利用servlet的監聽器實現在線人數監控的功能

1、分析：

做一個網站在線人數統計，可以通過ServletContextListener監聽，當Web應用上下文啓動時，在ServletContext中添加一個List.用來準備存放在線的用戶名，然後通過HttpSessionAttributeListener監聽，當用戶登錄成功，把用戶名設置到Session中。同時將用戶名方法到ServletContext的List中，最後通過HttpSessionListener監聽，當用戶註銷會話時，講用戶名從應用上下文範圍中的List列表中刪除。

2、注意事項

測試時，需要啓動不同的瀏覽器來登陸不同的用戶，只有點擊註銷按鈕才能減少在線用戶，關閉瀏覽器不能減少在線用戶。

3、項目源代碼

（1）java代碼

OnlineListener類

package com.smalle.listener;

import java.util.LinkedList;
import java.util.List;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class OnlineListener implements 
	ServletContextListener, HttpSessionAttributeListener, HttpSessionListener {
	
	private ServletContext application = null;

	//應用上下文初始時會回調的方法
	@Override
    public void contextInitialized(ServletContextEvent e) {
        //初始化一個application對象
        application = e.getServletContext();
        //設置一個列表屬性，用於保存在線用戶名
        this.application.setAttribute("online", new LinkedList<String>());
    }
	
	//往會話中添加屬性時的回調方法
	@Override
    public void attributeAdded(HttpSessionBindingEvent e) {
        //取得用戶名列表
        List<String> onlines = (List<String>) this.application.getAttribute("online");
        if("username".equals(e.getName())){
            onlines.add((String) e.getValue());
        }
        //將添加後的列表重新設置列application屬性中.
        this.application.setAttribute("online", onlines);
    }

    //會話銷燬時會回調的方法
	@Override
    public void sessionDestroyed(HttpSessionEvent e) {
        //取得用戶名列表
        List<String> onlines = (List<String>) this.application.getAttribute("online");
        //取得當前用戶名
        String username = (String) e.getSession().getAttribute("username");
        //將此用戶從列表中刪除
        onlines.remove(username);
        //講刪除後的列表重新設置到application屬性中.
        this.application.setAttribute("online", onlines);
    }
	
	public void sessionCreated(HttpSessionEvent e) {}
	public void attributeRemoved(HttpSessionBindingEvent e) {}
	public void attributeReplaced(HttpSessionBindingEvent e) {}
	public void contextDestroyed(ServletContextEvent e) {}

}

LoginServlet類

package com.smalle.listener;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class LoginServlet extends HttpServlet {

	private static final long serialVersionUID = 1L;

	public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doPost(request, response);
    }
 
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
		request.setCharacterEncoding("utf-8");    //設置響應內容類型
		         
		        String username= request.getParameter("username");    //獲取請求參數中的用戶名
		 
		        //往session中添加屬性,會觸發HttpSessionAttributeListener中的attributeAdded方法
		        if(username != null && !username.equals("")) {  
		            request.getSession().setAttribute("username",username);    
		        }
		        //從應用上下文中獲取在線用戶名列表
		        List<String> online = (List<String>)getServletContext().getAttribute("online");  
System.out.println("LoginServlet" + online);
		        response.setContentType("text/html;charset=utf-8");
		        PrintWriter out = response.getWriter();
		        out.println("");
		        out.println("  <title>用戶列表</title>");
		        out.println("  ");
		        out.println("當前用戶是：" + username);
		        out.print("    <hr><h3>在線用戶列表</h3>");
		 
		        int size = online == null ? 0 : online.size();
		        for (int i = 0; i < size; i++) {
		            if(i > 0){
		                out.println("<br>");
		            }
		            out.println(i + 1 + "." + online.get(i));
		        }
		         
		        //注意: 要對鏈接URL進行自動重寫處理
		        out.println("<hr/><a href=\"" + response.encodeURL("logoutListener") + "\">註銷</a>");
		        out.println("  ");
		        out.println("");
		        out.flush();
		        out.close();
	}	
}

LogoutServlet類

package com.smalle.listener;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;
 
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class LogoutServlet extends HttpServlet{
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        this.doPost(request, response);
    }
 
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        request.setCharacterEncoding("utf-8");    //設置響應內容類型
         
        //銷燬會話,會觸發SessionLinstener中的sessionDestroyed方法
        request.getSession().invalidate();
         
        //從應用上下文中獲取在線用戶名列表
        List<String> online = (List<String>)getServletContext().getAttribute("online");    
        response.setContentType("text/html;charset=utf-8");
        PrintWriter out = response.getWriter();
        out.println("");
        out.println("  <title>用戶列表</title>");
        out.println("  ");
        out.print("    <h3>在線用戶列表</h3>");
 
        int size = online == null ? 0 : online.size();
        for (int i = 0; i < size; i++) {
            if(i > 0){
                out.println("<br>");
            }
            out.println(i + 1 + "." + online.get(i));
        }
         
        out.println("<hr><a href='\'index.html\''>主頁</a>");
        out.println("  ");
        out.println("");
        out.flush();
        out.close();
    }
}

（2）web.xml代碼

<?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>testServlet</display-name>
	
	<listener>
		<listener-class>com.smalle.listener.OnlineListener</listener-class>
	</listener>
	
	<servlet>
		<servlet-name>LoginServlet</servlet-name>
		<servlet-class>com.smalle.listener.LoginServlet</servlet-class>
	</servlet>
	<servlet-mapping>
		<servlet-name>LoginServlet</servlet-name>
		<url-pattern>/loginListener</url-pattern>
	</servlet-mapping>
	
	<servlet>
		<servlet-name>LogoutServlet</servlet-name>
		<servlet-class>com.smalle.listener.LogoutServlet</servlet-class>
	</servlet>
	<servlet-mapping>
		<servlet-name>LogoutServlet</servlet-name>
		<url-pattern>/logoutListener</url-pattern>
	</servlet-mapping>
		
	
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
</web-app>

（3）表現層代碼

<!DOCTYPE html>
<html>
  <head>
    <title>index.html</title>
    <meta name="content-type" content="text/html; charset=UTF-8">

  </head>
  
  <body>
    <form action="loginListener" method="post">
    	用戶名：<input type="text" name="username">
    <input type="submit" value="登錄"><br><br>
	</form>

  </body>
</html>