JPA - Using multiple instances of the same embeddable in an entity

Multiple instances of the same embeddable class can be embedded in the same entity. In that case we cannot use the default column names of the embeddable fields/properties; We must resolve the conflicts between the same attributes of the different instances. Although, we can use @Column annotation in the embeddable class, but in this specific case where we have multiple instances of the same embeddable, we must have a way to specify different column names for different 'instances'. For that we have to use @AttributeOverrides and @AttributeOverride annotations. Let's understand that with an example.

Example

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

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

    @AttributeOverrides({
            @AttributeOverride(name = "myStr", column = @Column(name = "MY_STR_COL1")),
            @AttributeOverride(name = "myInt", column = @Column(name = "MY_INT_COL1"))
    })
    @Embedded
    private ClassA classARef;

    @AttributeOverrides({
            @AttributeOverride(name = "myStr", column = @Column(name = "MY_STR_COL2")),
            @AttributeOverride(name = "myInt", column = @Column(name = "MY_INT_COL2"))
    })
    @Embedded
    private ClassA classARef2;
    .............
}

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");
            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]
'SHOW COLUMNS from EntityA'
[ID, INTEGER(10), NO, PRI, NULL]
[MY_INT_COL1, INTEGER(10), YES, , NULL]
[MY_STR_COL1, VARCHAR(255), YES, , NULL]
[MY_INT_COL2, INTEGER(10), YES, , NULL]
[MY_STR_COL2, 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();

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

        ClassA a2 = new ClassA();
        a2.setMyInt(20);
        a2.setMyStr("a test String 2");
        entityA.setClassARef2(a2);

        System.out.println(entityA);

        em.getTransaction().begin();
        em.persist(entityA);
        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");
    }

    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);
        em.close();
    }
}

Output

-- Persisting entities --
EntityA{id=0, classARef=ClassA{myStr='a test String', myInt=10}, classARef2=ClassA{myStr='a test String 2', myInt=20}}
-- native queries --
'Select * from EntityA'
[1, 10, a test String, 20, a test String 2]
-- Loading EntityA --
EntityA{id=1, classARef=ClassA{myStr='a test String', myInt=10}, classARef2=ClassA{myStr='a test String 2', myInt=20}}

org.hibernate.MappingException: Repeated column in mapping

If we don't use @AttributeOverrides then we will have MappingException. Let's check that out:

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

  @Embedded
  private ClassA classARef;

  @Embedded
  private ClassA classARef2;
    .............
}

Output

On running ExampleMain, we will have following output

Caused by: org.hibernate.MappingException: Repeated column in mapping for entity: com.logicbig.example.EntityA column: myInt (should be mapped with insert="false" update="false")
	at org.hibernate.mapping.PersistentClass.checkColumnDuplication(PersistentClass.java:835)
	at org.hibernate.mapping.PersistentClass.checkPropertyColumnDuplication(PersistentClass.java:853)
	at org.hibernate.mapping.PersistentClass.checkPropertyColumnDuplication(PersistentClass.java:849)
    ........................

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