JPA - java.util.Map entity values with ManyToMany relationship

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 CST 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 Project

Dependencies 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