Latest revision as of 11:53, 28 July 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. 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 relationships 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.

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.

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

The following example shows usages of a generic persistence mapped superclass.

Example: Using @MappedSuperClass annotation

@MappedSuperclass
public abstract class PersistentObject implements Serializable {
    @Id
    private Long id;
    @Version
    private Long version;
    ...
}

@Entity
@Table("EMP")
public class Employee extends PersistentObject {
    @Basic
    private String name;
    ...
}

@Entity
@Table("PROJECT")
@AttributeOverride(name="id" column=@Column(name="PROJ_ID"))
public class Project extends PersistentObject {
    @Basic
    private String name;
    ...
}

Example: Using <mapped-superclass> XML

<mapped-superclass class="PersistentObject">
    <attributes>
        <id name="id"/>
        <version name="version"/>
    </attributes>
</mapped-superclass>
<entity class="Employee">
    <table name="EMP">
    <attributes>
        <basic name="name"/>
        ...
    </attributes>
</entity>
<entity class="Project">
    <table name="PROJECT">
    <attribute-override name="id">
        <column name="PROJ_ID"/>
    </attribute-override>
    <attributes>
        ...
    </attributes>
</entity>

@Inheritance	Configuring Entities	@Embeddable
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