java.util.Map entity values with @ManyToMany relationship is similar to other collections ManyToMany mapping, where we need to use @ManyToMany on the map field/property. In this case, our Map can have duplicate values. The same values can also be shared among different Map instances.
Example
@Entity
public class Employee {
@Id
@GeneratedValue
private long id;
private String name;
@ManyToMany
private Map<Date, Task> tasks;
.............
}
@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 EMPLOYEE_TASK");
} 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] [EMPLOYEE_TASK, 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 EMPLOYEE_TASK' [EMPLOYEE_ID, BIGINT(19), NO, PRI, NULL] [TASKS_ID, BIGINT(19), NO, , NULL] [TASKS_KEY, TIMESTAMP(23), 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");
// A single task will be shared
Task task = new Task();
task.setName("task 1");
task.setDescription("task 1 desc");
SimpleDateFormat sdf = new SimpleDateFormat("MM-dd-yy");
e1.addTask(sdf.parse("15-01-17"), task);
e1.addTask(sdf.parse("20-01-17"), task);
System.out.println(e1);
Employee e2 = new Employee();
e2.setName("employee name 2");
e2.addTask(sdf.parse("02-05-2017"), task);
System.out.println(e2);
em.getTransaction().begin();
em.persist(task);
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 Employee_Task");
}
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', tasks={Thu Mar 01 00:00:00 CST 2018=Task{id=0, name='task 1', description='task 1 desc'}, Wed Aug 01 00:00:00 CDT 2018=Task{id=0, name='task 1', description='task 1 desc'}}} Employee{id=0, name='employee name 2', tasks={Sun Feb 05 00:00:00 CST 2017=Task{id=0, name='task 1', description='task 1 desc'}}} -- Native queries -- 'Select * from Employee' [2, employee name 1] [3, employee name 2] 'Select * from Task' [1, task 1 desc, task 1] 'Select * from Employee_Task' [2, 1, 2018-03-01 00:00:00.0] [2, 1, 2018-08-01 00:00:00.0] [3, 1, 2017-02-05 00:00:00.0] -- Loading entities -- Employee{id=2, name='employee name 1', tasks={2018-03-01 00:00:00.0=Task{id=1, name='task 1', description='task 1 desc'}, 2018-08-01 00:00:00.0=Task{id=1, name='task 1', description='task 1 desc'}}} Employee{id=3, name='employee name 2', tasks={2017-02-05 00:00:00.0=Task{id=1, name='task 1', description='task 1 desc'}}}
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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