GeneratorImpl.java

package com.example.generator;

import com.example.generator.entity.Brand;
import com.example.generator.entity.Model;
import com.example.generator.hibernate.HibernateSessionManager;
import org.hibernate.Session;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.ArrayList;
import java.util.List;
import java.util.Set;

public class GeneratorImpl implements Generator {

    private static final Logger LOGGER = LoggerFactory.getLogger(GeneratorImpl.class);
    private Brand brand;

    @Override
    public void generate(Brand brand) {
        Session session = HibernateSessionManager.getSessionFactory().openSession();
         // brand nesnesi yeniden aliniyor db'den
        brand = (Brand) session.load(Brand.class, brand.getId()); 
    }

    @Override
    public void run() {
        for (Model model : brand.getModels()) { // nullpointer exception

        }
    }

Main.java

package com.example.generator;

import com.example.entity.Brand;
import com.example.hibernate.HibernateSessionManager;
import org.hibernate.Session;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.List;

public class Main {

    private final static Logger LOGGER = LoggerFactory.getLogger(Main.class);

    public static void main(String[] args) {
        LOGGER.info("Hi!");

        // markaların alindigi session
        Session session = HibernateSessionManager.getSessionFactory().openSession(); 

        TaskExecutor taskExecutor = new TaskExecutorImpl();

        List brands = session.createCriteria(Brand.class).list();

        for (Object brand : brands) {
            Generator generator = new GeneratorImpl();
            generator.generate((Brand) brand); // 
            taskExecutor.execute(generator); // thread-pool(10)
        }

        taskExecutor.shutdown();
        session.close();
    }
}