Filter(过滤器)与Listener(监听器)详解

11、Filter（重点）

Filter：过滤器，用来过滤网站的数据；

处理中文乱码
登陆验证...

Filter开发步骤：

导包

编写过滤器

导包不要错

实现Filter接口，重写对应的方法即可

public class CharacterEncodingFilter implements Filter {

    //初始化：web服务器启动，就已经初始化了，随时等待过滤对象出现！
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("CharacterEncodingFilter初始化");
    }

    //Chain : 链
    /*
    1. 过滤器中的所有代码，在过滤特定请求的时候都会执行
    2. 必须要让过滤器继续通行
        filterChain.doFilter(servletRequest,servletResponse);
     */
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        servletRequest.setCharacterEncoding("utf-8");
        servletResponse.setCharacterEncoding("utf-8");
        servletResponse.setContentType("text/html;charset=UTF-8");

        System.out.println("CharacterEncodingFilter执行前...");
        filterChain.doFilter(servletRequest,servletResponse); //让我们的请求继续走，如果不写，程序到这里就被拦截停止！
        System.out.println("CharacterEncodingFilter执行后...");
    }

    //销毁：web服务器关闭的时候，过滤器会销毁
    public void destroy() {
        System.out.println("CharacterEncodingFilter销毁");
    }
}

在web.xml中配置 Filter

<servlet>
    <servlet-name>ShowServlet</servlet-name>
    <servlet-class>com.kuang.servlet.ShowServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>ShowServlet</servlet-name>
    <url-pattern>/servlet/show</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>ShowServlet</servlet-name>
    <url-pattern>/show</url-pattern>
</servlet-mapping>

<filter>
    <filter-name>CharacterEncodingFilter</filter-name>
    <filter-class>com.kuang.filter.CharacterEncodingFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>CharacterEncodingFilter</filter-name>
    <!--只要是 /servlet的任何请求，都会经过这个过滤器-->
    <url-pattern>/servlet/*</url-pattern>
    <!--<url-pattern>/*</url-pattern>-->
</filter-mapping>

12、监听器

实现一个监听器的接口；（有N中）

编写一个监听器

实现监听器的接口...

//统计网站在线人数 ： 统计session
public class OnlineCountListener implements HttpSessionListener {

    //创建session监听：看你的一举一动
    //一旦创建一个session就会触发一次这个事件！
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
        ServletContext context = httpSessionEvent.getSession().getServletContext();

        System.out.println(httpSessionEvent.getSession().getId());

        Integer onlineCount = (Integer) context.getAttribute("OnlineCount");

        if (onlineCount==null){
            onlineCount = new Integer(1);
        }else {
            int count = onlineCount.intValue();
            onlineCount = new Integer(count+1);
        }

        context.setAttribute("OnlineCount",onlineCount);
    }

    //销毁session监听
    //一旦销毁session就会触发一次这个事件！
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        ServletContext context = httpSessionEvent.getSession().getServletContext();
        Integer onlineCount = (Integer) context.getAttribute("OnlineCount");

        if (onlineCount==null){
            onlineCount = new Integer(0);
        }else {
            int count = onlineCount.intValue();
            onlineCount = new Integer(count-1);
        }

        context.setAttribute("OnlineCount",onlineCount);
    }

    /*
    Session销毁：
    1. 手动销毁 getSession.invalidate()
    2. 自动销毁 在web.xml中设置session过期时间
     */
}

web.xml中注册监听器

<!--注册监听器-->
<listener>
    <listener-class>com.kuang.listener.OnlineCountListener</listener-class>
</listener>

看情况是否使用！

13、过滤器、监听器常见应用

监听器：GUI编程中经常使用；

public class TestPanel1 {
    public static void main(String[] args) {
        Frame frame = new Frame("中秋节快乐"); //新建一个窗体
        Panel panel = new Panel(null); //面板
        frame.setLayout(null); //设置窗体的布局

        frame.setBounds(50,50,300,300);
        frame.setBackground(Color.blue); //设置背景颜色

        panel.setBounds(70,70,70,70); //设置背景颜色
        panel.setBackground(Color.GREEN);

        frame.add(panel);
        frame.setVisible(true);

        //监听事件，监听关闭事件，使用适配器
        frame.addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });
    }
}

实现：用户登录之后才能进入主页！用户注销后就不能进入主页了！

用户登陆之后，向Session中放入用户的数据

进入主页的时候要判断用户是否已经登录；要求：在过滤器中实现！

//ServletRequest    HttpServletRequest
HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;
HttpServletResponse httpServletResponse = (HttpServletResponse) servletResponse;

if (httpServletRequest.getSession().getAttribute(Constant.USER_SESSION)==null){
    httpServletResponse.sendRedirect("/error.jsp");
}

filterChain.doFilter(servletRequest, servletResponse);

项目全部代码：

public class Constant {
    public final static String USER_SESSION = "USER_SESSION";
}
=============================================================
public class LoginServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取前端请求的参数
        String username = req.getParameter("username");

        if (username.equals("admin")){ //登陆成功
            req.getSession().setAttribute(Constant.USER_SESSION,req.getSession().getId());
            resp.sendRedirect("/sys/success.jsp");
        }else { //登录失败
            resp.sendRedirect("/error.jsp");
        }
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}
==========================================================================
public class LogoutServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        Object user_session = req.getSession().getAttribute(Constant.USER_SESSION);
        
        if (user_session!=null){
            req.getSession().removeAttribute(Constant.USER_SESSION);
            resp.sendRedirect("/Login.jsp");
        }else {
            resp.sendRedirect("/Login.jsp");
        }
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}
===============================================================================
public class SysFilter implements Filter {

    public void init(FilterConfig filterConfig) throws ServletException {
    }

    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {

        /*
        实现会员分级：
            if (req.getSession().setAttribute(Constant.USER_SESSION).level==VIP1){
                response.sendRedirect("/vip1/index.jsp");
            }
            if (req.getSession().setAttribute(Constant.USER_SESSION).level==VIP2){
                response.sendRedirect("/vip2/index.jsp");
            }
            if (req.getSession().setAttribute(Constant.USER_SESSION).level==VIP3){
                response.sendRedirect("/vip3/index.jsp");
            }
         */

        //ServletRequest    HttpServletRequest
        HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;
        HttpServletResponse httpServletResponse = (HttpServletResponse) servletResponse;

        if (httpServletRequest.getSession().getAttribute(Constant.USER_SESSION)==null){
            httpServletResponse.sendRedirect("/error.jsp");
        }

        filterChain.doFilter(servletRequest, servletResponse);
    }

    public void destroy() {
    }
}

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>

<h1>错误</h1>
<h3>没有权限，用户名错误</h3>

<p><a href="/Login.jsp">返回登录页面</a></p>

</body>
</html>
====================================================================
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>

<h1>主页</h1>

<p><a href="/servlet/logout">注销</a></p>

</body>
</html>
=======================================================================
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>

<h1>登录</h1>

<form action="/servlet/login" method="post">
    用户名：<input type="text" name="username"><br>
    <input type="submit">
</form>

</body>
</html>
========================================================================
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <servlet>
        <servlet-name>LoginServlet</servlet-name>
        <servlet-class>com.kuang.servlet.LoginServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LoginServlet</servlet-name>
        <url-pattern>/servlet/login</url-pattern>
    </servlet-mapping>
    <servlet>
        <servlet-name>LogoutServlet</servlet-name>
        <servlet-class>com.kuang.servlet.LogoutServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LogoutServlet</servlet-name>
        <url-pattern>/servlet/logout</url-pattern>
    </servlet-mapping>
    <filter>
        <filter-name>SysFilter</filter-name>
        <filter-class>com.kuang.servlet.SysFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>SysFilter</filter-name>
        <url-pattern>/sys/*</url-pattern>
    </filter-mapping>
    
</web-app>

相关阅读:
（附源码）python云顶之弈数据分析系统毕业设计451545
实现Promise的原型方法--前端面试能力提升
 阿里云国际版邮件服务套餐购买流程
 C语言入门 Day_12 一维数组
 在ubuntu（20.04）上搭建基于docker的yolov5环境（GPU：3060）
asp毕业设计——基于asp+access的网上远程教育网设计与实现（毕业论文+程序源码）——网上远程教育网
 大数据技术基础实验九：Hive实验——部署Hive
北京易华录大数据高级研发师1103面试题
 SASE , sdp等
 webpack 打包优化 - splitChunks
原文地址：https://www.cnblogs.com/wydilearn/p/15911737.html