JPA - Bidirectional OneToMany/ManyToOne Example

Example

@Entity
public class EntityA {
  @Id
  @GeneratedValue
  private int myIdA;
  private String strA;
  @OneToMany(mappedBy = "refEntityA")
  private List<EntityB> entityBList;
    .............
}

@Entity
public class EntityB {
  @Id
  @GeneratedValue
  private int myIdB;
  private String strB;
  @ManyToOne
  private EntityA refEntityA;
    .............
}

public class ExampleMain {

      public static void main(String[] args) {
      EntityManagerFactory emf = Persistence.createEntityManagerFactory("test1");
      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("-----------------------------%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'
[ENTITYA, PUBLIC]
[ENTITYB, PUBLIC]
-----------------------------
'SHOW COLUMNS from EntityA'
[MYIDA, INTEGER(10), NO, PRI, NULL]
[STRA, VARCHAR(255), YES, , NULL]
-----------------------------
'SHOW COLUMNS from EntityB'
[MYIDB, INTEGER(10), NO, PRI, NULL]
[STRB, VARCHAR(255), YES, , NULL]
[REFENTITYA_MYIDA, INTEGER(10), YES, , NULL]

H2 database SHOW statements

Following is a quick overview of the relationship:

Persisting and loading entities

public class ExampleMain2 {

  public static void main(String[] args) {
      EntityManagerFactory emf = Persistence.createEntityManagerFactory("test1");
      try {
          persistEntity(emf);
          nativeQueries(emf);
          loadEntityA(emf);
          loadEntityB(emf);
      } finally {
          emf.close();
      }
  }

  private static void nativeQueries(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 persistEntity(EntityManagerFactory emf) {
      EntityManager em = emf.createEntityManager();

      EntityB entityB1 = new EntityB();
      entityB1.setStrB("testStringB");

      EntityB entityB2 = new EntityB();
      entityB2.setStrB("testStringB2");

      EntityA entityA = new EntityA();
      entityA.setStrA("testStringA");
      entityA.setEntityBList(Arrays.asList(entityB1, entityB2));

      entityB1.setRefEntityA(entityA);
      entityB2.setRefEntityA(entityA);

      System.out.println("-- persisting entities --");
      System.out.printf(" %s%n entityA#entityBList: %s%n", entityA, entityA.getEntityBList());
      System.out.printf(" %s%n entityB1#refEntityA: %s%n", entityB1, entityB1.getRefEntityA());
      System.out.printf(" %s%n entityB2#refEntityA: %s%n", entityB2, entityB2.getRefEntityA());

      em.getTransaction().begin();
      em.persist(entityA);
      em.persist(entityB1);
      em.persist(entityB2);
      em.getTransaction().commit();

      em.close();
  }

  private static void loadEntityA(EntityManagerFactory emf) {
      System.out.println("-- loading EntityA --");
      EntityManager em = emf.createEntityManager();
      List<EntityA> entityAList = em.createQuery("Select t from EntityA t").getResultList();
      entityAList.forEach(e -> System.out.printf(" %s%n entityA#entityBList: %s%n", e, e.getEntityBList()));
      em.close();
  }

  private static void loadEntityB(EntityManagerFactory emf) {
      System.out.println("-- Loading EntityB --");
      EntityManager em = emf.createEntityManager();
      List<EntityB> entityBList = em.createQuery("Select t from EntityB t").getResultList();
      entityBList.forEach(e -> System.out.printf(" %s%n entityB#refEntityA: %s%n", e, e.getRefEntityA()));
      em.close();
  }
}

Output

-- persisting entities --
 EntityA{myIdA=0, strA='testStringA'}
 entityA#entityBList: [EntityB{myIdB=0, strB='testStringB'}, EntityB{myIdB=0, strB='testStringB2'}]
 EntityB{myIdB=0, strB='testStringB'}
 entityB1#refEntityA: EntityA{myIdA=0, strA='testStringA'}
 EntityB{myIdB=0, strB='testStringB2'}
 entityB2#refEntityA: EntityA{myIdA=0, strA='testStringA'}
-- native queries --
-----------------------------
'Select * from EntityA'
[1, testStringA]
-----------------------------
'Select * from EntityB'
[2, testStringB, 1]
[3, testStringB2, 1]
-- loading EntityA --
 EntityA{myIdA=1, strA='testStringA'}
 entityA#entityBList: [EntityB{myIdB=2, strB='testStringB'}, EntityB{myIdB=3, strB='testStringB2'}]
-- Loading EntityB --
 EntityB{myIdB=2, strB='testStringB'}
 entityB#refEntityA: EntityA{myIdA=1, strA='testStringA'}
 EntityB{myIdB=3, strB='testStringB2'}
 entityB#refEntityA: EntityA{myIdA=1, strA='testStringA'}

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