web监听器

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。

  • httpSessionListner: 监听session的创建与销毁,用于收集在线用户信息。

  • 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。

  • servletRequestListener: 监听request 的创建与销毁。

  • httpSessionAttributeListener 监听session的种属性变化

  • ServletContextAttributeListener

  • ServletRequestAttributeListener

  • HttpSessionBindingListener,监听对象存入或者移除 session

  • 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。

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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:

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<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:

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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:

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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

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<%@ 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

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<%@ 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:

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@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:

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<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。

请我吃🍗