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EclipseLink/UserGuide/JPA/Basic JPA Development/Entities/MappedSuperclass

EclipseLink JPA


@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. A mapped superclass should normally be abstract but is not required to be, but cannot have any persistent instances.

Mapped superclasses cannot define a table, but can define mapping for its attributes and other common persistence behavior. Entities cannot have relatinoships to mapped superclasses, and mapped superclasses cannot be queried.

The @MappedSuperclass annotation does not have any attributes.

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

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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 override the column for a basic attribute in a mapped superclass. This allows for the column name to be different in each subclass.

@AttributeOverride Attributes
Attribute Description Default Required?
name The name of the attribute. Yes
column The column in the subclass table. column defined in mapped superclass No
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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 override the join column or join table for a relationship attribute in a mapped superclass. This allows for the join column name or join table to be different in each subclass.

@AssociationOverrideAttributes
Attribute Description Default Required?
name The name of the attribute. Yes
joinColumn The join column in the subclass table. join column defined in mapped superclass No
joinTable The join table for the subclass. join table defined in mapped superclass No
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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>


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Version: 2.2.0 DRAFT
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