An embeddable class may contain a relationship with an entity. Let's see that with an example
Example
@Entity
public class EntityA {
@Id
@GeneratedValue
private int id;
@Embedded
private ClassA classARef;
.............
}
The @Embeddable ClassA has ManyToOne relationship with entityB:
@Embeddable
public class ClassA {
private String myStr;
@ManyToOne
private EntityB entityBRef;
.............
}
@Entity
public class EntityB {
@Id
@GeneratedValue
private int myIdB;
private String myStr2;
.............
}
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 EntityA");
nativeQuery(em, "SHOW COLUMNS from EntityB");
emf.close();
} finally {
emf.close();
}
}
public static void nativeQuery(EntityManager em, String s) {
System.out.printf("'%s'%n", s);
Query query = em.createNativeQuery(s);
List list = query.getResultList();
for (Object o : list) {
if(o instanceof Object[]) {
System.out.println(Arrays.toString((Object[]) o));
}else{
System.out.println(o);
}
}
}
} Output'SHOW TABLES' [ENTITYA, PUBLIC] [ENTITYB, PUBLIC] 'SHOW COLUMNS from EntityA' [ID, INTEGER(10), NO, PRI, NULL] [MYSTR, VARCHAR(255), YES, , NULL] [ENTITYBREF_MYIDB, INTEGER(10), YES, , NULL] 'SHOW COLUMNS from EntityB' [MYIDB, INTEGER(10), NO, PRI, NULL] [MYSTR2, VARCHAR(255), YES, , NULL]
The foreign key column ENTITYBREF_MYIDB is in EntityA table, that's because EntityA having the embedded class in it is just like: EntityA has direct ManyToOne relationship to EntityB
Persisting and loading data
public class ExampleMain2 {
public static void main(String[] args) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory("example-unit");
try {
persistEntity(emf);
runNativeQueries(emf);
loadEntity(emf);
} finally {
emf.close();
}
}
private static void persistEntity(EntityManagerFactory emf) {
System.out.println("-- Persisting entities --");
EntityManager em = emf.createEntityManager();
EntityB entityB = new EntityB();
entityB.setMyStr2("a test string in EntityB");
ClassA a = new ClassA();
a.setMyStr("a test string in ClassA");
a.setEntityBRef(entityB);
EntityA entityA = new EntityA();
entityA.setClassARef(a);
System.out.println(entityA);
em.getTransaction().begin();
em.persist(entityA);
em.persist(entityB);
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 EntityA");
ExampleMain.nativeQuery(em, "Select * from EntityB");
}
private static void loadEntity(EntityManagerFactory emf) {
System.out.println("-- Loading EntityA --");
EntityManager em = emf.createEntityManager();
List<EntityA> entityAList = em.createQuery("Select t from EntityA t")
.getResultList();
entityAList.forEach(System.out::println);
System.out.println("-- Loading EntityB --");
List<EntityB> entityBList = em.createQuery("Select t from EntityB t")
.getResultList();
entityBList.forEach(System.out::println);
em.close();
}
} Output-- Persisting entities -- EntityA{id=0, classARef=ClassA{myStr='a test string in ClassA', entityBRef=EntityB{myIdB=0, myStr2='a test string in EntityB'}}} -- native queries -- 'Select * from EntityA' [1, a test string in ClassA, 2] 'Select * from EntityB' [2, a test string in EntityB] -- Loading EntityA -- EntityA{id=1, classARef=ClassA{myStr='a test string in ClassA', entityBRef=EntityB{myIdB=2, myStr2='a test string in EntityB'}}} -- Loading EntityB -- EntityB{myIdB=2, myStr2='a test string in EntityB'}
Example ProjectDependencies and Technologies Used: - h2 1.4.193: H2 Database Engine.
- hibernate-core 5.2.8.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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