JPA - Embeddable class with entity relationships

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 Project

Dependencies 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