What is the use of bag tag in Hibernate?

I need to know how to use the bag tag and what is th开发者_高级运维e purpose of it?

For collection mapping

If your table does not have an index column, and you still wish to use List as the property type, you can map the property as a Hibernate < bag>. A bag does not retain its order when it is retrieved from the database, but it can be optionally sorted or ordered.

http://docs.jboss.org/hibernate/core/3.3/reference/en/html/collections.html

From the API:

A bag is an unordered, unkeyed collection that can contain the same element multiple times. A bag permits duplicates, so it has no primary key. The Java collections API, curiously, has no Bag.

A short and simple answer:

You don't need to use the <index> tag when mapping an unsorted List and you also don't need an additional index column in database.

Unlike a List a Bag does not persist the order of its elements, but you can specify an order-by parameter to retrieve its elements in a specific order.