一、简介
Listener是Servlet规范的另一个高级特性，它用于监听java web程序的事件，例如创建、修改、删除session,request,context等，并触发相应的处理事件，这个处理事件是由web容器回掉的。
学过安卓开发的同学一定很熟悉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件。
二、Listerner的分类
Listerner分为八种，前三种是用于监听对象的创建和销毁，中间三种用于监听对象属性的变化，后两种用于监听Session内对象。
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httpSessionListner: 监听session的创建与销毁，用于收集在线用户信息。
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servletContextListener:监听context的创建与销毁，context代表当前web应用，该listener可用于启动时获取web.xml的初始化参数。
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servletRequestListener: 监听request 的创建与销毁。
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httpSessionAttributeListener 监听session的种属性变化
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ServletContextAttributeListener
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ServletRequestAttributeListener
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HttpSessionBindingListener,监听对象存入或者移除 session
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httpSessionActivationListener,钝化和重新加载 session的监听
三、监听session、request、servletContext
直接上代码，下面监听了这三个对象创建销毁。
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("time");
log.info("请求处理时间"+time);
}
public void requestInitialized(ServletRequestEvent sre) {
HttpServletRequest request=(HttpServletRequest) sre.getServletRequest();
String uri=request.getRequestURI();
uri=request.getQueryString()==null?uri:(uri+"?"+request.getQueryString());
log.info("ip"+request.getRemoteAddr()+uri);
request.setAttribute("time", System.currentTimeMillis());
}
public void contextDestroyed(ServletContextEvent sce) {
ServletContext servletContext=sce.getServletContext();
log.info("关闭："+servletContext.getContextPath());
}
public void contextInitialized(ServletContextEvent sce) {
ServletContext servletContext=sce.getServletContext();
log.info("启动："+servletContext.getContextPath());
}
public void sessionCreated(HttpSessionEvent se) {
HttpSession session=se.getSession();
log.info("创建：session:"+session.getId());
}
public void sessionDestroyed(HttpSessionEvent se) {
HttpSession session=se.getSession();
log.info("销毁建：session:"+session.getId());
}
}
需要在web.xml中配置：
<listener>
<listener-class>com.forezp.listener.ListenerTest</listener-class>
</listener>
四、监听对象属性的变化
- httpSessionAttributeListener 监听session的种属性变化
- ServletContextAttributeListener
- ServletRequestAttributeListener
以上三种方法用于监听session ,context,request的属性发生变化，例如添加、更新、移除。
下面以session的属性变化为例子：
public class SessionAttributeListener implements HttpSessionAttributeListener{
Log log=LogFactory.getLog(getClass());
public void attributeAdded(HttpSessionBindingEvent se) {
HttpSession httpSession=se.getSession();
log.info("新建属性："+se.getName()+"值："+se.getValue());
}
public void attributeRemoved(HttpSessionBindingEvent se) {
HttpSession httpSession=se.getSession();
log.info(" 删除属性："+se.getName()+"值："+se.getValue());
}
public void attributeReplaced(HttpSessionBindingEvent se) {
HttpSession httpSession=se.getSession();
log.info(" 修改属性："+se.getName()+"原来的值："+se.getValue()+"新值："+httpSession.getAttribute(se.getName()));
}
}
web.xml配置，此处省略。
五、监听session内的对象
- HttpSessionBindingListener,当对象被放到session里执行valueBond();当对象被移除，执行valueUnbond();
- httpSessionActivationListener,服务器关闭,会将session的内容保存在硬盘里，这个过程叫钝化；服务器重启，会将session的内容从硬盘中重新加载。钝化时执行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("valueBound Name:"+httpsessionbindingevent.getName());
}
public void valueUnbound(HttpSessionBindingEvent httpsessionbindingevent) {
System.out.println("valueUnbound Name:"+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("sessionWillPassivate "+httpsessionevent.getSource());
}
//活化
public void sessionDidActivate(HttpSessionEvent httpsessionevent) {
System.out.println("sessionDidActivate "+httpsessionevent.getSource());
}
}
init.jsp
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
request.getSession().setAttribute("currentUser", new com.forezp.entity.User());
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'init.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
这是初始化值的界面
<button onclick="location.href='<%=request.getContextPath()%>/init.jsp';">Init</button>
<button onclick="location.href='<%=request.getContextPath()%>/destory.jsp';">Destory</button>
</body>
</html>
destroy.jsp
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
request.getSession().removeAttribute("currentUser");
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'destory.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
这是销毁界面
<button onclick="location.href='<%=request.getContextPath()%>/init.jsp';">Init</button>
<button onclick="location.href='<%=request.getContextPath()%>/destory.jsp';">Destory</button>
</body>
</html>
当访问init.jsp，再访问destroy.jsp;再访问init,jsp，再关闭服务器，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：
@WebListener
public class MyHttpSessionListener implements HttpSessionListener {
private int userNumber = 0;
@Override
public void sessionCreated(HttpSessionEvent arg0) {
userNumber++;
arg0.getSession().getServletContext().setAttribute("userNumber", userNumber);
}
@Override
public void sessionDestroyed(HttpSessionEvent arg0) {
userNumber--;
arg0.getSession().getServletContext().setAttribute("userNumber", 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，如果你需要知道这些人来自哪里，需要配合httpRequestListener配合，也可以实现单登陆，在这里不写代码了。
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