Filter filter

Reference resources ：https://www.bilibili.com/video/BV1Y7411K7zz?p=293&vd_source=09e3f81b9f7ed8ee4306bb2e0ecd5538

One 、 What is? Filter filter ？

1、Filter Filter it's JavaWeb One of the three components of , Namely ：Servlet Program 、Listener Monitor 、Filter filter .
2、Filter Filter it's JavaEE The specification of , That's the interface .
3、Filter The function of the filter is ： Intercept request , Filtering response .

Common application scenarios for intercepting requests are ：

1、 Permission check
2、 Diary operation
3、 Business management
……

Two 、 How to use Filter

2.1 Write a Filter class , And identify @WebFilter annotation

package com.atguigu.filter;

import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import java.io.IOException;

/** * @author: sjmp1573 * @date: 2022/7/23 21:23 * @description: */
@WebFilter
public class AdminFilter implements Filter {
    

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    

    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
    
        HttpServletRequest httpServletRequest = (HttpServletRequest) request;
        HttpSession session = httpServletRequest.getSession();
        if (session.getAttribute("user") == null) {
    
            request.getRequestDispatcher("/login.html").forward(request, response);
        } else {
    
            chain.doFilter(request, response);
        }
    }

    @Override
    public void destroy() {
    
        Filter.super.destroy();
    }
}

2.2 stay web-xml Middle configuration

<?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">

    <filter>
        <filter-name>AdminFilter</filter-name>
        <filter-class>com.atguigu.filter.AdminFilter</filter-class>
        <init-param>
            <param-name>book1</param-name>
            <param-value>book1</param-value>
        </init-param>
        <init-param>
            <param-name>book2</param-name>
            <param-value>book2</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>AdminFilter</filter-name>
        <url-pattern>/admin/*</url-pattern> </filter-mapping> </web-app> 

2.3 Writing a login permission check is Servlet

package com.atguigu.servlet;

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

/** * @author: sjmp1573 * @date: 2022/7/23 22:30 * @description: */

@WebServlet(name = "adminServlet", value = "/adminServlet")
public class AdminServlet extends HttpServlet {
    
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        HttpSession session = req.getSession();
        if (username.equals("admin") && password.equals("admin")) {
    
            if (session.getAttribute("user") == null) {
    
                //  Request parameters  == null , Description is the first login 
                session.setAttribute("user",username);
            }
            resp.getWriter().write("ok!!!");
        } else {
    
            req.getRequestDispatcher("login.html").forward(req, resp);
        }
    }
}

3、 ... and 、Filter Life cycle of

Filter The life cycle of is composed of several methods
1、 Constructor method
2、init Initialization method
The first 1,2 Step , stay web When the project starts （Filter Created ）
3、doFilter Filtration method
The first 3 Step , Every time a request is intercepted , Will execute
4、destroy The destruction
The first 4 Step , stop it web During the project , Will execute （ stop it web engineering , It will be destroyed Filter filter ）

Four 、FilterConfig class

FilterConfig The name and meaning can be seen by category , It is Filter Filter profile class .

Tomcat Each creation Filter When , It will also create a FilterConfig class , It contains Filter Configuration information of the configuration file .

FilterConfig The role of a class is to get filter Filter configuration content

1、 obtain Filter The name of filter-name The content of
2、 To get in Filter Configured in init-param Initialize parameters
3、 obtain ServletContext object

5、 ... and 、FilterChain Filter chain

6、 ... and 、Filter The interception path of

• Exactly match
  <url-pattern>/target.jsp</url-pattern>
  The path of the above configuration , Indicates that the request address must be ：http://ip:port/ Project path /target.jsp

• Directory matching
  <url-pattern>/admin/*</url-pattern>
  The path of the above configuration , Indicates that the request address must be ：http://ip:port/ Project path /admin/*

• Suffix matches
  <url-pattern>*.html</url-pattern>
  The path of the above configuration , Indicates that the request address must be in .html The end will intercept

Filter The filter only cares about whether the requested address matches , Does not care if the requested resource exists ！！！