@Entity
@Table(name = "OBJECT")
public class DataObject {

   @Id
   @GeneratedValue(strategy = GenerationType.AUTO)
   private long id;

   @Column
   private String name;

   @ManyToMany(cascade = { CascadeType.ALL })
   private Set<Property> properties;

   @ManyToOne
   private DataObject parent;
   
   @ManyToMany(cascade = { CascadeType.ALL })
   private List<DataObject> children;
   
   public long getId() {
      return id;
   }

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

   public String getName() {
      return name;
   }

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

   public Set<Property> getProperties() {
      return properties;
   }

   public void setProperties(Set<Property> properties) {
      this.properties = properties;
   }

   public List<DataObject> getChildren() {
      return children;
   }

   public void setChildren(List<DataObject> children) {
      this.children = children;
   }

}

@Entity
@Inheritance (strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Property {

   @Id @GeneratedValue(strategy=GenerationType.AUTO)
   private long id = -1;

   @ManyToOne
   private PropertyType propertytype;
   
   public long getId() {
      return id;
   }

   public void setId(long id) {
      this.id = id;
   }
   
   public PropertyType getPropertytype() {
      return propertytype;
   }

   public void setPropertytype(PropertyType propertytype) {
      this.propertytype = propertytype;
   }

   public abstract void setValue(Object o);
   
   public abstract Object getValue();
   
}

@Entity @Table(name = "TEXTPROPERTY")
public class TextProperty extends Property {

   @Column @Type(type="org.hibernate.type.StringClobType")
   private String value;

   public Object getValue() {
      return value;
   }

   public void setValue(Object value) {
      this.value = (String) value;
   }
   
   
}

@Entity
@Table (name = "VARCHARPROP")
public class VarcharProperty extends Property {

   private String value;

   public Object getValue() {
      return value;
   }

   public void setValue(Object value) {
      this.value = (String) value;
   }
   
   
}

select value, propertytype_id, id, 1 as clazz_ from TEXTPROPERTY union all select value, propertytype_id, id, 2 as clazz_ from VARCHARPROP

<hibernate-mapping>

    <class name="events.Event" table="EVENTS">
        <id name="id" column="EVENT_ID">
            <generator class="native"/>
        </id>
        <property name="date" type="timestamp" column="EVENT_DATE"/>
        <property name="title"/>
    </class>

</hibernate-mapping>

Just as with the id element, the name attribute of the property element tells Hibernate which getter and setter methods to use. So, in this case, Hibernate will look for getDate()/setDate(), as well as getTitle()/setTitle().

Why does the date property mapping include the column attribute, but the title doesn't? Without the column attribute Hibernate by default uses the property name as the column name. This works fine for title. However, date is a reserved keyword in most database, so we better map it to a different name.

The next interesting thing is that the title mapping also lacks a type attribute. The types we declare and use in the mapping files are not, as you might expect, Java data types. They are also not SQL database types. These types are so called Hibernate mapping types, converters which can translate from Java to SQL data types and vice versa. Again, Hibernate will try to determine the correct conversion and mapping type itself if the type attribute is not present in the mapping. In some cases this automatic detection (using Reflection on the Java class) might not have the default you expect or need. This is the case with the date property. Hibernate can't know if the property (which is of java.util.Date) should map to a SQL date, timestamp, or time column. We preserve full date and time information by mapping the property with a timestamp converter.