public class HibernateUtil {

    private static SessionFactory sessionFactory;

    public static void initialize(String hibernateConfigFile) {
        try {
            // Create the SessionFactory from hibernate.cfg.xml
            sessionFactory = new AnnotationConfiguration().configure(new File(hibernateConfigFile)).buildSessionFactory();
        } catch (Throwable ex) {
            // Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }

}

@MappedSuperclass
public class Base {

    private static Logger log = Logger.getLogger(Base.class);

    @Id()
    private int id;


    private String name;


    private String decription;

    protected Base() {
        log.debug("Creando nuevo objeto.");
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDecription() {
        return decription;
    }

    public void setDecription(String decription) {
        this.decription = decription;
    }

    @Transient
    public void save() {

        log.debug("Guardando el objeto...");

        SessionFactory sf = HibernateUtil.getSessionFactory();
        Session session = sf.getCurrentSession();
        session.beginTransaction();
        session.save(this);
        session.getTransaction().commit();

        log.debug("Objeto guardado.");

    }

}

@Entity()
@Table(name="COMPU")
@AttributeOverride( name="id", column = @Column(name="COMPID") )
public class Computer extends Base {

    private Collection<ComputerGroup> computerGroupList;
    private String ipAddress;

    public Computer() {
    }

    @Transient
    @ManyToMany(
        cascade={CascadeType.PERSIST, CascadeType.MERGE},
        mappedBy="computersList",
        targetEntity=ComputerGroup.class
    )
    public Collection<ComputerGroup> getComputerGroupList() {
        return computerGroupList;
    }

    public void setComputerGroupList(Collection<ComputerGroup> computerGroupList) {
        this.computerGroupList = computerGroupList;
    }

    public String getIpAddress() {
        return ipAddress;
    }

    public void setIpAddress(String ipAddress) {
        this.ipAddress = ipAddress;
    }

}

@Entity()
@Table(name="COMPU")
@AttributeOverride( name="id", column = @Column(name="COMPID") )
public class Computer extends Base {

    private Collection<ComputerGroup> computerGroupList;
    private String ipAddress;

    public Computer() {
    }

    @Transient
    @ManyToMany(
        cascade={CascadeType.PERSIST, CascadeType.MERGE},
        mappedBy="computersList",
        targetEntity=ComputerGroup.class
    )
    public Collection<ComputerGroup> getComputerGroupList() {
        return computerGroupList;
    }

    public void setComputerGroupList(Collection<ComputerGroup> computerGroupList) {
        this.computerGroupList = computerGroupList;
    }

    public String getIpAddress() {
        return ipAddress;
    }

    public void setIpAddress(String ipAddress) {
        this.ipAddress = ipAddress;
    }

}

Initial SessionFactory creation failed.org.hibernate.MappingException: Could not determine type for: java.util.Collection, for columns: [org.hibernate.mapping.Column(computerGroupList)]

@javax.persistence.Entity()
@javax.persistence.Table(name="COMPUGR")
@AttributeOverride( name="id", column = @Column(name="COMPUGRID") )
public class ComputerGroup extends Base {

    private Collection<Computer> computersList;


    public ComputerGroup() {
    }

    @ManyToMany(targetEntity=Computer.class,
        cascade={CascadeType.PERSIST, CascadeType.MERGE}
    )
    @JoinTable(name="COMPUGRMEM",
        joinColumns={@JoinColumn(name="COMPUGRID")},
        inverseJoinColumns={@JoinColumn(name="COMPUID")}
    )
    public Collection<Computer> getComputersList() {
        return computersList;
    }

    public void setComputersList(Collection<Computer> computersList) {
        this.computersList = computersList;
    }

}