Difference between @OneToMany and @ElementCollection?

Question

ElementCollection is a standard JPA annotation, which is now preferred over the proprietary Hibernate annotation CollectionOfElements.

It means that the collection is not a collection of entities, but a collection of simple types (Strings, etc.) or a collection of embeddable elements (class annotated with @Embeddable).

It also means that the elements are completely owned by the containing entities: they're modified when the entity is modified, deleted when the entity is deleted, etc. They can't have their own lifecycle.

I believe @ElementCollection is mainly for mapping non-entities (embeddable or basic) while @OneToMany is used to map entities. So which one to use depend on what you want to achieve.

@ElementCollection allows you to simplify code when you want to implement one-to-many relationship with simple or embedded type. For instance in JPA 1.0 when you wanted to have a one-to-many relationship to a list of Strings, you had to create a simple entity POJO (StringWrapper) containing only primary key and the String in question:

@OneToMany
private Collection<StringWrapper> strings;

//...

public class StringWrapper {
  @Id
  private int id;

  private String string;
}

With JPA 2.0 you can simply write:

@ElementCollection
private Collection<String> strings;

Simpler, isn't it? Note that you can still control the table and column names using @CollectionTable annotation.

See also:

• Java Persistence/ElementCollection

Basic or Embedded: @ElementCollection
Entities: @OneToMany or @ManyToMany

@ElementCollection:

• the relation is managed (only) by the entity in which the relation is defined
• table contains id reference to the owning entity plus basic or embedded attributes

@OneToMany / @ManyToMany:

• can also be managed by the other entity
• join table or column(s) typically contains id references only