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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 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 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

map-with-many-to-many Select All Download
  • map-with-many-to-many
    • src
      • main
        • java
          • com
            • logicbig
              • example
                • Employee.java
          • resources
            • META-INF

    See Also