Java.Hibernate.Middle.When batch will not work ?

🚫 1. Detached entities or closed session

• Batch fetching works only if multiple lazy proxies are accessed within the same persistence context (i.e., the same open Hibernate Session or JPA EntityManager).
• If you load entities in one transaction but access their lazy associations later, outside of that session, you’ll get LazyInitializationException instead of batch fetching.

🚫 2. Single proxy accessed

• Batch fetching triggers only if Hibernate sees multiple proxies of the same type waiting for initialization.
• If you access just one lazy association, batching has nothing to batch → it’s still a single query.

🚫 3. Associations already eagerly loaded

• If you fetch an association with JOIN FETCH or it’s configured with FetchType.EAGER, there’s no lazy loading left → batch fetching has no effect.

🚫 4. Mixed session contexts

• If you scatter your entity access across multiple transactions or EntityManagers, batch fetching can’t group proxies because it only considers proxies in the same session.

🚫 5. Proxies of different types

• Batch fetching only batches same-type lazy proxies.
  • E.g., if you have 20 Author proxies and 20 Publisher proxies, they’re batched separately — not together.
  • So if you only have a few of each type, batching may have limited impact.

🚫 6. Direct queries ignoring proxies

• If you manually run queries on child entities (e.g., SELECT b FROM Book b WHERE ...) instead of accessing them through lazy proxies → batch fetching never triggers.

🚫 7. Using DTO projections

• If you bypass entity mappings entirely (e.g., mapping query results to DTOs), Hibernate doesn’t manage proxies → batch fetching doesn’t apply.

⚠️ Summary:
Batch fetching works only on lazy proxies accessed in the same session, and only if there’s more than one proxy of the same type to batch together.