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What it looks like

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.acme.Order.items, could not initialize proxy - no Session

Extends HibernateException in org.hibernate.

What it means

Associations such as @OneToMany are loaded lazily by default: Hibernate puts a proxy in the field and runs the query the first time you touch it. That only works while the session (the persistence context) is open. Once the transaction ends, and the entity is detached, touching the proxy fails with this exception. It typically appears in the controller or in JSON serialisation, after the service method returned.

Common causes

• Returning an entity from a transactional service and accessing a lazy collection in the controller or in Jackson.
• Spring Boot's open-in-view being disabled (or never relevant for async or batch code).
• Accessing a lazy field from another thread or after the transaction was committed.
• Serialising an entity directly instead of a DTO.

How to fix it

1. Fetch what you need inside the transaction: a JOIN FETCH query, an @EntityGraph, or Hibernate.initialize.
2. Map entities to DTOs inside the service and return the DTO.
3. Avoid switching associations to EAGER globally: it causes N+1 queries and large joins.
4. Keep spring.jpa.open-in-view=false and fix the access pattern, so database work stays in the service layer.

Broken and fixed

@Transactional
public Order get(Long id) { return repo.findById(id).orElseThrow(); }
// controller: order.getItems() -> no Session

@Query("select o from Order o join fetch o.items where o.id = :id")
Optional<Order> findWithItems(@Param("id") Long id);

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In Spring and enterprise code

Spring Boot enables open-in-view by default, which hides this error by keeping the session open during the web request but holds a database connection for the whole request. Many teams turn it off.

Debug it faster

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