web的监听器，你需要知道这些...

浅浅的花香味﹌ 2022-07-13 01:24 274阅读 0赞

### 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">
    	
    
      </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">
    	
    
      </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。

![SouthEast][]  
扫码关注公众号有惊喜

**（转载本站文章请注明作者和出处 [方志朋的博客][Link 1]）**

[SouthEast]: /images/20220713/047ea6f541464d25b524350505aeb5ef.png
[Link 1]: https://www.fangzhipeng.com