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549 words

javax.persistence.MappedSuperclass - JPA annotation

javax.persistence
Annotation MappedSuperclass

Target:
Classes
Designates a class whose mapping information is applied to the entities that inherit from it. A mapped superclass has no separate table defined for it.

A class designated with the MappedSuperclass annotation can be mapped in the same way as an entity except that the mappings will apply only to its subclasses since no table exists for the mapped superclass itself. When applied to the subclasses the inherited mappings will apply in the context of the subclass tables. Mapping information may be overridden in such subclasses by using the AttributeOverride and AssociationOverride annotations or corresponding XML elements.

Example: Concrete class as a mapped superclass

    @MappedSuperclass
    public class Employee {
 
        @Id protected Integer empId;
        @Version protected Integer version;
        @ManyToOne @JoinColumn(name="ADDR")
        protected Address address;
 
        public Integer getEmpId() { ... }
        public void setEmpId(Integer id) { ... }
        public Address getAddress() { ... }
        public void setAddress(Address addr) { ... }
    }
 
    // Default table is FTEMPLOYEE table
    @Entity
    public class FTEmployee extends Employee {
 
        // Inherited empId field mapped to FTEMPLOYEE.EMPID
        // Inherited version field mapped to FTEMPLOYEE.VERSION
        // Inherited address field mapped to FTEMPLOYEE.ADDR fk
 
        // Defaults to FTEMPLOYEE.SALARY
        protected Integer salary;
 
        public FTEmployee() {}
 
        public Integer getSalary() { ... }
 
        public void setSalary(Integer salary) { ... }
    }
 
    @Entity @Table(name="PT_EMP")
    @AssociationOverride(
        name="address", 
        joincolumns=@JoinColumn(name="ADDR_ID"))
    public class PartTimeEmployee extends Employee {
 
        // Inherited empId field mapped to PT_EMP.EMPID
        // Inherited version field mapped to PT_EMP.VERSION
        // address field mapping overridden to PT_EMP.ADDR_ID fk
        @Column(name="WAGE")
        protected Float hourlyWage;
 
        public PartTimeEmployee() {}
 
        public Float getHourlyWage() { ... }
        public void setHourlyWage(Float wage) { ... }
    }

Since:
JPA 1.0
See Also:
AttributeOverride
AssociationOverride
This annotation is a marker annotation (with no elements).
This documentation page is derived (with some adjustments) from the open source JPA 2 RI (EclipseLink)
and is available under the terms of the Eclipse Public License, v. 1.0 and Eclipse Distribution License, v. 1.0.
Object Relational Mapping (ORM) JPA 2 providers include Hibernate, EclipseLink, TopLink, OpenJPA and DataNucleus.
Object DB is not an ORM JPA implementation but an Object Database for Java with built in JPA 2 support.