Creating Mappings with Hibernate XML Files - Beginning Hibernate: From Novice to Professional

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This technique offers the best performance—for simple queries on simple classes even

in the deepest of inheritance hierarchies, a single select may suffice to gather all the fields to

populate the entity.

Conversely, this is not a satisfying representation of the attribute. Changes to members of

the hierarchy will usually require a column to be altered, added, or deleted from the table. This

will often be a very slow operation. As the hierarchy grows (horizontally as well as vertically),

so too will the number of columns required by this table.

Each mapped subclass must specify the class that it extends and a value that can be used

to discriminate this subclass from the other classes held in the same table. Thus, this is known

as the discriminator value, and is mapped with a discriminator-value attribute in the

<subclass> element (see Listing 7-10).

Listing 7-10. Mapping a Property Advert with the One-Table-per-Class-Hierarchy Approach

<hibernate-mapping>

<subclass

name="com.hibernatebook.xmlmapping.Property"

extends="com.hibernatebook.xmlmapping.Advert"

discriminator-value="property">

</subclass>

</hibernate-mapping>

Note that this also requires the specification of a discriminator column for the root of the

class hierarchy, from which the discriminator values identifying the types of the child classes

can be obtained (see Listing 7-11).

Listing 7-11. The Addition to Advert.hbm.xml Required to Support a One-Table-per-Class-

Hierarchy Approach

A subclass mapping cannot contain <joined-subclass> elements and vice versa—the two

strategies are not compatible.

More Exotic Mappings

The Hibernate mapping DTD is large. We have discussed the core set of mappings that you

will use on a day-to-day basis; but before we move on, we will take a very quick tour around

four of the more interesting remaining mapping types.

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