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过滤器和监听器

Filter和Listener

Filter和Listener

1. Filter概述

过滤器,用来过滤网站的数据

graph LR id1[Web浏览器] --> id2[Web服务器] id2 --> id1 id2 --> id3[过滤器 web服务有一些垃圾请求,后台不应该处理,或者应该报错 处理乱码问题] id3 --> id2 id3 --> id4[Servlet JSP HTML 静态资源 ......] id4 --> id3

2. Filter开发步骤

导包不要错:javax.servlet包
编写过滤器:实现Filter接口,重写对应方法

package com.wang.filter;

import javax.servlet.*;
import java.io.IOException;

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");
    }

    //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

<?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>ShowServlet</servlet-name>
        <servlet-class>com.wang.Servlet.ShowServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ShowServlet</servlet-name>
        <url-pattern>/Servlet/show</url-pattern>
    </servlet-mapping>
    
    <filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>com.wang.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>
    </filter-mapping>
    
</web-app>

3. 监听器

监听器是实现一个监听器的接口

编写一个监听器:实现监听器的接口,重写里面的方法

package com.wang.listener;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import java.awt.*;

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

    //创建session监听
    //一旦创建一个session,就会触发一次这个事件!
    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
        //每次来一个session,取出上下文
        ServletContext ctx = httpSessionEvent.getSession().getServletContext();
        //将上下文中的OnlineCount属性取出!
        Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");

        //如果一开始没有人,第一个session来的时候将onlineCount设定为1
        if (onlineCount == null) {
            onlineCount = new Integer(1);
        }else {
            //如果之前有人,来一个session就将onlineCount加1
            int count = onlineCount;
            onlineCount = new Integer(count + 1);
        }

        //将计算得到的onlineCount值放入OnlineCount属性中!
        ctx.setAttribute("OnlineCount", onlineCount);

    }

    //销毁session监听
    //一旦session销毁,就会触发这个事件!
    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        //每次来一个session,取出上下文
        ServletContext ctx = httpSessionEvent.getSession().getServletContext();
        //将上下文中的OnlineCount属性取出!
        Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");

        if (onlineCount == null) {
            onlineCount = new Integer(0);
        }else {
            //如果之前有人,来一个session就将onlineCount减1
            int count = onlineCount;
            onlineCount = new Integer(count - 1);
        }

        //将计算得到的onlineCount值放入OnlineCount属性中!
        ctx.setAttribute("OnlineCount", onlineCount);
    }

    /*
    session销毁:
    1. 手动销毁     httpSessionEvent.getSession().invalidate();
    2. 自动销毁     在web.xml下配置:设定1分钟后自动销毁
                    <session-config>
                        <session-timeout>1</session-timeout>
                    </session-config>
     */
}

在web.xml中注册监听器

<?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>ShowServlet</servlet-name>
        <servlet-class>com.wang.Servlet.ShowServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ShowServlet</servlet-name>
        <url-pattern>/Servlet/show</url-pattern>
    </servlet-mapping>
    
    <filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>com.wang.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>
    </filter-mapping>
    
    <!--注册监听器-->
    <listener>
        <listener-class>com.wang.listener.OnlineCountListener</listener-class>
    </listener>

</web-app>

看情况是否使用

此处在前台展示session数目

<%--
  Created by IntelliJ IDEA.
  User: Wang
  Date: 2020/8/20
  Time: 16:58
  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>
  <%--
    JSP中,this关键字代表JSP中的page对象
    通过getServletConfig获取Servlet的初始化参数
    通过getServletContext获取Servlet的上下文
    通过getAttribute获取上下文中对应名称的属性值
  --%>
  <h1>当前有 <span style="color: aqua"><%=this.getServletConfig().getServletContext().getAttribute("OnlineCount")%></span> 人在线</h1>
  </body>
</html>

首次启动时有可能不为1,这是由于浏览器使用了多个session,只有最后一个才成功!

4. 过滤器,监听器的常见应用

1. 在GUI中使用监听器关闭窗口

package com.wang.listener;

import java.awt.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;

public class PagePanel {
    public static void main(String[] args) {
        Frame frame = new Frame("这是一个窗口");       //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(300, 300, 500, 500);
        frame.setBackground(Color.BLUE);                 //设置背景颜色
        panel.setBounds(50, 50, 300, 300);  //设置背景颜色
        panel.setBackground(Color.GREEN);

        frame.add(panel);

        frame.setVisible(true);     //设置窗体可见性

        //监听事件,监听关闭时间
        frame.addWindowListener(new WindowListener() {
            @Override
            public void windowOpened(WindowEvent e) {
                System.out.println("打开");
            }

            @Override
            public void windowClosing(WindowEvent e) {
                System.out.println("正在关闭");
                //0:程序正常关闭
                System.exit(0);
            }

            @Override
            public void windowClosed(WindowEvent e) {
                System.out.println("已关闭");
            }

            @Override
            public void windowIconified(WindowEvent e) {

            }

            @Override
            public void windowDeiconified(WindowEvent e) {

            }

            @Override
            public void windowActivated(WindowEvent e) {
                System.out.println("激活");
            }

            @Override
            public void windowDeactivated(WindowEvent e) {
                System.out.println("未激活");
            }
        });

        //监听窗口的另一种写法,避免对所有方法重写
        //new 一个 WindowAdapter(),选择要重写的方法
        frame.addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {
                super.windowClosing(e);
            }
        });
    }
}

2. Filter实现权限拦截

需求:用户登录之后才能进入主页,用户注销之后就不能进入了!

用户登录之后,想Session中放入用户数据
进入主页的时候要判断用户是否已经登录(在过滤器中实现)

package com.wang.Servlet;

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

public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取前端的请求
        String username = req.getParameter("username");

        //如果用户名是admin,则登录成功,将用户信息放到session中
        if (username.equals("admin")) {
            //将键值对为USER_SESSION:当前用户的session id的信息放入session中
            req.getSession().setAttribute("USER_SESSION", req.getSession().getId());
            //重定向,用response,给客户端响应一个url
            //转发用的是request,给服务器响应一个url
            resp.sendRedirect("/sys/success.jsp");
        }else {
            resp.sendRedirect("/error.jsp");
            //登录失败
        }
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

logout

package com.wang.Servlet;

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

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

        //获得session中USER_SESSION的值
        Object user_session = req.getSession().getAttribute("USER_SESSION");

        if (user_session != null) {
            //注销,移除session中存放的USER_SESSION属性
            req.getSession().removeAttribute("USER_SESSION");
            //注销后重定向到登录页面
            resp.sendRedirect("/login.jsp");
        }else {
            resp.sendRedirect("/login.jsp");
        }
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

过滤器

package com.wang.filter;

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class SysFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {

    }

    @Override
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {

        //过滤器中的为上一层的类型,为了能够取到session,我们要强转为HttpServletResponse和HttpServletRequest
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) resp;

        //如果session的USER_SESSION字段为空,重定向到error.jsp
        if (request.getSession().getAttribute("USER_SESSION") == null) {
            response.sendRedirect("/error.jsp");
        }

        chain.doFilter(request, response);
    }

    @Override
    public void destroy() {

    }
}

前端(login)

<%--
  Created by IntelliJ IDEA.
  User: Wang
  Date: 2020/8/21
  Time: 10:43
  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>

<%--action:提交后,会跳转到对应的页面--%>
<form action="Servlet/login" method="post">
    <input type="text" name="username">
    <input type="submit">
</form>

</body>
</html>

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原文地址：https://www.cnblogs.com/wang-sky/p/13540990.html