Listener采用了观察者模式（24种模式之一），Listener是servlet的监听器，他可以监听客户端的请求、服务器端的操作等， 通过监听器，可以自动激发一些操作。比如：监听在线用户数量
当增加一个HttpSession时，就会激发sessinCreated(HttpSessionEvent sce)方法，这样就可以给在线人数+1了。
常见的监听器接口：
ServletContextAttributeListener 监听对servletContext属性的操作，比如删除、增加、修改属性等
ServletContextListener 监听ServletContext，当创建ServletContext时，激发contextInitialized(ServletContextEvent sce)方法，当销毁ServletContext时，激发ContextDestory(ServletContextEvent sce)方法、
实例：
首先配置web.xml
<!--servlet 监听器 start--> <listener> <listener-class>com.listener.MyServletContextListener</listener-class> </listener> <listener> <listener-class>com.listener.MyServletContextAttributeListener</listener-class> </listener> <!-- servlet 监听器 end -->
2.ServletContextListener接口的调用
/**
* 在xml中配置监听器
* ServletContextListener 坚挺servletContext，当创建servletContext时，激发ContextInitialized(ServletContextEvent sce)方法
* 当销毁servletContext时，激发contextDestoryed(ServletContextEvent sce)方法
*/
package com.listener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class MyServletContextListener implements ServletContextListener
{
@Override
public void contextDestroyed(ServletContextEvent arg0)
{
System.out.println(arg0.toString());
}
@Override
public void contextInitialized(ServletContextEvent arg0)
{
System.out.println(arg0.toString());
}
}
3.ServletContextAttributeListener接口的调用
package com.listener;
import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
public class MyServletContextAttributeListener implements ServletContextAttributeListener
{
@Override
public void attributeAdded(ServletContextAttributeEvent arg0)
{
System.out.println("attributeAdd ");
System.out.println(arg0.getName() + ":" + arg0.getValue());
}
@Override
public void attributeRemoved(ServletContextAttributeEvent arg0)
{
System.out.println("attributeRemoved");
System.out.println(arg0.getName() + ":" + arg0.getValue());//value值将显示之前的 值
}
@Override
public void attributeReplaced(ServletContextAttributeEvent arg0)
{
System.out.println("attributeRepaced");
System.out.println(arg0.getName() + ":" + arg0.getValue());//value值将显示之前的 值
}
}
为了方便，没有用servlet举例，直接写的jsp
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%> <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Insert title here</title> </head> <body> <% application.setAttribute("name1","value1"); application.setAttribute("name2","value2"); application.setAttribute("name1","value3"); application.setAttribute("name2","value4"); %> <a href="listener2.jsp">next</a> </body> </html>
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%> <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>Insert title here</title> </head> <body> <% application.removeAttribute("name1"); %> </body> </html>
结束。listener在实际项目开发中，用到的不多。这里最好知道有这么回事就成。