化和重新加载 session的监听三、监听session、request、servletContext
直接上代码，下面监听了这三个工具创建销毁。
public class ListenerTest implements HttpSessionListener ,ServletContextListener,ServletRequestListener{Log log=LogFactory.getLog(getClass());public void requestDestroyed(ServletRequestEvent sre) {HttpServletRequest request=(HttpServletRequest) sre.getServletRequest();long time=System.currentTimeMillis()-(Long)request.getAttribute(\公众time\"大众);log.info(\"大众要求处理韶光\公众+time);}public void requestInitialized(ServletRequestEvent sre) {HttpServletRequest request=(HttpServletRequest) sre.getServletRequest();String uri=request.getRequestURI();uri=request.getQueryString()==null?uri:(uri+\公众?\"大众+request.getQueryString());log.info(\"大众ip\公众+request.getRemoteAddr()+uri);request.setAttribute(\公众time\"大众, System.currentTimeMillis());}public void contextDestroyed(ServletContextEvent sce) {ServletContext servletContext=sce.getServletContext();log.info(\"大众关闭：\"大众+servletContext.getContextPath());}public void contextInitialized(ServletContextEvent sce) {ServletContext servletContext=sce.getServletContext();log.info(\"大众启动：\"大众+servletContext.getContextPath());}public void sessionCreated(HttpSessionEvent se) {HttpSession session=se.getSession();log.info(\"大众创建：session:\"大众+session.getId());}public void sessionDestroyed(HttpSessionEvent se) {HttpSession session=se.getSession();log.info(\公众销毁建：session:\"大众+session.getId());}}
须要在web.xml中配置：
<listener> <listener-class>com.forezp.listener.ListenerTest</listener-class> </listener>
四、监听工具属性的变革
httpSessionAttributeListener 监听session的种属性变革ServletContextAttributeListenerServletRequestAttributeListener以上三种方法用于监听session ,context,request的属性发生变革，例如添加、更新、移除。 下面以session的属性变革为例子：
public class SessionAttributeListener implements HttpSessionAttributeListener{Log log=LogFactory.getLog(getClass());public void attributeAdded(HttpSessionBindingEvent se) {HttpSession httpSession=se.getSession();log.info(\公众新建属性：\公众+se.getName()+\"大众值：\"大众+se.getValue());}public void attributeRemoved(HttpSessionBindingEvent se) {HttpSession httpSession=se.getSession();log.info(\"大众 删除属性：\"大众+se.getName()+\"大众值：\"大众+se.getValue());}public void attributeReplaced(HttpSessionBindingEvent se) {HttpSession httpSession=se.getSession();log.info(\"大众 修正属性：\"大众+se.getName()+\公众原来的值：\"大众+se.getValue()+\"大众新值：\"大众+httpSession.getAttribute(se.getName()));}}
web.xml配置，此处省略。
五、监听session内的工具
HttpSessionBindingListener,当工具被放到session里实行valueBond();当工具被移除，实行valueUnbond();httpSessionActivationListener,做事器关闭,会将session的内容保存在硬盘里，这个过程叫钝化；做事看重启，会将session的内容从硬盘中重新加载。钝化时实行sesionWillPassivate(),重新加载sessionDidActivate();举个例子：
public class User implements HttpSessionBindingListener,HttpSessionActivationListener,Serializable {
private String username;
private String password;
public void valueBound(HttpSessionBindingEvent httpsessionbindingevent) {
System.out.println(\"大众valueBound Name:\"大众+httpsessionbindingevent.getName());
}
public void valueUnbound(HttpSessionBindingEvent httpsessionbindingevent) {
System.out.println(\"大众valueUnbound Name:\公众+httpsessionbindingevent.getName());
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
//钝化
public void sessionWillPassivate(HttpSessionEvent httpsessionevent) {
System.out.println(\"大众sessionWillPassivate \"大众+httpsessionevent.getSource());
}
//活化
public void sessionDidActivate(HttpSessionEvent httpsessionevent) {
System.out.println(\公众sessionDidActivate \"大众+httpsessionevent.getSource());
}
}
init.jsp
<%@ page language=\"大众java\"大众 import=\"大众java.util.\公众 pageEncoding=\"大众UTF-8\公众%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+\公众://\"大众+request.getServerName()+\公众:\公众+request.getServerPort()+path+\"大众/\"大众;
request.getSession().setAttribute(\公众currentUser\"大众, new com.forezp.entity.User());
%>
<!DOCTYPE HTML PUBLIC \公众-//W3C//DTD HTML 4.01 Transitional//EN\"大众>
<html>
<head>
<base href=\"大众<%=basePath%>\"大众>
<title>My JSP 'init.jsp' starting page</title>
<meta http-equiv=\公众pragma\"大众 content=\"大众no-cache\公众>
<meta http-equiv=\"大众cache-control\公众 content=\"大众no-cache\公众>
<meta http-equiv=\公众expires\"大众 content=\"大众0\"大众>
<meta http-equiv=\公众keywords\公众 content=\"大众keyword1,keyword2,keyword3\"大众>
<meta http-equiv=\公众description\"大众 content=\"大众This is my page\公众>
<!--
<link rel=\"大众stylesheet\公众 type=\"大众text/css\"大众 href=\"大众styles.css\"大众>
-->
</head>
<body>
这是初始化值的界面
<button onclick=\"大众location.href='<%=request.getContextPath()%>/init.jsp';\"大众>Init</button>
<button onclick=\"大众location.href='<%=request.getContextPath()%>/destory.jsp';\"大众>Destory</button>
</body>
</html>
destroy.jsp
<%@ page language=\"大众java\"大众 import=\公众java.util.\"大众 pageEncoding=\"大众UTF-8\"大众%><%String path = request.getContextPath();String basePath = request.getScheme()+\"大众://\"大众+request.getServerName()+\"大众:\"大众+request.getServerPort()+path+\公众/\"大众;request.getSession().removeAttribute(\公众currentUser\"大众);%><!DOCTYPE HTML PUBLIC \公众-//W3C//DTD HTML 4.01 Transitional//EN\公众><html> <head> <base href=\"大众<%=basePath%>\"大众> <title>My JSP 'destory.jsp' starting page</title> <meta http-equiv=\"大众pragma\公众 content=\"大众no-cache\"大众><meta http-equiv=\"大众cache-control\"大众 content=\"大众no-cache\公众><meta http-equiv=\"大众expires\"大众 content=\公众0\公众> <meta http-equiv=\"大众keywords\公众 content=\"大众keyword1,keyword2,keyword3\"大众><meta http-equiv=\"大众description\"大众 content=\公众This is my page\"大众><!--<link rel=\"大众stylesheet\"大众 type=\公众text/css\"大众 href=\"大众styles.css\"大众>--> </head> <body> 这是销毁界面 <button onclick=\"大众location.href='<%=request.getContextPath()%>/init.jsp';\公众>Init</button> <button onclick=\"大众location.href='<%=request.getContextPath()%>/destory.jsp';\"大众>Destory</button> </body></html>
当访问init.jsp，再访问destroy.jsp;再访问init,jsp，再关闭做事器，重启；log日志如下：
valueBound Name:currentUser
valueUnbound Name:currentUser
sessionWillPassivate org.apache.catalina.session.StandardSessionFacade@33f3be1
sessionDidActivate org.apache.catalina.session.StandardSessionFacade@33f3be1
六、显示在线人数：
@WebListenerpublic class MyHttpSessionListener implements HttpSessionListener {private int userNumber = 0;@Overridepublic void sessionCreated(HttpSessionEvent arg0) {userNumber++;arg0.getSession().getServletContext().setAttribute(\"大众userNumber\公众, userNumber);}@Overridepublic void sessionDestroyed(HttpSessionEvent arg0) {userNumber--;arg0.getSession().getServletContext().setAttribute(\"大众userNumber\"大众, userNumber);}}
jsp中显示：
<body> 当前在线用户人数:${userNumber }<br/></body>
这是一个简答的统计在线人数的方法，如果你须要知道这些人来自哪里，须要合营httpRequestListener合营，也可以实现单上岸，在这里不写代码了。
