Introduction to servlet listener & filter

1： Monitor Listener
Need to see Servlet Can see my last article , There is a detailed introduction

Listener Introduce ：
A listener is to monitor the state change of an object
Event source ： The three domain objects being monitored request,session,servletcontext

The writing steps of the listener :
1: Write a listener class to implement the listener interface
Use annotations to configure ：@WebListener

 Another configuration method is to use web.xml To configure the , But it's more troublesome , I like to use notes 

package com.zking.lister;

import javax.servlet.Servlet;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

/** *  Monitor  * @author JIE * * @date 2022 year 4 month 22 Japan   Afternoon 8:21:42 */
@WebListener // Annotation mode 
public class onLineListenr implements ServletContextListener,HttpSessionListener {
    

	
	ServletContext applicaton;
	@Override
	public void contextDestroyed(ServletContextEvent sce) {
    
		// application Destroyed 
		System.out.println(" The service was destroyed ");
		
	}

	@Override
	public void contextInitialized(ServletContextEvent sce) {
    
		//application Was created 
		System.out.println(" The service created ");
		applicaton = sce.getServletContext();
		applicaton.setAttribute("onlineCount", 0);
		
		
	}

	@Override
	public void sessionCreated(HttpSessionEvent arg0) {
    
		//  The main page of the project was visited 
		// Number of people obtained 
		Integer count = (Integer)applicaton.getAttribute("onlineCount");
		// The number of +1
		// Set number of people 
		applicaton.setAttribute("onlineCount", ++count);
		System.out.println(" Someone came in ");
	}

	@Override
	public void sessionDestroyed(HttpSessionEvent arg0) {
    
		// 1. The survival time has expired 
		//2. Destroy by hand 
		// Number of people obtained 
				Integer count = (Integer)applicaton.getAttribute("onlineCount");
				// The number of -1
				// Set number of people 
				applicaton.setAttribute("onlineCount", --count);
				System.out.println(" Someone's out ");
	}
	
}

2: filter

Filter introduction :
stay Servlet in , We need to Filter To compare and filter information , For example, in a project , We can limit the functions of tourists through filters .
advantage ：

 The function of the filter is described ： While working on the project , Usually divided into several packages , The tourist interface is put in the tourist bag ,
 The user interface is placed in the user package , The administrator interface is placed in the administrator package .
 So when we write a project, we can use the path matching of the filter to determine , This efficiency will greatly increase 

Filter Configuration of :
There are two ways :
1: adopt web.xml To configure the filter
2： Configure filters through annotations （ Personal recommendations ）
The first way is more troublesome , The second way is relatively simple , Personally, I like the simpler

package com.zking.filter;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/** *  Permission filter 【 authentication 】 * @author JIE * * @date 2022 year 4 month 22 Japan   Afternoon 8:50:11 * */

@WebFilter("/*")
public class Rolefilter implements Filter{
    
	// The path to allow access 
	List<String> paths=new ArrayList<String>();
	
	{
    
		paths.add("/index.jsp");
		paths.add("/tourists.jsp");
		paths.add("/login.do");
		paths.add("/exit.do");
		
	}
	
	
	@Override
	public void destroy() {
    
		// TODO Auto-generated method stub
		
	}

	@Override
	public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)
			throws IOException, ServletException {
    
		//  All operations of the filter are completed here 
		HttpServletRequest request =(HttpServletRequest)req;
		HttpServletResponse response  =(HttpServletResponse)resp;
		// Get the path of the current request 
		String path=request.getServletPath();
		// Path judgment , Use the hypothesis method 
		boolean f=false;
		for (String p : paths) {
    
			if(p.equals(path)) {
    
				f=true;
				break;
			}
		}
		
		if(f) {
    
			// Filter release operation 
			chain.doFilter(req, resp);
			return;
		}
		
		Object isLogin= request.getSession().getAttribute("isLogin");
		if(isLogin==null) {
    
			response.sendRedirect("index.jsp");
			return;
		}
		
		
		
	}

	@Override
	public void init(FilterConfig arg0) throws ServletException {
    
		// TODO Auto-generated method stub
		
	}
	
	
}

Java Development Manual （ Development guidelines ）:https://pan.baidu.com/s/18cNu2W5p9EumiBZW_yxWIA

Extraction code ：a4cb