What causes LazyInitializationException, and should Open Session in View be used?

Fixes lazy access outside the persistence context with explicit fetch and DTO mapping while explaining the tradeoff of Open Session in View.

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TL;DR

LazyInitializationException occurs when code accesses a lazy proxy or collection after its Hibernate session or JPA persistence context is no longer available. I fix the use case by loading or projecting the data it actually needs inside the application transaction, then mapping to a DTO before returning. Making every relationship eager hides the boundary and can create much larger query problems.

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Lazy proxies, persistence-context lifetime, OSIV, serialization queries, DTO mapping, and transaction boundaries.

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Core answer
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Overview
The exception says code touched an unfetched association after the context capable of loading it was gone.
LazyInitializationException occurs when code accesses a lazy proxy or collection after its Hibernate session or JPA persistence context is no longer available. I fix the use case by loading or projecting the data it actually needs inside the application transaction, then mapping to a DTO before returning. Making every relationship eager hides the boundary and can create much larger query problems.
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Open Session in View keeps a persistence context available through web rendering or serialization, which can prevent the exception but allows database queries to happen outside the intended service operation. A serializer touching getters may then produce N+1 queries and make endpoint cost depend on response traversal. OSIV also keeps persistence resources associated with a request longer. I generally disable it for APIs and make repository fetch plans explicit.

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Fixing the exception by making every association eager.
Enabling OSIV without checking serialization queries.
Returning managed entities as an API contract.

What causes LazyInitializationException, and should Open Session in View be used?

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