JPA - Customizing the mapping of java.util.Map

The mapping of java.util.Map can be customized by using one or more of the followings annotations on the map field/property:

• @CollectionTable:

  To customize the map table (the the foreign key table containing map's key/value columns).
  
  

• @MapKeyColumn:

  To customize the 'key' column in the map table.
  
  

• @Column:

  To customize the 'value' column in the map table.
  
  

Example

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

    private String name;
    @ElementCollection
    @MapKeyColumn(name = "item")
    @Column(name = "qty")
    @CollectionTable(name = "ITEM_QTY")
    private Map<String, Integer> itemQtyMap;
    .............
}

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 ITEM_QTY");

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

Output

'SHOW TABLES'
[CUSTOMER, PUBLIC]
[ITEM_QTY, PUBLIC]
'SHOW COLUMNS from Customer'
[ID, INTEGER(10), NO, PRI, NULL]
[NAME, VARCHAR(255), YES, , NULL]
'SHOW COLUMNS from ITEM_QTY'
[CUSTOMER_ID, INTEGER(10), NO, PRI, NULL]
[QTY, INTEGER(10), YES, , NULL]
[ITEM, VARCHAR(255), 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");
        Map<String, Integer> itemPurchased = new HashMap<>();
        itemPurchased.put("XYZ Blender", 1);
        itemPurchased.put("ZZZ Beer Glass", 4);
        c1.setItemQtyMap(itemPurchased);
        System.out.println(c1);

        Customer c2 = new Customer();
        c2.setName("Morgan Philips");
        Map<String, Integer> orderMap2 = new HashMap<>();
        orderMap2.put("AA Glass Cleaner",3);
        c2.setItemQtyMap(orderMap2);
        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 ITEM_QTY");
    }

    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', itemQtyMap={XYZ Blender=1, ZZZ Beer Glass=4}}
Customer{id=0, name='Morgan Philips', itemQtyMap={AA Glass Cleaner=3}}
-- Native queries --
'Select * from Customer'
[1, Lindsey Craft]
[2, Morgan Philips]
'Select * from ITEM_QTY'
[1, 1, XYZ Blender]
[1, 4, ZZZ Beer Glass]
[2, 3, AA Glass Cleaner]
-- Loading Customer --
Customer{id=1, name='Lindsey Craft', itemQtyMap={XYZ Blender=1, ZZZ Beer Glass=4}}
Customer{id=2, name='Morgan Philips', itemQtyMap={AA Glass Cleaner=3}}

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