@Entity
@Table (name="firmen")
public class Firma implements Serializable {
  private static final long serialVersionUID = -6665434043077843943L;

  private int firmaId;
  private String name;
  private List<Person> personen = new ArrayList<Person>();

  @Id @GeneratedValue(strategy = GenerationType.AUTO)
  @Column (name="FIRMA_ID")
  public int getFirmaId() {
    return firmaId;
  }
  public void setFirmaId(int firmaId) {
    this.firmaId = firmaId;
  }
 
  @Column (name="NAME", length=30)
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
 
  @OneToMany (targetEntity=Person.class, mappedBy="firma", cascade=CascadeType.ALL)
  public List<Person> getPersonen() {
    return personen;
  }
  public void setPersonen(List<Person> personen) {
    this.personen = personen;
  } 

@Entity
@Table (name="personen")
public class Person implements Serializable {
  private static final long serialVersionUID = 2135362023133590877L;
  private int personId;
  private String name;
  private Firma firma;
 
  public static long getSerialVersionUID() {
    return serialVersionUID;
  }
 
  @Id @GeneratedValue(strategy = GenerationType.AUTO)   
  public int getPersonId() {
    return personId;
  }
  public void setPersonId(int personId) {
    this.personId = personId;
  }
 
  @Column (name="NAME", length=20)
  public String getName() {
    return name;
  } 
  public void setName(String name) {
    this.name = name;
  }

  @ManyToOne (cascade=CascadeType.ALL)
  @JoinColumn (name="FIRMA_FK", referencedColumnName = "FIRMA_ID", insertable=false, updatable=false)
  public Firma getFirma() {
    return firma;
  }

  public void setFirma(Firma firma) {
    this.firma = firma;
  } 
}

  ...
  public static void main(String[] args) {
    Session session = HibernateUtil.currentSession();
    Transaction tx = session.beginTransaction();
   
    Firma firma = new Firma();
    firma.setName("Dummy Corp.");
   
    List<Person> personen = new ArrayList<Person>();
   
    Person p1 = new Person();
    p1.setName("Barnes");
    personen.add(p1);

    Person p2 = new Person();
    p2.setName("Smith");
    personen.add(p2);
   
    firma.setPersonen(personen);
    session.save(firma);
    session.flush();

    tx.commit();   
    session.close();
  }