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EclipseLink/UserGuide/JPA/Basic JPA Development/Entities/MappedSuperclass
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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. 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.
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 override the column for a basic attribute in a mapped superclass. This allows for the column name to be different in each subclass.
Attribute | Description | Default | Required? |
---|---|---|---|
name | The name of the attribute. | Yes | |
column | The column in the subclass table. | column defined in mapped superclass |
No |
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.
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 |
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>