index.html：從用戶處獲取輸入。
MyListener.java：一個偵聽器類，用于計算當(dāng)前登錄的用戶數(shù)量，并將此信息作為屬性存儲在ServletContext對象中。
First.java：一個創(chuàng)建會話并打印登錄用戶數(shù)量的Servlet類。
Logout.java：一個使會話無效的Servlet類。

打開Eclipse，創(chuàng)建一個動態(tài)Web項目：HttpSessionEvent，其完整的目錄結(jié)構(gòu)如下所示 -

以下是這個項目中的幾個主要的代碼文件。

文件：index.html -

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>統(tǒng)計在線用戶數(shù)量</title>
</head>
<body>
    <div style="font-align: center;">
        <form action="first" method="post">
            用戶名:<input type="text" name="username" value="admin">密碼:<input
                type="password" name="password"><input
                type="submit" value="登錄" />
        </form>
    </div>
</body>
</html>

監(jiān)聽器文件：CountUserListener.java -

package com.yiibai;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class CountUserListener implements HttpSessionListener {
    ServletContext ctx = null;
    static int total = 0, current = 0;

    public void sessionCreated(HttpSessionEvent e) {
        total++;
        current++;

        ctx = e.getSession().getServletContext();
        ctx.setAttribute("totalusers", total);
        ctx.setAttribute("currentusers", current);

    }

    public void sessionDestroyed(HttpSessionEvent e) {
        current--;
        ctx.setAttribute("currentusers", current);
    }

}

文件：First.java -

package com.yiibai;

import java.io.IOException;
import java.io.PrintWriter;

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

public class FirstServlet extends HttpServlet {
    // 顯示用戶數(shù)
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        response.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=UTF-8");
        request.setCharacterEncoding("UTF-8");
        PrintWriter out = response.getWriter();

        // retrieving data from ServletContext object
        ServletContext ctx = getServletContext();
        int t = (Integer) ctx.getAttribute("totalusers");
        int c = (Integer) ctx.getAttribute("currentusers");
        out.print("<br>用戶總數(shù)： " + t);
        out.print("<br>當(dāng)前用戶數(shù)： " + c);
        out.print("<br><a href='logout'>注銷</a>");
        out.close();
    }

    // 執(zhí)行用戶登錄
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        response.setCharacterEncoding("UTF-8");
        response.setContentType("text/html;charset=UTF-8");
        request.setCharacterEncoding("UTF-8");
        PrintWriter out = response.getWriter();

        String n = request.getParameter("username");
        String password = request.getParameter("password");
        // 假設(shè)輸入密碼為：123456時，此用戶登錄成功
        if (password == null) {
            password = "";
        }
        if (password.equals("123456")) {
            out.print("您好, " + n);
            HttpSession session = request.getSession();
            session.setAttribute("uname", n);
        } else {
            out.print("登陸失敗 ~");
        }
        // retrieving data from ServletContext object
        ServletContext ctx = getServletContext();
        int t = (Integer) ctx.getAttribute("totalusers");
        int c = (Integer) ctx.getAttribute("currentusers");
        out.print("<br>用戶總數(shù)： " + t);
        out.print("<br>當(dāng)前用戶數(shù)： " + c);
        out.print("<br><a href='logout'>注銷</a>");
        out.close();
    }

}

文件：LogoutServlet.java -

package com.yiibai;

import java.io.IOException;
import java.io.PrintWriter;

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

public class LogoutServlet extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        HttpSession session = request.getSession(false);
        session.invalidate();// invalidating session

        out.print("You are successfully logged out");

        out.close();
    }

}

文件：web.xml -

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://xmlns.jcp.org/xml/ns/javaee"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
    id="WebApp_ID" version="3.1">
    <display-name>HttpSessionEvent</display-name>
    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
        <listener>
        <listener-class>com.yiibai.CountUserListener</listener-class>
    </listener>

    <servlet>
        <servlet-name>FirstServlet</servlet-name>
        <servlet-class>com.yiibai.FirstServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>FirstServlet</servlet-name>
        <url-pattern>/first</url-pattern>
    </servlet-mapping>    
    <servlet>
        <servlet-name>LogoutServlet</servlet-name>
        <servlet-class>com.yiibai.LogoutServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LogoutServlet</servlet-name>
        <url-pattern>/logout</url-pattern>
    </servlet-mapping>
</web-app>

在編寫上面代碼后，部署此Web應(yīng)用程序(在項目名稱上點(diǎn)擊右鍵->”Run On Server…”)，打開瀏覽器訪問URL： http://localhost:8080/HttpSessionEvent/ ，如果沒有錯誤，應(yīng)該會看到以下結(jié)果 -

打開另一個瀏覽器訪問URL： http://localhost:8080/HttpSessionEvent/ ，登錄應(yīng)用，然后注銷，再次訪問： http://localhost:8080/HttpSessionEvent/first ，應(yīng)該看到以下結(jié)果 -

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Servlet HttpSessionEvent統(tǒng)計在線用戶數(shù)實(shí)例

HttpSessionEvent和HttpSessionListener統(tǒng)計當(dāng)前登錄用戶數(shù)的示例