JPA map collection of Enums

Question

Is there a way in JPA to map a collection of Enums within the Entity class? Or the only solution is to wrap Enum with another domain class and use it to map the collection?<

Answer 1

I'm using a slight modification of java.util.RegularEnumSet to have a persistent EnumSet:

@MappedSuperclass
@Access(AccessType.FIELD)
public class PersistentEnumSet> 
    extends AbstractSet {
  private long elements;

  @Transient
  private final Class elementType;

  @Transient
  private final E[] universe;

  public PersistentEnumSet(final Class elementType) {
    this.elementType = elementType;
    try {
      this.universe = (E[]) elementType.getMethod("values").invoke(null);
    } catch (final ReflectiveOperationException e) {
      throw new IllegalArgumentException("Not an enum type: " + elementType, e);
    }
    if (this.universe.length > 64) {
      throw new IllegalArgumentException("More than 64 enum elements are not allowed");
    }
  }

  // Copy everything else from java.util.RegularEnumSet
  // ...
}

This class is now the base for all of my enum sets:

@Embeddable
public class InterestsSet extends PersistentEnumSet {
  public InterestsSet() {
    super(InterestsEnum.class);
  }
}

And that set I can use in my entity:

@Entity
public class MyEntity {
  // ...
  @Embedded
  @AttributeOverride(name="elements", column=@Column(name="interests"))
  private InterestsSet interests = new InterestsSet();
}

Advantages:

• Working with a type safe and performant enum set in your code (see java.util.EnumSet for a description)
• The set is just one numeric column in the database
• everything is plain JPA (no provider specific custom types)
• easy (and short) declaration of new fields of the same type, compared with the other solutions

Drawbacks:

• Code duplication (RegularEnumSet and PersistentEnumSet are nearly the same)
  • You could wrap the result of EnumSet.noneOf(enumType) in your PersistenEnumSet, declare AccessType.PROPERTY and provide two access methods which use reflection to read and write the elements field
• An additional set class is needed for every enum class that should be stored in a persistent set
  • If your persistence provider supports embeddables without a public constructor, you could add @Embeddable to PersistentEnumSet and drop the extra class (... interests = new PersistentEnumSet<>(InterestsEnum.class);)
• You must use an @AttributeOverride, as given in my example, if you've got more than one PersistentEnumSet in your entity (otherwise both would be stored to the same column "elements")
• The access of values() with reflection in the constructor is not optimal (especially when looking at the performance), but the two other options have their drawbacks as well:
  • An implementation like EnumSet.getUniverse() makes use of a sun.misc class
  • Providing the values array as parameter has the risk that the given values are not the correct ones
• Only enums with up to 64 values are supported (is that really a drawback?)
  • You could use BigInteger instead
• It's not easy to use the elements field in a criteria query or JPQL
  • You could use binary operators or a bitmask column with the appropriate functions, if your database supports that