过滤器和监听器
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[<strong>过滤器</strong> <br>web服务有一些垃圾请求,后台不应该处理,或者应该报错<br>处理乱码问题]
id3 --> id2
id3 --> id4[Servlet<br>JSP<br>HTML<br>静态资源<br>......]
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)
login
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>
