　　Web容器管理Servlet/JSP相关的生命周期，若对HttpServletRequest对象、HttpSession对象、ServletContxt对象在生成、销毁或相关属性设置发生的时机点有兴趣，可以实现对应的监听器(Listener)。

一、ServletContext事件、监听器

与ServletContext相关的监听器有两个，ServletContextListener、ServletContextAttributeListener

1、ServletContextListener

它是生命周期监听器，如果想要知道何时web应用程序已经初始化或者即将结束销毁，可以实现该监听器。

例如，可以实现ServletContextListener，在此应用程序初始过程中，准备好数据库连接线对象、读取应用程序设置等动作，如放置使用头像的目录信息，就不适合将目录名称写死，这时候可以这么做：

 1 package cc.openhome;
 2 import javax.servlet.*;
 3 @WebListener  //使用@weblistener标注
 4 public class ContextParameterReader implements ServletContextListener{
 5        public void contextInitialized(ServletContextEvent e){
 6            ServletContext context = sce.getServletContext();//取得ServletContext
 7            String avatars = context.getInitParameter("AVATAR");//取得初始参数
 8            context.setAttribute("avatars",avatars);//设置ServletContext属性
 9 }
10       public void contextDestroyed(ServletContextEvent sce){}
11 }

　　整个web应用程序生命周期，Servlet共享的资料可以设置为ServletContext属性。这些属性数据会一直存在于web应用程序中。可以通过setAttribute()方法设置对象为ServletContext属性，之后可通过ServletContext的getAttribute()方法取出该属性。如果移除属性，用removeAttribute()。

 1 package ServletAPI;
 2 
 3 import java.io.IOException;
 4 import java.io.PrintWriter;
 5 
 6 import javax.servlet.ServletException;
 7 import javax.servlet.annotation.WebInitParam;
 8 import javax.servlet.annotation.WebServlet;
 9 import javax.servlet.http.HttpServlet;
10 import javax.servlet.http.HttpServletRequest;
11 import javax.servlet.http.HttpServletResponse;
12 
13 /**
14  * Servlet implementation class Avatar
15  */
16 @WebServlet(
17         urlPatterns = { "/avatar.view" }, 
18         initParams = { 
19                 @WebInitParam(name = "AVATAR_DIR", value = "/image")//根目录WebContent内部
20         })
21 public class Avatar extends HttpServlet {
22     private static final long serialVersionUID = 1L;
23     private String AVATAR_DIR;
24     /**
25      * @see HttpServlet#HttpServlet()
26      */
27     public Avatar() {
28         super();
29         // TODO Auto-generated constructor stub
30     }
31     public void init() throws ServletException{
32         //AVATAR_DIR=getInitParameter("AVATAR_DIR");
33         AVATAR_DIR=(String) getServletContext().getAttribute("avatars");
34     }
35     
36 
37     /**
38      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
39      */
40     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
41         // TODO Auto-generated method stub
42         response.setContentType("text/html;charset=UTF-8");
43         PrintWriter out=response.getWriter();
44         out.println("<!DOCTYPE html>");
45         out.println("<html>");
46         out.println("<head>");
47         out.println("<meta content='text/html; charset=UTF-8' http-equiv='content-type'>");
48         out.println("<title>头像显示</title>");
49         out.println("</head>");
50         out.println("<body>");
51         for(String avatar:getServletContext().getResourcePaths(AVATAR_DIR)){//取得头像路径
52             avatar=avatar.replaceFirst("/", "");
53             out.println("<img src='"+avatar+"'>");
54         }
55         out.println("</body>");
56         out.println("</html>");
57         out.close();
58     }
59 
60     /**
61      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
62      */
63     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
64         // TODO Auto-generated method stub
65     }
66 
67 }

avatar.java

2、ServletContextAttributeListener

它是监听属性改变的监听器，如果想要对象被设置、移除或替换ServletContext属性，也可以收到通知以进行一些操作，可以实现该监听器。

当在ServletContext中添加属性、移除或者替换属性时，对应的attributeAdded()、attributeRemoved()、attributeReplaced()方法就会被调用。

二、HttpSession事件、监听器

与HttpSession相关的有四个监听器：HttpSessionListener、HttpSessionAttributeListener、HttpSessionBindingListener与HttpSessionActivationListener.

1、HttpSessionListener

它是生命周期监听器，如果想要在HttpSession对象创建或者结束时，做些相对应动作，可以实现该监听。

在HttpSession对象初始化或结束之前，会分别调用sessionCreated()、sessionDestroyed()方法，

可以通过传入的HttpSessionEvent，使用getSession()取得HttpSession，以针对会话对象做出相应操作。例如：除非用户注销，否则登录后的用户在另个浏览器上登录会显示登录失败。

如果用户不小心再注销前关闭了浏览器，那么数据库中代表登录与否的字段就不会被重置。为此，可是实现HttpSessionListener，由于HttpSession有存活期限，当容器销毁某个HttpSession时，就会调用sessionDestroyed(),就可以再当中判断要重置哪个用户的数据库中代表登录与否的字段。

例1：显示目前已经在线人数的统计。

 1 package ServletAPI;
 2 
 3 import java.io.IOException;
 4 import java.io.PrintWriter;
 5 import javax.servlet.ServletException;
 6 import javax.servlet.annotation.WebServlet;
 7 import javax.servlet.http.HttpServlet;
 8 import javax.servlet.http.HttpServletRequest;
 9 import javax.servlet.http.HttpServletResponse;
10 import javax.servlet.http.HttpSession;
11 
12 /**
13  * Servlet implementation class Welcome
14  */
15 @WebServlet("/welcome.view")
16 public class Welcome extends HttpServlet {
17     private static final long serialVersionUID = 1L;
18 
19     /**
20      * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
21      */
22     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
23         // TODO Auto-generated method stub
24         response.setContentType("text/html;charset=UTF-8");
25         PrintWriter out=response.getWriter();
26         HttpSession session=request.getSession(true);
27         out.println("<html>");
28         out.println("<head>");
29         out.println("<title>欢迎</title>");
30         out.println("</head>");
31         out.println("<body>");
32         out.println("<h1>目前在线人数"+OnlineUserCounter.getCounter()+"人</h1>");//获取在线人数
33         if(session!=null){
34             String user=(String) session.getAttribute("user");
35             out.println("<h1>欢迎："+user+"</h1>");
36             out.println("<a href='logout.do'>注销</a>");
37         }
38         out.println("</body>");
39         out.println("</html>");
40     }
41 
42 }

Welcome.java

 1 package ServletAPI;
 2 
 3 import javax.servlet.annotation.WebListener;
 4 
 5 import javax.servlet.http.HttpSessionEvent;
 6 import javax.servlet.http.HttpSessionListener;
 7 
 8 /**
 9  * Application Lifecycle Listener implementation class OnlineUserCounter
10  *
11  */
12 @WebListener()
13 public class OnlineUserCounter implements HttpSessionListener {
14     private static int counter;
15 
16     public static int getCounter(){
17         return counter;
18     }
19 
20     /**
21      * @see HttpSessionListener#sessionCreated(HttpSessionEvent)
22      */
23     public void sessionCreated(HttpSessionEvent se) {
24         // TODO Auto-generated method stub
25         OnlineUserCounter.counter++;
26     }
27 
28     /**
29      * @see HttpSessionListener#sessionDestroyed(HttpSessionEvent)
30      */
31     public void sessionDestroyed(HttpSessionEvent se) {
32         // TODO Auto-generated method stub
33         OnlineUserCounter.counter--;
34     }
35     
36 }

OnlineUserCounter.java

 1 package ServletAPI;
 2 
 3 import java.io.IOException;
 4 import java.util.HashMap;
 5 import java.util.Map;
 6 
 7 import javax.print.attribute.standard.PagesPerMinute;
 8 import javax.servlet.ServletException;
 9 import javax.servlet.annotation.WebServlet;
10 import javax.servlet.http.HttpServlet;
11 import javax.servlet.http.HttpServletRequest;
12 import javax.servlet.http.HttpServletResponse;
13 
14 import org.apache.catalina.User;
15 
16 /**
17  * Servlet implementation class loginlistener
18  */
19 @WebServlet("/listener.view")
20 public class loginlistener extends HttpServlet {
21     private static final long serialVersionUID = 1L;
22     private Map<String, String> users;
23     /**
24      * @see HttpServlet#HttpServlet()
25      */
26     public loginlistener() {
27         //super();
28         users=new HashMap<String, String>();
29         users.put("nihao", "nihaonihao");
30         users.put("momor", "123456789");
31         users.put("hamimi", "123456789");
32         // TODO Auto-generated constructor stub
33     }
34 
35     /**
36      * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
37      */
38     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
39         // TODO Auto-generated method stub
40         String name=request.getParameter("user");
41         String passwd=request.getParameter("passwd");
42         String page="listenerlogin.html";
43         if(users.containsKey(name) && users.get(name).equals(passwd)){
44             User user=(User) new ServletAPI.User(name);
45             request.getSession().setAttribute("user", user);        
46             page="welcome.view";
47         }
48         response.sendRedirect(page);
49     }
50 
51 }