JPA - Mapping java.util.Map with @Embeddable keys with @CollectionTable and @Column

By default, mapping a Map of embeddable type keys and basic type values, JPA naming conventions are used for join table/columns. we can use @CollectionTable to customize the join table. The column in the join table, which maps to the map values, can be customized by using @Column.

Note that @MapKeyColumn has not effect, because there's no single column which points to the map key.

Example

@Entity
public class Customer {
  @Id
  @GeneratedValue
  private int id;
  private String name;

  @ElementCollection
  @CollectionTable(name = "ORDERS", joinColumns = @JoinColumn(name = "CUST_ID"))
  @Column(name = "ORDER_TYPE")
  private Map<Order, String> orders;
    .............
}

@Embeddable
public class Order {
  private String item;
  private int qty;
  private Date date;
    .............
}

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 CUSTOMER");
          nativeQuery(em, "SHOW COLUMNS from ORDERS");
      } 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'
[CUSTOMER, PUBLIC]
[ORDERS, PUBLIC]
'SHOW COLUMNS from CUSTOMER'
[ID, INTEGER(10), NO, PRI, NULL]
[NAME, VARCHAR(255), YES, , NULL]
'SHOW COLUMNS from ORDERS'
[CUST_ID, INTEGER(10), NO, PRI, NULL]
[ORDER_TYPE, VARCHAR(255), YES, , NULL]
[DATE, TIMESTAMP(23), NO, PRI, NULL]
[ITEM, VARCHAR(255), NO, PRI, NULL]
[QTY, INTEGER(10), NO, PRI, 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();

      Customer c1 = new Customer();
      c1.setName("Lindsey Craft");
      c1.addOrder("online", "XYZ Blender", 2);
      c1.addOrder("store", "ZZZ Beer Glass", 4);
      System.out.println(c1);

      Customer c2 = new Customer();
      c2.setName("Morgan Philips");
      c2.addOrder("online", "AA Glass Cleaner", 3);
      System.out.println(c2);

      em.getTransaction().begin();
      em.persist(c1);
      em.persist(c2);
      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 Customer");
      ExampleMain.nativeQuery(em, "Select * from Orders");
  }

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

Output

-- Persisting entities --
Customer{id=0, name='Lindsey Craft', orders={Order{item='XYZ Blender', qty=2, date=Sat Jun 10 13:17:12 CDT 2017}=online, Order{item='ZZZ Beer Glass', qty=4, date=Sat Jun 10 13:17:12 CDT 2017}=store}}
Customer{id=0, name='Morgan Philips', orders={Order{item='AA Glass Cleaner', qty=3, date=Sat Jun 10 13:17:12 CDT 2017}=online}}
-- Native queries --
'Select * from Customer'
[1, Lindsey Craft]
[2, Morgan Philips]
'Select * from Orders'
[1, online, 2017-06-10 13:17:12.762, XYZ Blender, 2]
[1, store, 2017-06-10 13:17:12.762, ZZZ Beer Glass, 4]
[2, online, 2017-06-10 13:17:12.766, AA Glass Cleaner, 3]
-- Loading Customer --
Customer{id=1, name='Lindsey Craft', orders={Order{item='XYZ Blender', qty=2, date=2017-06-10 13:17:12.762}=online, Order{item='ZZZ Beer Glass', qty=4, date=2017-06-10 13:17:12.762}=store}}
Customer{id=2, name='Morgan Philips', orders={Order{item='AA Glass Cleaner', qty=3, date=2017-06-10 13:17:12.766}=online}}

Example Project

Dependencies and Technologies Used:

• h2 1.4.194: H2 Database Engine.
• hibernate-core 5.2.10.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