Originally posted by Sankar Subbiramaniam:
OK. Please explain why b2 is empty ?
Also what is the difference between these two statements
[ December 15, 2005: Message edited by: Sankar Subbiramaniam ]
The methods of the java.util.Collection API for the container-managed collections used for collection-valued cmr-fields have the usual semantics, with the following exception: the add and addAll methods applied to container-managed collections in one-to-many relationships have a special semantics that is determined by the referential integrity of one-to-many relationships.
If the argument to the add method is already an element of a collection-valued relationship field of the same relationship type as the target collection (as defined by the ejb-relation and ejb-relationship-role deployment descriptor elements), it is removed from this first relationship and added, in the same transaction context, to the target relationship (i.e., it is effectively moved from one collection of the relationship type to the other). For example, if there is a one-to-many relationship between field "person" and "telephones", adding a "telephone" to a new field "person" will have the effect of removing it from its current field "person".
The semantics of a set accessor method, when applied to a collection-valued cmr-field, is also determined by the referential integrity semantics associated with the multiplicity of the relationship. The identity of the collection object referenced by a cmr-field does not change when a set accessor method is executed.
In the case of a one-to-many relationship, if a collection of entity objects is assigned from a cmr-field of in one instance to a cmr-field of the same relationship type in another instance, the objects in the collection are effectively MOVED. The CONTENTS of the collection of the target instance are REPLACED with the contents of the collection of the source instance, but the identity of the collection object containing the instances in the relationship does not change. The source cmr-field references the same collection object as before (i.e., the identity of the collection object is preserved), but the collection is empty.
The Bean Provider can thus use the set(...) method to MOVE objects between the collections referenced by cmr-fields of the same relationship type in different instances. The set accessor method, when applied to a cmr-field in a one-to-many relationship thus has the semantics of the java.util.Collection methods clear, followed by addAll, applied to the TARGET collection; and clear, applied to the SOURCE collection. It is the responsibility of the container to transfer the contents of the collection instances in the same transaction context.