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JPA - Persisting a Map of entity keys and entity values

[Updated: Jun 26, 2017, Created: Jun 20, 2017]

This example will show how to map a java.util.Map having entity keys and entity values.

As we have already learnt in previous tutorials, to map entity values, we need to use @OneToMany/@ManyToMany and to map entity keys, we need to use @ElementCollection. This example is simply a combination of the two cases.

@OneToMany Example

@Entity
public class Employee {
  @Id
  @GeneratedValue
  private long id;
  private String name;
  @ElementCollection
  @OneToMany
  private Map<AssignInfo, Task> taskMap;
    .............
}
@Entity
public class AssignInfo {
  @Id
  @GeneratedValue
  private long id;
  private Date startDate;
  private Date endDate;
    .............
}
@Entity
public class Task {
  @Id
  @GeneratedValue
  private long id;
  private String name;
  private String description;
    .............
}

Note that using both @OneToMany and @ElementCollection together is redundant. Hibernate can recognise the intended mapping, if we use just one of them.


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");
          nativeQuery(em, "SHOW COLUMNS from ASSIGNINFO");
      } finally {
          emf.close();
      }
  }
    .............
}

Output

'SHOW TABLES'
[ASSIGNINFO, PUBLIC]
[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]
[TASKMAP_ID, BIGINT(19), NO, UNI, NULL]
[TASKMAP_KEY, BIGINT(19), NO, PRI, NULL]
'SHOW COLUMNS from ASSIGNINFO'
[ID, BIGINT(19), NO, PRI, NULL]
[ENDDATE, TIMESTAMP(23), YES, , NULL]
[STARTDATE, TIMESTAMP(23), YES, , NULL]

H2 database SHOW statements

A quick overview of the mapping:

In the join table (EMPLOYEE_TASK) the foreign key pointing to the map value's primary key (TASK table in above example) is unique. That's because it's a OneToMany relation, which does not allow to share the same target entity. If we use @ManyToMany, then there won't be any unique constraint.

Complete Example

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();

      SimpleDateFormat sdf = new SimpleDateFormat("MM-dd-yy");

      Employee e1 = new Employee();
      e1.setName("employee name 1");
      Task task1 = new Task();
      task1.setName("task 1");
      task1.setDescription("task 1 desc");
      AssignInfo assignInfo1 = new AssignInfo();
      assignInfo1.setStartDate(sdf.parse("15-01-17"));
      assignInfo1.setEndDate(sdf.parse("20-01-17"));
      e1.addTask( assignInfo1, task1);
      Task task2 = new Task();
      task2.setName("task 2");
      task2.setDescription("task 2 desc");
      AssignInfo assignInfo2 = new AssignInfo();
      assignInfo2.setStartDate(sdf.parse("02-01-17"));
      assignInfo2.setEndDate(sdf.parse("11-02-17"));
      e1.addTask(assignInfo2, 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");
      AssignInfo assignInfo3 = new AssignInfo();
      assignInfo3.setStartDate(sdf.parse("9-02-17"));
      assignInfo3.setEndDate(sdf.parse("19-03-17"));
      e2.addTask(assignInfo3, task3);
      System.out.println(e2);

      em.getTransaction().begin();
      em.persist(task1);
      em.persist(task2);
      em.persist(task3);
      em.persist(assignInfo1);
      em.persist(assignInfo2);
      em.persist(assignInfo3);
      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");
      ExampleMain.nativeQuery(em, "Select * from AssignInfo");
  }

  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={AssignInfo{id=0, startDate=Thu Mar 01 00:00:00 CST 2018, endDate=Wed Aug 01 00:00:00 CDT 2018}=Task{id=0, name='task 2', description='task 2 desc'}}}
Employee{id=0, name='employee name 2', taskMap={AssignInfo{id=0, startDate=Sat Sep 02 00:00:00 CDT 2017, endDate=Tue Jul 03 00:00:00 CDT 2018}=Task{id=0, name='task 3', description='task 3 desc'}}}
-- Native queries --
'Select * from Employee'
[7, employee name 1]
[8, 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 Employee_Task'
[7, 2, 4]
[8, 3, 6]
'Select * from AssignInfo'
[4, 2018-08-01 00:00:00.0, 2018-03-01 00:00:00.0]
[5, 2017-11-02 00:00:00.0, 2017-02-01 00:00:00.0]
[6, 2018-07-03 00:00:00.0, 2017-09-02 00:00:00.0]
-- Loading entities --
Employee{id=7, name='employee name 1', taskMap={AssignInfo{id=4, startDate=2018-03-01 00:00:00.0, endDate=2018-08-01 00:00:00.0}=Task{id=2, name='task 2', description='task 2 desc'}}}
Employee{id=8, name='employee name 2', taskMap={AssignInfo{id=6, startDate=2017-09-02 00:00:00.0, endDate=2018-07-03 00:00:00.0}=Task{id=3, name='task 3', description='task 3 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 Entity Keys And Values Example Select All Download
  • map-with-entity-keys-and-values
    • src
      • main
        • java
          • com
            • logicbig
              • example
                • Employee.java
          • resources
            • META-INF

    See Also