Filter filter details (listeners and their applications)

Filter Filter details

1. filter

• Fillter： filter , It's used to filter the data of the website ;

  • Deal with Chinese code
  • validate logon …

  ​

• web The service has some garbage requests , When the background should not process or should report an error, the filter will filter

2. Filter Development steps

1. Guide pack
2. Write filters

package com.qian.filter;

import javax.servlet.*;
import java.io.IOException;
// initialization ：web Server startup , It's already initialized , Wait for the filter object to appear at any time ！
// The destruction ：eb When the server is down , Just destroy 
public class CharacterEncodingFilter implements Filter {
    
    //init:  initialization  destroy： The destruction 
    public void init(FilterConfig filterConfig) throws ServletException {
    
        System.out.println("CharacterEncodingFilter initialization ");
    }
    //chain: chain 
 /* 1. Filter all code in the , When filtering specific requests, it will execute  2. You have to keep the filter going  */
    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 Before execution ...");
        filterChain.doFilter(servletRequest,servletResponse); //  Let our request go on , If you don't write , The program is intercepted and stopped here ！
        System.out.println(" CharacterEncodingFilter After execution ...");
    }

    public void destroy() {
    
        System.out.println("CharacterEncodingFilter The destruction ");
    }
}

3. stay web.xml Register filters in

<filter>
        <filter-name>CharacterEncodingFilter</filter-name>
        <filter-class>com.qian.filter.CharacterEncodingFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>CharacterEncodingFilter</filter-name>
<!--         As long as it is /servlet Any request under , It goes through this filter -->
        <url-pattern>/servlet/*</url-pattern> </filter-mapping> 

3. Monitor

1. Implement a listener interface ：（ There are many kinds of ）

• Write a listener , Implement monitor interface

package com.qian.listener;

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

// Count the number of people online ： Statistics session
public class OnlineCountListener implements HttpSessionListener {
    
   // establish session monitor ： Look at your every move 
   // Once created Session This event will be triggered once 
    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);
    }
    // The destruction session monitor 
    // Once destroyed Seesion This event will be triggered once 
    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 The destruction ： * 1. Destroy by hand ：getSession().invalidate(); * * 2. Automatically destroy ---->web.xml Middle configuration  * */
    }
}

• web.xml Registering listener

<!--  Register listener -->
    <listener>
        <listener-class>com.qian.listener.OnlineCountListener</listener-class>
    </listener>


<!-- session Automatically destroy -->
<session-config>
        <session-timeout>1</session-timeout>
    </session-config>

4. filter 、 Common applications of listeners

1. Monitor ：GUI Programming often uses

2. Log in and log out

   ​

• Users can enter the home page only after logging in , Users cannot enter the home page after logging out

// Sign in 

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 {
    
        // Get the parameters of the front-end request 
        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);

    }
}


// Cancellation 
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);
    }
}

register

<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 Design

<%--   Home page 
  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> Login successful </h1>
<p><a href="/servlet/logout"> Cancellation </a></p>

</body>
</html>


<%--  The login page 
  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> Sign in </title>
</head>
<body>
<h1> Sign in </h1>
<form action=${pageContext.request.contextPath}/servlet/login method="post">
    <input type="text" name="username">
    <input type="submit">
</form>
<%-- The login page ----> Submit a request to /servlet/login----> Operation request , Successful jump success--%>
</body>
</html>


<%-- Error page 
  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> Login error </title>
</head>
<body>
<h1> error </h1>
<h3> No permission or wrong user name </h3>
<a href="/login.jsp"> Return to login page </a>
</body>
</html>

• When entering the home page, judge whether the user has logged in , requirement ： Implement... In the filter

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() {
    

    }
}