Mapping class inheritance in Java

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Mapping class inheritance
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You can override column mappings from the superclass in a subclass with the @AttributeOverride annotation. You rename the OWNER column to CC_OWNER in the CREDIT_CARD table. The database identifier can also be declared in the superclass, with a shared column name and generator strategy for all subclasses. Let s repeat the same mapping in a JPA XML descriptor:
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<entity-mappings> <mapped-superclass class="auction.model.BillingDetails" access="FIELD"> <attributes> ... </attributes> </mapped-superclass> <entity class="auction.model.CreditCard" access="FIELD"> <attribute-override name="owner"> <column name="CC_OWNER" nullable="false"/> </attribute-override> <attributes> ... </attributes> </entity> ... </entity-mappings>
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A component is a value type; hence, the normal entity inheritance rules presented in this chapter don t apply. However, you can map a subclass as a component by including all the properties of the superclass (or interface) in your component mapping. With annotations, you use the @MappedSuperclass annotation on the superclass of the embeddable component you re mapping just like you would for an entity. Note that this feature is available only in Hibernate Annotations and isn t standardized or portable.
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With the help of the SQL UNION operation, you can eliminate most of the issues with polymorphic queries and associations, which are present with this mapping strategy.
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Table per concrete class with unions
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First, let s consider a union subclass mapping with BillingDetails as an abstract class (or interface), as in the previous section. In this situation, we again have two tables and duplicate superclass columns in both: CREDIT_CARD and BANK_ACCOUNT. What s new is a special Hibernate mapping that includes the superclass, as you can see in listing 5.1.
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Inheritance and custom types
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Listing 5.1 Using the <union-subclass> inheritance strategy
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<hibernate-mapping> <class name="BillingDetails" abstract="true"> <id name="id" column="BILLING_DETAILS_ID" type="long"> <generator class="native"/> </id>
<property name="name" column="OWNER" type="string"/> ...
<union-subclass name="CreditCard" table="CREDIT_CARD"> <property name="number" column= NUMBER /> <property name="expMonth" column="EXP_MONTH"/> <property name="expYear" column="EXP_YEAR"/> </union-subclass> <union-subclass name="BankAccount" table="BANK_ACCOUNT"> ... </class> </hibernate-mapping>
An abstract superclass or an interface has to be declared as abstract="true"; otherwise a separate table for instances of the superclass is needed. The database identifier mapping is shared for all concrete classes in the hierarchy. The CREDIT_CARD and the BANK_ACCOUNT tables both have a BILLING_DETAILS_ID primary key column. The database identifier property now has to be shared for all subclasses; hence you have to move it into BillingDetails and remove it from CreditCard and BankAccount. Properties of the superclass (or interface) are declared here and inherited by all concrete class mappings. This avoids duplication of the same mapping. A concrete subclass is mapped to a table; the table inherits the superclass (or interface) identifier and other property mappings.
Mapping class inheritance
The first advantage you may notice with this strategy is the shared declaration of superclass (or interface) properties. No longer do you have to duplicate these mappings for all concrete classes Hibernate takes care of this. Keep in mind that the SQL schema still isn t aware of the inheritance; effectively, we ve mapped two unrelated tables to a more expressive class structure. Except for the different primary key column name, the tables look exactly alike, as shown in figure 5.1. In JPA annotations, this strategy is known as TABLE_PER_CLASS:
@Entity @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) public abstract class BillingDetails { @Id @GeneratedValue @Column(name = "BILLING_DETAILS_ID") private Long id = null; @Column(name = "OWNER", nullable = false) private String owner; ... }
The database identifier and its mapping have to be present in the superclass, to be shared across all subclasses and their tables. An @Entity annotation on each subclass is all that is required:
@Entity @Table(name = "CREDIT_CARD") public class CreditCard extends BillingDetails { @Column(name = "NUMBER", nullable = false) private String number; ... }
Note that TABLE_PER_CLASS is specified in the JPA standard as optional, so not all JPA implementations may support it. The actual implementation is also vendor dependent in Hibernate, it s equivalent to a <union-subclass> mapping in XML files. The same mapping looks like this in a JPA XML descriptor:
<entity-mappings> <entity class="auction.model.BillingDetails" access="FIELD"> <inheritance strategy="TABLE_PER_CLASS"/> ... </entity> <entity class="auction.model.CreditCard" access="FIELD"/>
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