Hibernate 'org.hibernate.LazyInitializationException: could not initialize proxy [...] - no Session' / 'failed to lazily initialize a collection of role ... could not initialize proxy - no Session': how do I fix it?

Cause: A lazily loaded association (a proxy or a collection) was touched after the Hibernate Session / JPA persistence context that loaded it was closed. Proxies only load their state while the entity is still attached to an open session; afterwards their methods throw LazyInitializationException. Typical situations: returning an entity from a @Transactional service method and reading a lazy field in a controller or while serializing JSON, or using the entity in a different thread. Fix (best first): 1. Fetch what you need inside the unit of work. In HQL/JPQL use join fetch, for example select o from Order o join fetch o.items where o.id = :id, or request an EntityGraph so the association is loaded with the parent. Hibernate's documentation recommends mapping associations lazy and fetching the data needed at the start of a unit of work. 2. Return a DTO or projection built inside the transaction, instead of exposing entities to the web layer. This also avoids accidentally serializing whole object graphs. 3. Widen the transaction boundary so access happens within it (for example put @Transactional on the service method that does the whole job), rather than reaching into lazy fields later. 4. For collections, consider batch fetching or subselect fetching to avoid N+1 queries while still being lazy. 5. Avoid switching the association to eager everywhere, relying on Spring's open-in-view setting as a design, or enabling hibernate.enable_lazy_load_no_trans. They hide the problem and cause extra queries, long connection use or inconsistent behavior. Verify: The call path that failed now returns data without the exception, and the SQL log shows the expected number of queries (not one per row). Sources: https://docs.hibernate.org/orm/6.6/introduction/html_single/Hibernate_Introduction.html https://docs.hibernate.org/orm/6.6/userguide/html_single/Hibernate_User_Guide.html