What does "Illegal attempt to associate a collection with two open sessions" mean in the context of Spring-Hibernate?

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This error message means that there is an attempt to associate a collection object with two different Hibernate sessions simultaneously. In the context of Spring-Hibernate integration, this can happen when two different transactions try to access the same collection object at the same time, or when a collection is loaded in one transaction and then passed to another transaction. To fix this issue, ensure that transactions are properly managed and that collections are synchronized with the current Hibernate session.