Hibernate Many to Many Relations Set Or List?

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.

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