监听器Listener -，钝化，活化，利用监听器实现简易版统计网站在线用户功能

监听绑定到HttpSession域中某个对象状态

1.HttpSessionBindingListener

实体类

package com.etime.enetity;

import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;

//此处需要实现HttpSessionBindingListener接口，并重新里面方法
public class Student implements HttpSessionBindingListener{
    private String name;
    private int age;

    public Student() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    @Override
    public void valueBound(HttpSessionBindingEvent event) {
        System.out.println("该"+event.getValue()+"学生被session绑定了");
    }

    @Override
    public void valueUnbound(HttpSessionBindingEvent event) {
        System.out.println("该"+event.getValue()+"学生被session解绑了");
    }
}

<%@ page import="com.etime.enetity.Student" %><%--
  Created by IntelliJ IDEA.
  User: xx
  Date: 2023/9/2
  Time: 20:01
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%
    Student student1 = new Student("张三", 23);
    Student student2 = new Student("李四", 21);
    Student student3 = new Student("zh", 21);
    session.setAttribute("stu1",student1);
    session.setAttribute("stu2",student2);
    session.setAttribute("stu1",student3);
    session.removeAttribute("stu1");
    session.removeAttribute("stu2");
%>
</body>
</html>

2.. HttpSessionActivationListener

package com.etime.enetity;

import javax.servlet.http.HttpSessionActivationListener;
import javax.servlet.http.HttpSessionEvent;
import java.io.Serializable;

//实现HttpSessionActivationListener和Serializable接口
public class Teacher implements HttpSessionActivationListener, Serializable {
    private String name;
    private int age;

    public Teacher() {
    }

    public Teacher(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Teacher{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    @Override
    public void sessionWillPassivate(HttpSessionEvent se) {
        System.out.println("钝化了");
    }

    @Override
    public void sessionDidActivate(HttpSessionEvent se) {
        System.out.println("活化了");
    }
}

<%@ page import="com.etime.enetity.Teacher" %><%--
  Created by IntelliJ IDEA.
  User: xx
  Date: 2023/9/2
  Time: 20:20
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%--测试钝化，访问此页面，然后关闭服务器，查看是否保存在文件中--%>
<%
    Teacher teacher = new Teacher("张三", 23);
    session.setAttribute("teacher",teacher);
%>
</body>
</html>

<%--
  Created by IntelliJ IDEA.
  User: xx
  Date: 2023/9/2
  Time: 20:20
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%--测试活化，再次重启服务其然后直接访问此页面，观察数据是否还存在--%>
${teacher}
</body>
</html>

统计网站在线用户功能

思路分析：通过ServletContextListener监听，当Web应用上下文启动时，在ServletContext中添加一个List集合，用来准备存放在线的用户名；然后，可以通过HttpSessionAttributeListener监听器，当用户登录成功，把用户名设置到Session中时，同时将用户名存放到ServletContext中的 List列表中；最后通过HttpSessionListener监听，当用户注销会话时，将用户名从应用上下文范围中的List列表中删除。

登陆界面

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<form action="LoginServlet.ser" method="post">
    用户名：<input type="text" name="username" ><br>
    密码：<input type="password" name="password" ><br>
    <input type="submit" value="登陆">
</form>
</body>
</html>

主页面

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
  $END$<a href="OutLoginServlet.ser" >退出登陆</a>
  欢迎您，${sessionScope.username}
  现在访问本网站的用户有：
  <c:forEach var="str" items="${list}">
    ${str}
  </c:forEach>
  </body>
</html>

servlet

@WebServlet(name = "LoginServlet", value = "/LoginServlet.ser")
public class LoginServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("username");
        String password = request.getParameter("password");

        boolean flag=true;
        if (flag){
            //登陆成功
            HttpSession session = request.getSession();
            System.out.println(username);
            session.setAttribute("username",username);
            request.getRequestDispatcher("index.jsp").forward(request,response);
        }else {
            //登陆失败
        }
    }
}

退出登陆

@WebServlet(name = "OutLoginServlet", value = "/OutLoginServlet.ser")
public class OutLoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession();
        session.invalidate();
        response.sendRedirect("login.jsp");
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

监听器


@WebListener
public class UserLoginListener implements ServletContextListener, HttpSessionAttributeListener, HttpSessionListener {
    ServletContext servletContext = null;

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        //服务器启动时就创建一个集合，存储访问本网站的用户
        List<String> list = new LinkedList<>();
        //获取ServletContext对象
        servletContext = sce.getServletContext();
        //将集合放到ServletContext对象中
        servletContext.setAttribute("list", list);
    }

    @Override
    public void attributeAdded(HttpSessionBindingEvent se) {
        //用户登陆时将用户名放入集合中
        //获取session对象
        HttpSession session = se.getSession();
        //获取KEY
        if ("username".equals(se.getName())) {
            //放入session中名字为username的才存入集合中
            //获取用户名
            String username = (String) se.getValue();
            //String username = (String)session.getAttribute("username");
            //获取集合
            List<String> list =(List<String>) servletContext.getAttribute("list");
            //将数据存入集合中
            list.add(username);
            //最后再将集合放回去
            servletContext.setAttribute("list",list);
        }
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        //用户退出时，就是session对象的销毁，将退出用户从集合中移出去
        //获取session对象
        HttpSession session = se.getSession();
        String username =(String) session.getAttribute("username");
        //从集合中移除掉
        List<String> list =(List<String>) servletContext.getAttribute("list");
        list.remove(username);
        servletContext.setAttribute("list",list);
    }
}

过滤器---解决中文乱码问题-和未登录不能访问主页面

@WebFilter("*.ser")
public class ChainFilter implements javax.servlet.Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("init");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        //处理中文乱码
        servletRequest.setCharacterEncoding("utf-8");
        servletResponse.setContentType("text/html;charset=utf-8");
        //放行
        filterChain.doFilter(servletRequest, servletResponse);
    }

    @Override
    public void destroy() {
        System.out.println("destroy");
    }
}

@WebFilter("/index.jsp")
public class indexFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest)servletRequest;
        HttpServletResponse response = (HttpServletResponse)servletResponse;
        HttpSession session = request.getSession();
        Object username = session.getAttribute("username");
        if (username==null){
            response.sendRedirect("login.jsp");
        }else {
            filterChain.doFilter(servletRequest, servletResponse);
        }
    }
}