Illegal attempt to associate a collection with two open sessions

public class Hibernate
{
 protected static final SessionFactory sessionFactory;

 private Session session;

 static
 {
  try
  {
   // Create the SessionFactory from hibernate.cfg.xml
   sessionFactory = new Configuration().configure().buildSessionFactory();
   session
  }
  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 void create(Object obj)
 {
  this.session = sessionFactory.openSession();
  session.getTransaction().begin();
  session.save(obj);
  session.getTransaction().commit();
  session.close();
 }

 public void refresh(Object obj)
 {
  this.session = sessionFactory.openSession();
  session.getTransaction().begin();
  session.refresh(obj);
  session.getTransaction().commit();
  session.close();
 }

 public void update(Object obj)
 {
  this.session = sessionFactory.openSession();
  session.getTransaction().begin();
  session.saveOrUpdate(obj);
  session.getTransaction().commit();
  session.close();

 }
 public void delete(Object obj)
 {
  this.session = sessionFactory.openSession();
  session.getTransaction().begin();
  session.delete(obj);
  session.flush();
  session.getTransaction().commit();
  session.close();
 }
 protected String protectString(String toProtect)
 {
  return (toProtect.replace("'", "''"));
 }

}

public class DAOPerson extends Hibernate
{

 public void remove(Person p)
 {
  if (p instanceof Student)
  {
   Student s = (Student)p;
   Set<persistenceClass.Class> set = s.getClasses();
   Iterator<persistenceClass.Class> it = set.iterator();
   while (it.hasNext())
   {
    persistenceClass.Class r = it.next();
    r.getStudents().remove(s);
   }
   p.getBirthCountry();
   p.getCountry();
   this.delete(p);
  }
  else
   this.delete(p);
}

<class name="persistenceClass.Person" table="T_PERSON">
  <id name="Id" column="PERSON_ID">
   <generator class="native" />
  </id>
  <property name="FirstName" column="PERSON_FIRST_NAME" not-null="true" />
  <property name="LastName" column="PERSON_LAST_NAME" not-null="true" />
  <property name="Type" column="PERSON_TYPE" not-null="true" />
  <property name="BirthDate" column="PERSON_BIRTH_DATE" />
  <property name="BirthCity" column="PERSON_BIRTH_CITY" />
  <property name="PhoneNumber" column="PERSON_PHONE_NUMBER" />
  <property name="MobileNumber" column="PERSON_MOBILE_NUMBER" />
  <property name="Mail" column="PERSON_MAIL" />
  <property name="Address" column="PERSON_ADDRESS_ADDRESS" />
  <property name="ZipCode" column="PERSON_ADDRESS_ZIPCODE" />
  <property name="City" column="PERSON_ADDRESS_CITY" />
  <property name="Image" column="PERSON_IMAGE" type="image" />
  <many-to-one name="Country" column="PERSON_ADDRESS_COUNTRY" class="persistenceClass.Country" />
  <many-to-one name="BirthCountry" column="PERSON_BIRTH_COUNTRY" class="persistenceClass.Country" />
  <many-to-one name="Civility" column="PERSON_CIVILITY" class="persistenceClass.Civility" />
  <many-to-one name="Sex" column="PERSON_SEX" class="persistenceClass.Sex" />
  <joined-subclass name="persistenceClass.Student" table="T_STUDENT">
   <key column="PERSON_ID" />
   <set name="Classes" table="T_CLASS_STUDENT" inverse="true" >
    <key column="PERSON_ID" />
    <many-to-many class="persistenceClass.Class" column="CLASS_ID" />
   </set>
  </joined-subclass>
  <joined-subclass name="persistenceClass.Teacher" table="T_TEACHER">
   <key column="PERSON_ID" />
  </joined-subclass>
 </class>

<hibernate-configuration>
<session-factory>
<!-- Database connection settings -->
<property name="connection.driver_class">org.gjt.mm.mysql.Driver</property>
<property name="connection.url">jdbc:mysql://localhost/projet</property>
<property name="connection.username">root</property>
<property name="connection.password"></property>
<!-- JDBC connection pool (use the built-in) -->
<property name="connection.pool_size">10</property>
<!-- SQL dialect -->
<property name="dialect">org.hibernate.dialect.MySQLInnoDBDialect</property>
<!-- Drop and re-create the database schema on start-up, also try with “update” to keep the previous values -->
<property name="hbm2ddl.auto">update</property>
<!-- Echo all executed SQL to stdout -->
<property name="show_sql">true</property>
<mapping resource="persistenceConfigurations/Person.hbm.xml"/>
<mapping resource="persistenceConfigurations/Country.hbm.xml"/>
<mapping resource="persistenceConfigurations/Civility.hbm.xml"/>
<mapping resource="persistenceConfigurations/Sex.hbm.xml"/>
<mapping resource="persistenceConfigurations/Formation.hbm.xml"/>
<mapping resource="persistenceConfigurations/Year.hbm.xml"/>
<mapping resource="persistenceConfigurations/Class.hbm.xml"/>
<mapping resource="persistenceConfigurations/Subject.hbm.xml"/>
<mapping resource="persistenceConfigurations/Room.hbm.xml"/>
<mapping resource="persistenceConfigurations/Lesson.hbm.xml"/>
</session-factory>
</hibernate-configuration>

public void remove(Person p)
 {
  if (p instanceof Student)
  {
   Student s = (Student)p;
   Set<persistenceClass.Class> set = s.getClasses();
   Iterator<persistenceClass.Class> it = set.iterator();
   while (it.hasNext())
    {
     persistenceClass.Class r = it.next();
     r.getStudents().remove(s);

     //now effectively removing the student from the class.
     //this will update the class and its persistent set of students.
     this.update(r);
    }
   }
   this.delete(p); //now remove the student (or whatever comes as argument)
 }

//Example
public void create(Object obj){
  this.session = sessionFactory.openSession();
  try{
    session.getTransaction().begin();
    session.save(obj);
    session.getTransaction().commit();
  } catch (Exception e) {
    e.printStackTrace();
    session.getTransaction().rollback();
  } finally {
    session.flush();
    session.close();
    //keep in mind that too many flushes might cause db memory shortage.
    //So if you are treating list of objects and so on iterate and save them flushing
    //only when your batch size is reached.
  }
}

@ManyToOne(cascade = CascadeType.ALL)
@JoinColumn(name = “user_id”)
public User getAuthor() {
return author;
}