Following example shows how to use a type conversion involving key or value of a Map.

Example

The Converter

@Converter
public class FileConverter implements AttributeConverter<File, String> {

  @Override
  public String convertToDatabaseColumn(File attribute) {
      return attribute.getAbsolutePath();
  }

  @Override
  public File convertToEntityAttribute(String dbData) {
      return new File(dbData);
  }
}

The Entity

@Entity
public class Journal {
  @Id
  @GeneratedValue
  private long id;
  @ElementCollection
  @Convert(converter = FileConverter.class, attributeName = "value")
  private Map<String, File> reportMap;
    .............
}

As seen above @Convert placed on Map attribute must define attributeName of 'key' or 'value'

The main class

In case of the persisting Map with @ElementCollection, the entity and the map are persisted to two separate foreign/primary-key tables (check out the related tutorial here)

public class ExampleMain {
  private static EntityManagerFactory entityManagerFactory =
          Persistence.createEntityManagerFactory("example-unit");

  public static void main(String[] args) {
      try {
          nativeQuery("Show Columns from Journal");
          nativeQuery("Show Columns from Journal_ReportMap");
          persistEntity();
          nativeQuery("Select * from Journal");
          nativeQuery("Select * from Journal_ReportMap");
          loadEntity();
      } finally {
          entityManagerFactory.close();
      }
  }

  public static void persistEntity() {
      Journal journal = new Journal();
      journal.addReport("test report 1", new File("c:/temp/report-1.txt"));
      journal.addReport("test report 2", new File("c:/temp/report-2.txt"));
      System.out.println("Persisting journal: " + journal);
      EntityManager em = entityManagerFactory.createEntityManager();
      em.getTransaction().begin();
      em.persist(journal);
      em.getTransaction().commit();
      em.close();
  }

  private static void loadEntity() {
      EntityManager em = entityManagerFactory.createEntityManager();
      Journal journal = em.find(Journal.class, 1L);
      System.out.println("Journal loaded: " + journal);
      em.close();
  }

  public static void nativeQuery(String s) {
      EntityManager em = entityManagerFactory.createEntityManager();
      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);
          }
      }
      em.close();
  }
}

'Show Columns from Journal'
[ID, BIGINT(19), NO, PRI, NULL]
'Show Columns from Journal_ReportMap'
[JOURNAL_ID, BIGINT(19), NO, PRI, NULL]
[REPORTMAP, VARCHAR(255), YES, , NULL]
[REPORTMAP_KEY, VARCHAR(255), NO, PRI, NULL]
Persisting journal: Journal{id=0, reportMap={test report 1=c:\temp\report-1.txt, test report 2=c:\temp\report-2.txt}}
'Select * from Journal'
1
'Select * from Journal_ReportMap'
[1, c:\temp\report-1.txt, test report 1]
[1, c:\temp\report-2.txt, test report 2]
Journal loaded: Journal{id=1, reportMap={test report 1=c:\temp\report-1.txt, test report 2=c:\temp\report-2.txt}}

Example Project

Dependencies and Technologies Used:

• h2 1.4.196: H2 Database Engine.
• hibernate-core 5.2.12.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