Revision as of 14:21, 30 June 2011

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Key API

• @MappedSuperclass
• @AttributeOverride
• @AssociationOverride

Examples

• Advanced Inheritance Examples

@MappedSuperclass

You can use the @MappedSuperclass annotation or <mapped-superclass> XML element to define mappings for an abstract or non-persistent superclass, that are inherited by its subclass entities. A mapped superclass is a special type of class that is not persistent itself, but has subclasses that are persistent. A mapped superclass is useful for defined a common persistence superclass that defines common behavior across a set of classes, such as an id or version attribute.

The @MappedSuperclass annotation does not have any attributes.

For more information, see Section 2.1.9 "Inheritance" in the JPA Specification.

For more information, see Section 11.1.34 "MappedSuperclass Annotation" in the JPA Specification.

@AttributeOverride

You can use the @AttributeOverride and @AttributeOverrides annotations, or <attribute-override> XML element to configure the name or type of the inheritance discriminator column. The discriminator column is required for SINGLE_TABLE and JOINED inheritance and stores the associated entity type for the row. The default name for the discriminator column is DTYPE. JPA only allows String or Integer values for discriminators. Through the EclipseLink API, it is possible to use other discriminator types, and it is possible to not have a discriminator, or use custom discriminator, see Advanced Inheritance Configuration.

For more information, see Section 11.1.4 "AttributeOverride Annotation" in the JPA Specification.

@AssociationOverride

You can use the @AssociationOverride and @AssociationOverrides annotations, or <association-override> XML element to configure the name or type of the inheritance discriminator column. The discriminator column is required for SINGLE_TABLE and JOINED inheritance and stores the associated entity type for the row. The default name for the discriminator column is DTYPE. JPA only allows String or Integer values for discriminators. Through the EclipseLink API, it is possible to use other discriminator types, and it is possible to not have a discriminator, or use custom discriminator, see Advanced Inheritance Configuration.

For more information, see Section 11.1.2 "AssociationOverride Annotation" in the JPA Specification.

The following examples shows usages of the three different inheritance strategies for mapping an Account hierarchy.

Example: Using SINGLE_TABLE with @Inheritance annotation

CREATE TABLE ACCOUNT (ID NUMBER, ACCOUNT_TYPE VARCHAR(31), BALANCE NUMBER, INTERESTRATE NUMBER, RETURNCHECKS BOOLEAN)

@Entity
@Table(name="ACCOUNT")
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name="ACCOUNT_TYPE")
public abstract class Account implements Serializable {
    @Id
    private Long id;
    @Basic
    private BigDecimal balance;
    ...
}

@Entity
@DiscriminatorValue("SAVINGS")
public class SavingAccount extends Account {
    @Basic
    private BigDecimal interestRate;
}

@Entity
@DiscriminatorValue("CHECKING")
public class CheckingAccount extends Account {
    @Basic
    private boolean returnChecks;
}

Example: Using SINGLE_TABLE with <inheritance> XML

<entity class="Account">
    <table name="ACCOUNT"/>
    <inheritance strategy="SINGLE_TABLE"/>
    <discriminator-column name="ACCOUNT_TYPE"/>
    <attributes>
        <id name="id"/>
        <basic name="balance"/>
    </attributes>
</entity>

<entity class="SavingAccount">
    <discriminator-value>SAVINGS</discriminator-value>
    <attributes>
        <basic name="interestRate"/>
    </attributes>
</entity>

<entity class="CheckingAccount">
    <discriminator-value>CHECKING</discriminator-value>
    <attributes>
        <basic name="returnChecks"/>
    </attributes>
</entity>

@Inheritance	Configuring Entities	Configuring Mappings
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