By default, mapping a java.util.Map of entity type keys and basic type values, JPA naming conventions are used for join table/columns. we can use @CollectionTable, @Column and @MapKeyJoinColumns to customize that.
@CollectionTable: For the join table name. We can also use it's element 'joinColumns' for the main entity's id reference column.
@MapKeyJoinColumn: For the map key column, the id ref of key entity.
@Column: For the map value column.
Example
@Entity
public class Employee {
@Id
@GeneratedValue
private long id;
private String name;
@ElementCollection
@CollectionTable(name = "EMP_TASKS", joinColumns = @JoinColumn(name = "EMP_ID"))
@MapKeyJoinColumn(name = "TASK_ID")
@Column(name = "TASK_DATE")
private Map<Task, Date> taskMap;
.............
}
@Entity
public class Task {
@Id
@GeneratedValue
private long id;
private String name;
private String description;
.............
}
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 EMPLOYEE");
nativeQuery(em, "SHOW COLUMNS from TASK");
nativeQuery(em, "SHOW COLUMNS from EMP_TASKS");
} 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' [EMPLOYEE, PUBLIC] [EMP_TASKS, PUBLIC] [TASK, PUBLIC] 'SHOW COLUMNS from EMPLOYEE' [ID, BIGINT(19), NO, PRI, NULL] [NAME, VARCHAR(255), YES, , NULL] 'SHOW COLUMNS from TASK' [ID, BIGINT(19), NO, PRI, NULL] [DESCRIPTION, VARCHAR(255), YES, , NULL] [NAME, VARCHAR(255), YES, , NULL] 'SHOW COLUMNS from EMP_TASKS' [EMP_ID, BIGINT(19), NO, PRI, NULL] [TASK_DATE, TIMESTAMP(23), YES, , NULL] [TASK_ID, BIGINT(19), NO, PRI, NULL]
Persisting and loading data
public class ExampleMain2 {
public static void main(String[] args) throws Exception {
EntityManagerFactory emf =
Persistence.createEntityManagerFactory("example-unit");
try {
persistEntity(emf);
runNativeQueries(emf);
loadEntity(emf);
} finally {
emf.close();
}
}
private static void persistEntity(EntityManagerFactory emf) throws Exception {
System.out.println("-- Persisting entities --");
EntityManager em = emf.createEntityManager();
Employee e1 = new Employee();
e1.setName("employee name 1");
Task task1 = new Task();
task1.setName("task 1");
task1.setDescription("task 1 desc");
SimpleDateFormat sdf = new SimpleDateFormat("MM-dd-yy");
Task task2 = new Task();
task2.setName("task 2");
task2.setDescription("task 2 desc");
e1.addTask(sdf.parse("15-01-17"), task1);
e1.addTask(sdf.parse("20-01-17"), task2);
System.out.println(e1);
Employee e2 = new Employee();
e2.setName("employee name 2");
Task task3 = new Task();
task3.setName("task 3");
task3.setDescription("task 3 desc");
e2.addTask(sdf.parse("02-05-2017"), task3);
System.out.println(e2);
em.getTransaction().begin();
em.persist(task1);
em.persist(task2);
em.persist(task3);
em.persist(e1);
em.persist(e2);
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 Employee");
ExampleMain.nativeQuery(em, "Select * from Task");
ExampleMain.nativeQuery(em, "Select * from Emp_Tasks");
}
private static void loadEntity(EntityManagerFactory emf) {
System.out.println("-- Loading entities --");
EntityManager em = emf.createEntityManager();
List<Employee> entityAList = em.createQuery("Select t from Employee t")
.getResultList();
entityAList.forEach(System.out::println);
em.close();
}
} Output-- Persisting entities -- Employee{id=0, name='employee name 1', taskMap={Task{id=0, name='task 1', description='task 1 desc'}=Thu Mar 01 00:00:00 CST 2018, Task{id=0, name='task 2', description='task 2 desc'}=Wed Aug 01 00:00:00 CDT 2018}} Employee{id=0, name='employee name 2', taskMap={Task{id=0, name='task 3', description='task 3 desc'}=Sun Feb 05 00:00:00 CST 2017}} -- Native queries -- 'Select * from Employee' [4, employee name 1] [5, employee name 2] 'Select * from Task' [1, task 1 desc, task 1] [2, task 2 desc, task 2] [3, task 3 desc, task 3] 'Select * from Emp_Tasks' [4, 2018-03-01 00:00:00.0, 1] [4, 2018-08-01 00:00:00.0, 2] [5, 2017-02-05 00:00:00.0, 3] -- Loading entities -- Employee{id=4, name='employee name 1', taskMap={Task{id=1, name='task 1', description='task 1 desc'}=2018-03-01 00:00:00.0, Task{id=2, name='task 2', description='task 2 desc'}=2018-08-01 00:00:00.0}} Employee{id=5, name='employee name 2', taskMap={Task{id=3, name='task 3', description='task 3 desc'}=2017-02-05 00:00:00.0}}
Example ProjectDependencies and Technologies Used: - h2 1.4.195: 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
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