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

[Updated: May 31, 2017, Created: May 20, 2017]

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.
  • JDK 1.8
  • Maven 3.3.9

Override Embeddable Attributes Select All Download
  • embeddable-multiple-instances
    • src
      • main
        • java
          • com
            • logicbig
              • example
        • resources
          • META-INF

See Also