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Filter过滤器详解(监听器以及它们的应用)

2022-07-25 09:25:00 【Zero摄氏度】

Filter过滤器详解

1.过滤器

  • Fillter：过滤器，用来过滤网站的数据；

    • 处理中文乱码
    • 登录验证…

    ​

  • web服务有一些垃圾请求，后台不应该处理或者应该报错时由过滤器来过滤

2. Filter开发步骤

  1. 导包
  2. 编写过滤器
package com.qian.filter;

import javax.servlet.*;
import java.io.IOException;
//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！
//销毁：eb服务器关闭的时候，才销毁
public class CharacterEncodingFilter implements Filter {
    
    //init: 初始化 destroy：销毁
    public void init(FilterConfig filterConfig) throws ServletException {
    
        System.out.println("CharacterEncodingFilter初始化");
    }
    //chain:链
 /* 1.过滤中的所有代码，在过滤特定请求的时候都会执行 2.必须要让过滤器继续同行 */
    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...");
    }

    public void destroy() {
    
        System.out.println("CharacterEncodingFilter销毁");
    }
}
  1. 在web.xml中注册过滤器
<filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>com.qian.filter.CharacterEncodingFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
<!--        只要是/servlet下的任何请求，都会经过这个过滤器-->
        <url-pattern>/servlet/*</url-pattern> </filter-mapping>

3.监听器

  1. 实现一个监听器的接口：（有很多种）
  • 编写一个监听器，实现监听接口
package com.qian.listener;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

//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Session就会触发一次这个事件
    public void sessionCreated(HttpSessionEvent se) {
    
     ServletContext ctx = se.getSession().getServletContext();
     System.out.println(se.getSession().getId());
     Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
     if (onlineCount==null){
    
      onlineCount = new Integer(1);
     }else {
    
      int count = onlineCount.intValue();
      onlineCount = new Integer(count+1);
     }
      ctx.setAttribute("OnlineCount",onlineCount);
    }
    //销毁session监听
    //一旦销毁Seesion就会触发一次这个事件
    public void sessionDestroyed(HttpSessionEvent se) {
    
     ServletContext ctx = se.getSession().getServletContext();
     se.getSession().invalidate();
     Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
     if (onlineCount==null){
    
      onlineCount = new Integer(0);
     }else {
    
      int count = onlineCount.intValue();
      onlineCount = new Integer(count-1);
     }
     ctx.setAttribute("OnlineCount",onlineCount);
/* * Session销毁： * 1.手动销毁：getSession().invalidate(); * * 2.自动销毁---->web.xml中配置 * */
    }
}
  • web.xml中注册监听器
<!-- 注册监听器-->
    <listener>
        <listener-class>com.qian.listener.OnlineCountListener</listener-class>
    </listener>


<!-- session自动销毁-->
<session-config>
        <session-timeout>1</session-timeout>
    </session-config>

4.过滤器、监听器的常见应用

  1. 监听器：GUI编程中经常使用

  2. 登录注销

    ​

  • 用户登录之后才能进入主页，用户注销后就不能进入主页
//登录

package com.qian.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 {
    
        //获取前端请求的参数
        String username = req.getParameter("Username");
        if (username.equals("admin")){
    
            req.getSession().setAttribute("USER_SESSION",req.getSession().getId());
            resp.sendRedirect("/sys/success.jsp");
        }


    }

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

    }
}


//注销
package com.qian.servlet;

import com.qian.util.Constant;

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

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

注册

<servlet>
        <servlet-name>LoginServlet</servlet-name>
        <servlet-class>com.qian.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.qian.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.qian.filter.SysFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>SysFilter</filter-name>
        <url-pattern>/sys/*</url-pattern>
    </filter-mapping>

jsp设计

<%--  主页
  Created by IntelliJ IDEA.
  User: HUAWEI
  Date: 2022/4/11
  Time: 16:52
  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>
<h1>登录成功</h1>
<p><a href="/servlet/logout">注销</a></p>

</body>
</html>


<%-- 登录页面
  Created by IntelliJ IDEA.
  User: HUAWEI
  Date: 2022/4/11
  Time: 16:53
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录</title>
</head>
<body>
<h1>登录</h1>
<form action=${pageContext.request.contextPath}/servlet/login method="post">
    <input type="text" name="username">
    <input type="submit">
</form>
<%--页面登录---->提交请求到/servlet/login---->操作请求，成功跳转success--%>
</body>
</html>


<%--错误页面
  Created by IntelliJ IDEA.
  User: HUAWEI
  Date: 2022/4/11
  Time: 20:37
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>登录错误</title>
</head>
<body>
<h1>错误</h1>
<h3>没有权限或者用户名错误</h3>
<a href="/login.jsp">返回登录页面</a>
</body>
</html>
  • 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
package com.qian.filter;

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

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

    }

    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    
        HttpServletRequest request1 = (HttpServletRequest)servletRequest;
        HttpServletResponse response1 = (HttpServletResponse)servletResponse;
        Object user_session = request1.getSession().getAttribute("USER_SESSION");
        if (request1.getSession().getAttribute("USER_SESSION")==null){
    
            response1.sendRedirect("/Error.jsp");
        }



        filterChain.doFilter(servletRequest,servletResponse);
    }

    public void destroy() {
    

    }
}
原网站

版权声明
本文为[Zero摄氏度]所创，转载请带上原文链接，感谢
https://blog.csdn.net/m0_56116754/article/details/124110584

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