Java中的过滤器和监听器

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过滤器

一个过滤器可以附加到一个或者多个Servlet上，一个Servlet可以附加一个或者多个Filter。一个过滤器必须实现javax.servlet.Filter接口并实现其中的三个方法。

  • init()方法：

    这个方法在实例化过滤器时被调用，主要用于使过滤器为处理做准备。容器为这个方法传递一个FilterConfig对象，其中包含配置信息。

  • doFilter()：

    这个方法用于处理请求和相应。它接受3个输入参数：ServletRequest、ServletReponse、FilterChain对象。FilterChain对于正确的过滤操作至关重要。doFilter()方法必须调用FilterChain的doFilter()方法，除非该方法用来拦截以后的下游处理

  • destroy()：

    该方法由容器在销毁过滤器实例之前调用。

    package com.tz.filter;
import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
public class EncodingFilter implements Filter{
	String encoding = null;
	FilterConfig config = null;
	@Override
	public void destroy() {
		// TODO Auto-generated method stub
		this.config=null;
		this.encoding=null;
	}
	@Override
	public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {
		// TODO Auto-generated method stub
		if(encoding!=null){
			//对请求进行编码设置
			request.setCharacterEncoding(encoding);
		}
		chain.doFilter(request, response);
	}
	@Override
	public void init(FilterConfig config) throws ServletException {
		// TODO Auto-generated method stub
		this.config = config;
		this.encoding = config.getInitParameter("encoding");
	}
	
}

    web.xml配置：

    <?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
		 xmlns="http://java.sun.com/xml/ns/javaee" 
		 xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
		 					 http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" 
		 					 id="WebApp_ID" version="3.0">
  <display-name>test</display-name>
  <filter>
  	<filter-name>encodingFilter</filter-name>
  	<filter-class>com.tz.filter.EncodingFilter</filter-class>
  	<init-param>
  		<param-name>encoding</param-name>
  		<param-value>utf-8</param-value>
  	</init-param>
  </filter>
  <filter-mapping>
  	<filter-name>encodingFilter</filter-name>
  	<url-pattern>/*</url-pattern>
  </filter-mapping>
  
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>

    Jsp文件：

    <%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML>
<html>
  <head>
    <title>编码过滤器测试</title>
	<meta http-equiv="content-type" content="text/html;charset=utf-8">
  </head>
  
  <body>
    <form action="process.jsp" name="form1" method="post">
    	姓名：<input type="text" name="userName" />
    	<input type="submit" value="提交" name="submit">
    </form>
  </body>
</html>

    process.jsp：

    <%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML>
<html>
  <head>
    <title>编码过滤器测试</title>
	<meta http-equiv="content-type" content="text/html;charset=utf-8">
  </head>
  
  <body>
    <p><%=request.getParameter("userName") %>,你好！</p>
    <%=request.getCharacterEncoding() %>
  </body>
</html>

监听器

在定义监听器类时，只需要实现相应的监听器接口即可。如实现了 ServletContextListener 和ServletContextAttributeListener的类就可以监听到ServletContext对象的生命周期变化情况和ServletContext对象中属性的变化情况。实现了HttpSessionListener和HttpServletAttributeListener接口的类就可以监听到HttpSession对象的生命周期变化和HttpSession对象中的属性变化。

package com.tz.listener;

import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class ContextListenerTest implements ServletContextListener,ServletContextAttributeListener{

	//当应用被移除到容器时触发
	@Override
	public void contextDestroyed(ServletContextEvent arg0) {
		System.out.println("监听到应用被移除。。。。。。");
	}

	//当应用在容器中启动时触发
	@Override
	public void contextInitialized(ServletContextEvent arg0) {
		System.out.println("监听到应用被启动。。。。。。");
	}

	//ServletContext中新增属性时执行此方法
	@Override
	public void attributeAdded(ServletContextAttributeEvent event) {
		System.out.println("监听到新增了一个叫"+event.getName()+"的属性！");
	}

	@Override
	public void attributeRemoved(ServletContextAttributeEvent arg0) {
		System.out.println("removed......");
	}

	@Override
	public void attributeReplaced(ServletContextAttributeEvent arg0) {
		System.out.println("replaced......");
	}

}

package com.tz.listener;

import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class SessionListenerTest implements HttpSessionListener,HttpSessionAttributeListener{
	
	//在线人数统计
	int count = 0;
	
	//当产生一个新的HttpSession对象时(新用户上线)执行此方法
	@Override
	public void sessionCreated(HttpSessionEvent event) {
		event.getSession().getServletContext().setAttribute("count",++count);
		System.out.println("监听到新用户"+event.getSession().getId()+"上线了！");
	}
	
	//当一个HttpSession销毁(用户下线)时执行此方法
	@Override
	public void sessionDestroyed(HttpSessionEvent event) {
		event.getSession().getServletContext().setAttribute("count",--count);
		System.out.println("监听到新用户"+event.getSession().getId()+"下线了！");
	}
	
	@Override
	public void attributeAdded(HttpSessionBindingEvent event) {
		System.out.println("监听到"+ event.getName()+"属性新增了,属性值是"+event.getValue());
	}

	@Override
	public void attributeRemoved(HttpSessionBindingEvent event) {
		// TODO Auto-generated method stub
		System.out.println("监听到"+ event.getName()+"属性被删除了");
		
	}

	@Override
	public void attributeReplaced(HttpSessionBindingEvent event) {
		// TODO Auto-generated method stub
		System.out.println("监听到"+ event.getName()+"属性被修改了");
		
	}
	

}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
		 xmlns="http://java.sun.com/xml/ns/javaee" 
		 xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
		 					 http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" 
		 					 id="WebApp_ID" version="3.0">
  <display-name>test</display-name>
  
  <listener>
  	<listener-class>com.tz.listener.ContextListenerTest</listener-class>
  </listener>
  
  <listener>
  	<listener-class>com.tz.listener.SessionListenerTest</listener-class>
  </listener>
  
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>

jsp:

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML>
<html>
  <head>
    <title>编码过滤器测试</title>
	<meta http-equiv="content-type" content="text/html;charset=utf-8">
  </head>
  
  <body>
    
    <h2>Listener应用实例测试</h2>
    <%
    	application.setAttribute("admin", "liubin");
    	application.setAttribute("admin", "xiaohuang");
    	application.removeAttribute("admin");
    	session.setAttribute("now", new Date());
    	session.setAttribute("now", "2016-05-09");
    	session.removeAttribute("now");
     %>
            目前在线人数：<%=application.getAttribute("count") %>人！
     <a href="logout.jsp" style="list-style: none">注销</a>
  </body>
</html>

logout.jsp:

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML>
<html>
  <head>
    <title>Listener应用实例</title>
	<meta http-equiv="content-type" content="utf-8">
  </head>
  
  <body>
  	<h2>Listener应用实例</h2>
    <%
    	//销毁session对象
    	session.invalidate();
     %>
     	恭喜你注销成功！目前在线人数：<%=application.getAttribute("count") %>
  </body>
</html>
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