Mapping with Annotations - Beginning Hibernate: From Novice to Professional - page 121

name is the name of the discriminator column.
discriminatorType is the type of value to be stored in the column as selected from the
javax.persistence.DiscriminatorType enumeration of STRING , CHAR , or INTEGER .
columnDefinition is a fragment of DDL defining the column type. Using this is liable
to reduce the portability of your code across databases.
length is the column length of STRING discriminator types. It is ignored for CHAR and
INTEGER types.
All of these (and the annotation itself ) are optional, but we recommend supplying at least
the name attribute. If no @DiscriminatorColumn is specified in the hierarchy, a default column
name of DTYPE and type of STRING will be used.
Hibernate will supply an appropriate discriminator value for each of your entities. For
example, if the STRING discriminator type is used, the value this column contains will be the
name of the entity (which defaults to the class name). You can also override this behavior with
specific values using the @DiscriminatorValue annotation. If the discriminator type is INTEGER ,
any value provided via the @DiscriminatorValue annotation must be convertible directly into
an integer.
In Listing 6-22, we specify that an INTEGER discriminator type should be stored in the
column named DISCRIMINATOR . Rows representing Book entities will have a value of 1 in
this column, whereas the following mapping in Listing 6-23 requires that rows represent-
ing ComputerBook entities should have a value of 2 in the same column.
Listing 6-22. The Root of the Inheritance Hierarchy Mapped with the SINGLE_TABLE Strategy
@Entity
@Inheritance(strategy = SINGLE_TABLE)
@DiscriminatorColumn(
name="DISCRIMINATOR",
discriminatorType=INTEGER
)
@DiscriminatorValue("1")
public class Book {
...
}
Listing 6-23. A Derived Entity in the Inheritance Hierarchy
@Entity
@DiscriminatorValue("2")
public class ComputerBook extends Book {
...
}
An alternative to the monolithic single table approach is the otherwise similar joined
table approach. Here a discriminator column is used, but the fields of the various derived
types are stored in distinct tables. Other than the differing strategy, this inheritance type is
specified in the same way (as shown in Listing 6-24).