Hibernate Inheritance and polymorphism

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Hibernate Tutorial 08 Inheritance Mapping
1. Inheritance and polymorphism
Suppose that our bookshop is also selling CDs and DVDs to our customers. We
first create a class
Disc and provide a mapping definition for it.
public class Disc {
private Long id;
private String name;
private Integer price;
// Getters and Setters
}
<hibernate-mapping package="com.metaarchit.bookshop">
<class name="Disc" table="DISC">
<id name="id" type="long" column="ID">
<generator class="native"/>
</id>
<property name="name" type="string" column="NAME" />
<property name="price" type="int" column="PRICE" />
</class>
</hibernate-mapping>
1.1. Inheritance
There are mainly two kinds of discs we are selling, audio discs and video
discs. Each kind has
different properties from the other. From the orient-oriented perspective,
we should model these two
kinds of discs, AudioDisc and VideoDisc, as "subclasses" of Disc to
represent an "is-a" relationship.
In Java, we use the "extends" keyword to define subclass of a class.
Reversely, the class Disc is
called the "superclass" or "parent class" of AudioDisc and VideoDisc.
public class AudioDisc extends Disc {
private String singer;
private Integer numOfSongs;
// Getters and Setters
}
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public class VideoDisc extends Disc {
private String director;
private String language;
// Getters and Setters
}
The relationship from a subclass (e.g. AudioDisc and VideoDisc) to its
parent class (e.g. Disc) is
called "inheritance". All the subclasses and their parents make up a "class
hierarchy".
In relational model, there's not a concept of inheritance. That means we
must define a mapping
mechanism to persist the inheritance relationships of our object model.
However, Hibernate is
providing several strategies to make it easy.
1.2. Polymorphism
For our disc hierarchy, we can use the following query to find all the
discs in our system, no matter
audio or video ones. This kind of query is called "polymorphic query".
Session session = factory.openSession();
try {
Query query = session.createQuery("from Disc");
List discs = query.list();
return discs;
} finally {
session.close();
}
Suppose we would like to support disc reservation for our online bookshop.
We create a class
Reservation and define a many-to-one association to the Disc class. As the
concrete class of the disc
may be AudioDisc or VideoDisc and it can only be determined at runtime,
this kind of association is
called "polymorphic association".
public class Reservation {
private Long id;
private Disc disc;
private Customer customer;
private int quantity;
}
<hibernate-mapping package="com.metaarchit.bookshop">
<class name="Reservation" table="RESERVATION">
<id name="id" type="long" column="ID">
<generator class="native" />
</id>
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<many-to-one name="disc" class="Disc" column="DISC_ID" />
...
</class>
</hibernate-mapping>
2. Mapping inheritance
Hibernate is providing three main strategies for mapping inheritance
relationships. Each of them has
particular advantages and disadvantages.
2.1. Table per class hierarchy
This strategy assigns a single table to store all the properties of all the
subclasses/non-subclasses
within a class hierarchy. In addition, a special column called
"discriminator" is used to distinguish
the type of the object.
<hibernate-mapping package="com.metaarchit.bookshop">
<class name="Disc" table="Disc">
<id name="id" type="long" column="ID">
<generator class="native"/>
</id>
<discriminator column="DISC_TYPE" type="string" />
...
<subclass name="AudioDisc" discriminator-value="AUDIO">
<property name="singer" type="string" column="SINGER" />
<property name="numOfSongs" type="int" column="NUM_OF_SONGS" />
</subclass>
<subclass name="VideoDisc" discriminator-value="VIDEO">
<property name="director" type="string" column="DIRECTOR" />
<property name="language" type="string" column="LANGUAGE" />
</subclass>
</class>
</hibernate-mapping>
The main advantage of this strategy is simple and efficient, especially for
polymorphic queries and
associations, since one table contains all the data and no table join is
required. However, this
strategy has a fatal limitation that all the properties in subclasses must
not have not-null constraint.
2.2. Table per subclass
This strategy uses one table for each subclass and non-subclass. It defines
a foreign key in the table
of subclass that references the table of its parent. You can imagine
there's a one-to-one association
from the subclass to its parent.
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<hibernate-mapping package="com.metaarchit.bookshop">
<class name="Disc" table="DISC">
...
<joined-subclass name="AudioDisc" table="AUDIO_DISC">
<key column="DISC_ID" />
<property name="singer" type="string" column="SINGER" />
<property name="numOfSongs" type="int" column="NUM_OF_SONGS" />
</joined-subclass>
<joined-subclass name="VideoDisc" table="VIDEO_DISC">
<key column="DISC_ID" />
<property name="director" type="string" column="DIRECTOR" />
<property name="language" type="string" column="LANGUAGE" />
</joined-subclass>
</class>
</hibernate-mapping>
This strategy has no limitation on not-null constraint but it is less
efficient, since several tables need
to be joined for retrieving a single object. For polymorphic queries and
associations, more tables
need to be joined.
2.3. Table per concrete class
The last strategy is to assign each concrete class an isolated table that
duplicates all the columns for
inherited properties. Notice that we can no longer use identity id
generation since the id need to be
unique across several tables. This strategy is not efficient for
polymorphic queries and associations
since several tables need to be navigated for retrieving the objects.
<hibernate-mapping package="com.metaarchit.bookshop">
<class name="Disc" table="DISC">
<id name="id" type="long" column="ID">
<generator class="sequence">
<param name="sequence">DISC_SEQUENCE</param>
</generator>
</id>
...
<union-subclass name="AudioDisc" table="AUDIO_DISC">
<property name="singer" type="string" column="SINGER" />
<property name="numOfSongs" type="int" column="NUM_OF_SONGS" />
</union-subclass>
<union-subclass name="VideoDisc" table="VIDEO_DISC">
<property name="director" type="string" column="DIRECTOR" />
<property name="language" type="string" column="LANGUAGE" />
</union-subclass>
</class>
</hibernate-mapping>

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