本文实例讲述了Java基于servlet监听器实现在线人数监控功能的方法。分享给大家供大家参考，具体如下：
1、分析：
做一个网站在线人数统计，可以通过ServletContextListener监听，当Web应用上下文启动时，在ServletContext中添加一个List.用来准备存放在线的用户名，然后通过HttpSessionAttributeListener监听，当用户登录成功，把用户名设置到Session中。同时将用户名方法到ServletContext的List中，最后通过HttpSessionListener监听，当用户注销会话时，讲用户名从应用上下文范围中的List列表中删除。
2、注意事项
测试时，需要启动不同的浏览器来登陆不同的用户，只有点击注销按钮才能减少在线用户，关闭浏览器不能减少在线用户。
3、项目源代码
（1）java代码
OnlineListener类
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;
//应用上下文初始时会回调的方法
@Override
public void contextInitialized(ServletContextEvent e) {
//初始化一个application对象
application = e.getServletContext();
//设置一个列表属性，用于保存在线用户名
this.application.setAttribute("online", new LinkedList<String>());
}
//往会话中添加属性时的回调方法
@Override
public void attributeAdded(HttpSessionBindingEvent e) {
//取得用户名列表
List<String> onlines = (List<String>) this.application.getAttribute("online");
if("username".equals(e.getName())){
onlines.add((String) e.getValue());
}
//将添加后的列表重新设置列application属性中.
this.application.setAttribute("online", onlines);
}
//会话销毁时会回调的方法
@Override
public void sessionDestroyed(HttpSessionEvent e) {
//取得用户名列表
List<String> onlines = (List<String>) this.application.getAttribute("online");
//取得当前用户名
String username = (String) e.getSession().getAttribute("username");
//将此用户从列表中删除
onlines.remove(username);
//讲删除后的列表重新设置到application属性中.
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类
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"); //设置响应内容类型
String username= request.getParameter("username"); //获取请求参数中的用户名
//往session中添加属性,会触发HttpSessionAttributeListener中的attributeAdded方法
if(username != null && !username.equals("")) {
request.getSession().setAttribute("username",username);
}
//从应用上下文中获取在线用户名列表
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>用户列表</title>");
out.println(" ");
out.println("当前用户是：" + username);
out.print(" <hr><h3>在线用户列表</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: 要对链接URL进行自动重写处理
out.println("<hr/><a href=\"" + response.encodeURL("logoutListener") + "\">注销</a>");
out.println(" ");
out.println("");
out.flush();
out.close();
}
}
LogoutServlet类
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"); //设置响应内容类型
//销毁会话,会触发SessionLinstener中的sessionDestroyed方法
request.getSession().invalidate();
//从应用上下文中获取在线用户名列表
List<String> online = (List<String>)getServletContext().getAttribute("online");
response.setContentType("text/html;charset=utf-8");
PrintWriter out = response.getWriter();
out.println("");
out.println(" <title>用户列表</title>");
out.println(" ");
out.print(" <h3>在线用户列表</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\''>主页</a>");
out.println(" ");
out.println("");
out.flush();
out.close();
}
}
（2）web.xml代码
<?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）表现层代码
<!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">
用户名：<input type="text" name="username">
<input type="submit" value="登录"><br><br>
</form>
</body>
</html>
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