LazyInitializationException when accessing lazy association after Hibernate Session.close()

Answer

Do not rely on hibernate.enable_lazy_load_no_trans to allow lazy association access after the Hibernate Session is closed. Use a DTO projection (or another explicit fetch strategy) to control what data is serialized for the REST response.

Confirmed facts

• Accessing an uninitialized lazy association after Session.close() throws LazyInitializationException.
• Setting hibernate.enable_lazy_load_no_trans=true permits lazy loading outside a transaction, but it bypasses transactional isolation, can fire additional SQL unexpectedly, and may expose stale or sensitive data.
• In Hibernate 6.x the property is deprecated and effectively ignored; the recommended approach is to avoid it.
• Serializing a managed entity directly (e.g., with Jackson) can trigger lazy initialization during property access, leading to exceptions or unintended queries.
• A DTO projection guarantees that only the selected attributes are loaded while the Session is open, eliminating accidental lazy loads during serialization.

Steps for this case

1. Identify the data needed for the REST endpoint.
2. Write a JPQL/HQL constructor expression, a CriteriaBuilder tuple, or an @EntityGraph that selects exactly those attributes.
3. Execute the query inside a transaction (or Spring‑managed @Transactional method) and return the resulting DTO list.
4. Serialize the DTO collection with Jackson (or your preferred JSON library); no lazy proxies are involved.

Missing diagnostic detail

Which Hibernate version are you targeting (5.x vs 6.x)? If you are on 6.x, the enable_lazy_load_no_trans property is deprecated and should not be used; on 5.x it still works but is discouraged for the reasons above.