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

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;

public class Main {
   
   public static void main(String[] args) {
      SessionFactory sf = createSessionFactory();
      Session s = sf.openSession();
      
      Transaction tx = s.beginTransaction();
      try {
         Master master = new Master();
         master = (Master)s.merge(master);
         
         Item i = new Item();
         i.setMaster(master);
   
         s.merge(i);
         
         Master data = (Master)s.get(Master.class, master.getId());
         System.out.println(data.getItens().size());
      } finally {
         tx.rollback();
      }
   }
   
   private static SessionFactory createSessionFactory(){
      Configuration configuration = new Configuration();
      
      configuration.addAnnotatedClass(Master.class);
      configuration.addAnnotatedClass(Item.class);
      configuration.setProperty("hibernate.dialect", "org.hibernate.dialect.H2Dialect");
      configuration.setProperty("hibernate.show_sql", "true");
      configuration.setProperty("hibernate.hbm2ddl.auto","create");
      configuration.setProperty("hibernate.connection.url", "jdbc:h2:mem:xxx;DB_CLOSE_DELAY=1000");
      configuration.setProperty("hibernate.connection.driver_class", "org.h2.Driver");
      
       ServiceRegistry serviceRegistry = new ServiceRegistryBuilder().applySettings(configuration.getProperties()).buildServiceRegistry();       
       SessionFactory sessionFactory = configuration.buildSessionFactory(serviceRegistry);
       return sessionFactory;
   }
}

@Entity
class Master {

   @Id
   @GeneratedValue(strategy=GenerationType.AUTO)
   private Integer id;
   
   @OneToMany(mappedBy="master", fetch=FetchType.LAZY, cascade=CascadeType.ALL)
   private List<Item> itens = new ArrayList<Item>();

   public Integer getId() {
      return id;
   }

   public void setId(Integer id) {
      this.id = id;
   }

   public List<Item> getItens() {
      return itens;
   }

   public void setItens(List<Item> itens) {
      this.itens = itens;
   }
}

@Entity
class Item {

   @Id
   @GeneratedValue(strategy=GenerationType.AUTO)
   private Integer id;
   
   @ManyToOne()
   @JoinColumn(name="master", referencedColumnName="id")
   private Master master;

   public Integer getId() {
      return id;
   }

   public void setId(Integer id) {
      this.id = id;
   }

   public Master getMaster() {
      return master;
   }

   public void setMaster(Master master) {
      this.master = master;
   }   
}

i.setMaster(master);

i.setMaster(master);       
master.getItens().add(i);