package com.mycompany.testweb;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.concurrent.TimeUnit;
import java.util.logging.Level;
import java.util.logging.Logger;

public class DaemonClass implements Runnable{

    @Override
    public void run() {
        String filePath = "D:\\myfile.txt";
        String text = "Ку!\n";

        while (true) {
            try {
                Files.write(Paths.get(filePath), text.getBytes(), StandardOpenOption.APPEND);
            } catch (IOException e) {
                System.out.println(e);
            }

            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException ex) {
                Logger.getLogger(DaemonClass.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }    
}

package com.mycompany.testweb;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class IniClass implements ServletContextListener {
    private Thread thread;
    
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        DaemonClass daemonClass = new DaemonClass();
         thread = new Thread(daemonClass);
         thread.start();
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        thread.interrupt();
    }
    
}

package com.mycompany.testweb;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class IniClass implements ServletContextListener, Runnable {
    private Thread mainThread;
    private Thread thread;
    private DaemonClass daemonClass = null;
    
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        mainThread = new Thread(this);
        mainThread.start();
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        daemonClass.setIsInterrupted(true);
        thread.interrupt();
    }

    @Override
    public void run() {
        daemonClass = new DaemonClass();
        thread = new Thread(daemonClass);
        thread.start();
    }
}