JPA - Embeddable class within another embeddable class Example

An embeddable class may include another embeddable class. Let's see that with an example.

Example

@Embeddable
public class ClassA {
    private String myStr;
    private int myInt;
    .............
}

@Embeddable
public class ClassB {
    private String myStrB;
    @Embedded
    private ClassA classARef;
    .............
}

@Entity
public class MyEntity {
    @Id
    @GeneratedValue
    private int id;

    @Embedded
    private ClassB classBRef;
    .............
}

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 MyEntity");
            emf.close();
        } finally {
            emf.close();
        }
    }

    public static void nativeQuery(EntityManager em, String s) {
        System.out.printf("-----------------------------%n'%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'
[MYENTITY, PUBLIC]
-----------------------------
'SHOW COLUMNS from MyEntity'
[ID, INTEGER(10), NO, PRI, NULL]
[MYINT, INTEGER(10), NO, , NULL]
[MYSTR, VARCHAR(255), YES, , NULL]
[MYSTRB, VARCHAR(255), YES, , NULL]

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();

        ClassA a = new ClassA();
        a.setMyInt(10);
        a.setMyStr("a test String");

        ClassB b = new ClassB();
        b.setMyStrB("a test string in ClassB");
        b.setClassARef(a);

        MyEntity entityB = new MyEntity();
        entityB.setClassBRef(b);

        System.out.println(entityB);

        em.getTransaction().begin();
        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 MyEntity");
    }

    private static void loadEntity(EntityManagerFactory emf) {
        System.out.println("-- Loading MyEntity --");
        EntityManager em = emf.createEntityManager();
        List<MyEntity> entityList = em.createQuery("Select t from MyEntity t")
                                      .getResultList();
        entityList.forEach(System.out::println);
        em.close();
    }
}

Output

-- Persisting entities --
EntityB{id=0, classBRef=ClassB{myStrB='a test string in ClassB', classARef=ClassA{myStr='a test String', myInt=10}}}
-- native queries --
-----------------------------
'Select * from MyEntity'
[1, 10, a test String, a test string in ClassB]
-- Loading MyEntity --
EntityB{id=1, classBRef=ClassB{myStrB='a test string in ClassB', classARef=ClassA{myStr='a test String', myInt=10}}}

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