Listener Servlet 的运用

Listener是Servlet的监听器，它可以监听客户端的请求、服务端的操作等。通过监听器，可以自动激发一些操作，比如监听在线的用户的数量。当增加一个HttpSession时，就激发sessionCreated(HttpSessionEvent se)方法，这样就可以给在线人数加1。常用的监听接口有以下几个：

ServletContextAttributeListener监听对ServletContext属性的操作，比如增加、删除、修改属性。

ServletContextListener监听ServletContext。当创建ServletContext时，激发contextInitialized(ServletContextEvent sce)方法；当销毁ServletContext时，激发contextDestroyed(ServletContextEvent sce)方法。

HttpSessionListener监听HttpSession的操作。当创建一个Session时，激发session Created(HttpSessionEvent se)方法；当销毁一个Session时，激发sessionDestroyed (HttpSessionEvent se)方法。

HttpSessionAttributeListener监听HttpSession中的属性的操作。当在Session增加一个属性时，激发attributeAdded(HttpSessionBindingEvent se) 方法；当在Session删除一个属性时，激发attributeRemoved(HttpSessionBindingEvent se)方法；当在Session属性被重新设置时，激发attributeReplaced(HttpSessionBindingEvent se) 方法。

package com.xling.tools.application;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;

public class AppListener
    extends HttpServlet implements ServletContextListener,
    ServletContextAttributeListener, HttpSessionListener,
    HttpSessionAttributeListener, ServletRequestListener,
    ServletRequestAttributeListener {
  //Notification that the web module is ready to process requests
  int userCount = 0;
  public void contextInitialized(ServletContextEvent sce) {   
    System.out.println("Application Run!");
  }

  //Notification that the servlet context is about to be shut down
  public void contextDestroyed(ServletContextEvent sce) {
    System.out.println("Application ended!");
  }

  //Notification that a new attribute has been added to the servlet context
  public void attributeAdded(ServletContextAttributeEvent scab) {

  }

  //Notification that an attribute has been removed from the servlet context
  public void attributeRemoved(ServletContextAttributeEvent scab) {

  }

  //Notification that an attribute of the servlet context has been replaced
  public void attributeReplaced(ServletContextAttributeEvent scab) {

  }

  //Notification that a session was created
  public void sessionCreated(HttpSessionEvent se) {
    userCount ++;
    System.out.println(userCount + " users on line!");
  }

  //Notification that a session was invalidated
  public void sessionDestroyed(HttpSessionEvent se) {
    userCount --;
    System.out.println(userCount + " users on line");
  }

  //Notification that a new attribute has been added to a session
  public void attributeAdded(HttpSessionBindingEvent se) {
    
  }

  //Notification that an attribute has been removed from a session
  public void attributeRemoved(HttpSessionBindingEvent se) {

  }

  //Notification that an attribute of a session has been replaced
  public void attributeReplaced(HttpSessionBindingEvent se) {
    System.out.println(se.getName() + " : " + se.getValue() + "  " + se.getSession().getAttribute(se.getName()));
  }

  //Notification that a request was created
  public void requestInitialized(ServletRequestEvent sre) {

  }

  //Notification that a request was invalidated
  public void requestDestroyed(ServletRequestEvent sre) {

  }

  //Notification that a new attribute has been added to a request
  public void attributeAdded(ServletRequestAttributeEvent srae) {

  }

  //Notification that an attribute has been removed from a request
  public void attributeRemoved(ServletRequestAttributeEvent srae) {

  }

  //Notification that an attribute of a request has been replaced
  public void attributeReplaced(ServletRequestAttributeEvent srae) {

  }
}