- Just like the Map of basic type keys and basic values, a Map of basic type keys and embeddable values can be persisted by using @ElementCollection on the Map reference in the entity class.
- The corresponding class of the map values, must have @Embeddable annotation.
- The Entity and the Map are mapped to two separate foreign/primary-key tables. One for the entity and other for keys and embeddable fields/properties values. The Map table has the foreign key pointing to the primary key of the entity.
- By default, JPA specific naming conventions are used for the mapping. We can customize that by using; @CollectionTable annotation and @MapKeyColumn along with @ElementCollection (check out our last tutorials for quick details).
- Note that all cases of @Embeddable class are applied here: we can use an embeddable class within another one, the embeddable can have entity relationship, and/or we can also use multiple instances of the maps having same embeddable values in our entity.
Example
@Entity
public class Customer {
@Id
@GeneratedValue
private int id;
private String name;
@ElementCollection
@CollectionTable(name = "ORDERS")
@MapKeyColumn(name = "TYPE")
private Map<String, Order> orderMap;
.............
}
@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();
}
}
.............
}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'
[CUSTOMER_ID, INTEGER(10), NO, PRI, NULL]
[DATE, TIMESTAMP(23), YES, , NULL]
[ITEM, VARCHAR(255), YES, , NULL]
[QTY, INTEGER(10), NO, , NULL]
[TYPE, VARCHAR(255), NO, PRI, NULL]
A quick overview (with the default column/table names):
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', orderMap={online=Order{item='XYZ Blender', qty=2, date=Wed May 31 15:21:00 CDT 2017}, store=Order{item='ZZZ Beer Glass', qty=4, date=Wed May 31 15:21:00 CDT 2017}}}
Customer{id=0, name='Morgan Philips', orderMap={online=Order{item='AA Glass Cleaner', qty=3, date=Wed May 31 15:21:00 CDT 2017}}}
-- Native queries --
'Select * from Customer'
[1, Lindsey Craft]
[2, Morgan Philips]
'Select * from Orders'
[1, 2017-05-31 15:21:00.363, XYZ Blender, 2, online]
[1, 2017-05-31 15:21:00.363, ZZZ Beer Glass, 4, store]
[2, 2017-05-31 15:21:00.366, AA Glass Cleaner, 3, online]
-- Loading Customer --
Customer{id=1, name='Lindsey Craft', orderMap={online=Order{item='XYZ Blender', qty=2, date=2017-05-31 15:21:00.363}, store=Order{item='ZZZ Beer Glass', qty=4, date=2017-05-31 15:21:00.363}}}
Customer{id=2, name='Morgan Philips', orderMap={online=Order{item='AA Glass Cleaner', qty=3, date=2017-05-31 15:21:00.366}}}
Example ProjectDependencies 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
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