Russ,
Weaving or enhancement is a process of bytecode manipulation.
TopLink weaves those entity classes that have LAZY loading for ToOne relationships. If you use some other JPA provider that might be doing something else.
Such bytecode modifications allow the provider to intercept access to the persistence fields or properties and e.g. load them on demand (i.e. lazily), others allow to detect changes as they are happening to an entity to avoid scanning all entities in the persistence context at the time of the flush or the commit request.
HTH,
-marina
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