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。

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。