- A mapped superclass provides persistent entity state and mapping information but is not itself an entity.
- A mapped superclass, unlike an entity, does not allow querying, persisting, or relationships to the superclass.
- @MappedSuperclass annotation is used to designate a class as mapped superclass.
- All mappings annotation can be used on the root class except for @Entity. Also persistent relationships defined by a mapped superclass must be unidirectional.
- Mapping information may be overridden in the subclasses by using the @AttributeOverride and @AssociationOverride annotations.
- Both abstract and concrete classes may be specified as mapped superclasses.
- It is similar to table per class inheritance but no table joins or inheritance exists in data model. There's no table for the mapped superclass. Inheritance only exists in object model.
- The main disadvantage of using mapped superclass is that we cannot load all entities (subclasses) represented by the mapped superclass, i.e., polymorphic queries are not possible.
Example
@MappedSuperclass
public class Employee {
@Id
@GeneratedValue
private long id;
private String name;
.............
}
@Entity
@Table(name = "FULL_TIME_EMP")
public class FullTimeEmployee extends Employee {
private int salary;
.............
}
@Entity
@Table(name = "PART_TIME_EMP")
public class PartTimeEmployee extends Employee {
private int hourlyRate;
.............
}
public class ExampleMain {
public static void main(String[] args) {
EntityManagerFactory emf =
Persistence.createEntityManagerFactory("example-unit");
try {
EntityManager em = emf.createEntityManager();
nativeQuery(em, "SHOW TABLES");
nativeQuery(em, "SHOW COLUMNS from FULL_TIME_EMP");
nativeQuery(em, "SHOW COLUMNS from PART_TIME_EMP");
} finally {
emf.close();
}
}
.............
}'SHOW TABLES'
[FULL_TIME_EMP, PUBLIC]
[PART_TIME_EMP, PUBLIC]
'SHOW COLUMNS from FULL_TIME_EMP'
[ID, BIGINT(19), NO, PRI, NULL]
[NAME, VARCHAR(255), YES, , NULL]
[SALARY, INTEGER(10), NO, , NULL]
'SHOW COLUMNS from PART_TIME_EMP'
[ID, BIGINT(19), NO, PRI, NULL]
[NAME, VARCHAR(255), YES, , NULL]
[HOURLYRATE, INTEGER(10), NO, , NULL]
A quick overview of the mapping:
Persisting and loading data
public class ExampleMain2 {
public static void main(String[] args) throws Exception {
EntityManagerFactory emf =
Persistence.createEntityManagerFactory("example-unit");
try {
persistEntities(emf);
runNativeQueries(emf);
loadEntities(emf);
} finally {
emf.close();
}
}
private static void persistEntities(EntityManagerFactory emf) throws Exception {
System.out.println("-- Persisting entities --");
EntityManager em = emf.createEntityManager();
FullTimeEmployee e1 = new FullTimeEmployee();
e1.setName("Sara");
e1.setSalary(100000);
System.out.println(e1);
PartTimeEmployee e2 = new PartTimeEmployee();
e2.setName("Robert");
e2.setHourlyRate(60);
System.out.println(e2);
em.getTransaction().begin();
em.persist(e1);
em.persist(e2);
em.getTransaction().commit();
em.close();
}
private static void runNativeQueries(EntityManagerFactory emf) {
System.out.println("-- Native queries --");
EntityManager em = emf.createEntityManager();
ExampleMain.nativeQuery(em, "Select * from FULL_TIME_EMP");
ExampleMain.nativeQuery(em, "Select * from PART_TIME_EMP");
}
private static void loadEntities(EntityManagerFactory emf) {
System.out.println("-- Loading entities --");
EntityManager em = emf.createEntityManager();
List<Employee> entityAList = em.createQuery("Select t from FullTimeEmployee t")
.getResultList();
entityAList.forEach(System.out::println);
entityAList = em.createQuery("Select t from PartTimeEmployee t")
.getResultList();
entityAList.forEach(System.out::println);
em.close();
}
}-- Persisting entities --
FullTimeEmployee{id=0, name='Sara', salary=100000}
PartTimeEmployee{id=0, name='Robert', hourlyRate='60'}
-- Native queries --
'Select * from FULL_TIME_EMP'
[1, Sara, 100000]
'Select * from PART_TIME_EMP'
[2, Robert, 60]
-- Loading entities --
FullTimeEmployee{id=1, name='Sara', salary=100000}
PartTimeEmployee{id=2, name='Robert', hourlyRate='60'}
Note that, we cannot create a JPQL query like 'select t from Employee t' which can load all subclasses. That is only possible when we use an inheritance strategy: SINGLE_TABLE or JOINED. or TABLE_PER_CLASS.
Example ProjectDependencies and Technologies Used: - h2 1.4.196: H2 Database Engine.
- hibernate-core 5.2.10.Final: The core O/RM functionality as provided by Hibernate.
Implements javax.persistence:javax.persistence-api version 2.1 - JDK 1.8
- Maven 3.3.9
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