Why do I get org.hibernate.HibernateException: No CurrentSessionContext configured
org.hibernate.HibernateException: Illegal attempt to associate a collection with two open sessions

Session sess = HibernateUtil.getSession();
                                    Transaction tx = sess.beginTransaction();
                                    try {
                                        tx.begin();
                                        sess.delete(ims);
                                    } catch (Exception e) {
                                        tx.rollback();
                                        throw e;
                                    }
                                    tx.commit();
                                    sess.flush();

public static Session getSession()
            throws HibernateException {
        Session sess = null;
        try {
            sess = sessionFactory.getCurrentSession();
        } catch (org.hibernate.HibernateException he) {
            sess = sessionFactory.openSession();
        }
            return sess;
        }

<hibernate-configuration>
    <session-factory >
    <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
    <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="hibernate.connection.url">jdbc:mysql://localhost/joptel</property>
    <property name="hibernate.connection.username">root</property>
    <property name="hibernate.connection.password">******</property>
    <property name="hibernate.connection.pool_size">1</property>
    <property name="show_sql">true</property>
    <property name="hibernate.hbm2ddl.auto">update</property>


    ..mappings..

    </session-factory>
</hibernate-configuration>

public static Session getSession()
    throws HibernateException {
    Session sess = null;
   try {
   sess = sessionFactory.getCurrentSession();
     } catch (org.hibernate.HibernateException he) {
      sess = sessionFactory.openSession();
   }             return sess;         }

<property name="hibernate.current_session_context_class">thread</property>

<property name="hibernate.current_session_context_class">...</property>