Hibernate Many to Many Relations Set Or List?

Question

I have a many to many relationship at my Java beans. When I use List to define my variables as like:

@Entity
@Table(name=\"ScD\")
public class G         


        

Answer 1

From relational databases perspective this is a set. Databases do not preserve order and using a List is meaningless, the order in them is unspecified (unless using so called indexed collections).

Using a Set also has great performance implications. When List is used, Hibernate uses PersistentBag collection underneath which has some terrible characteristics. I.e.: if you add a new relationship it will first delete all existing ones and then insert them back + your new one. With Set it just inserts the new record.

Third thing - you cannot have multiple Lists in one entity as you will get infamous cannot simultaneously fetch multiple bags exception.

See also:

• 19.5. Understanding Collection performance

• Why Hibernate does "delete all then re-insert" - its not so strange

Answer 2

How about the uniqueness requirement from Set? Doesn't this force Hibernate to retrieve all objects each time one is added to the collection to make sure a newly added one is unique? A List wouldn't have this limitation.

Answer 3

I know the question was made years ago but I wanted to comment on this topic, just in case someone is doubtful about the set vs list issue.

Regarding lazy fetching, I think a bag (list without index) would be a better option due to the fact that you avoid retrieving all objects each time one is added to the collection to:

• make sure a newly added one is unique, in case you are using a set
• preserve order, in case you are using a list (with index)

Please correct me if I'm mistaken.