How do lazy and eager fetching work in JPA?

Explains mapping fetch semantics while separating them from the query-specific data shape an endpoint actually needs.

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TL;DR

Lazy fetching defers loading an association until code accesses it, usually through a proxy or Hibernate collection. Eager fetching requires the association to be available with the entity, but it does not guarantee one SQL join; the provider may use additional queries. JPA defaults to eager for to-one associations and lazy for to-many associations, though I usually make important choices explicit. The right data shape depends on the use case, not on one global setting.

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Association loading, proxies and collections, query counts, default fetch rules, and use-case fetch planning.

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Core answer
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Overview
Fetch type is a mapping-level promise or hint, not a complete query-performance plan.
Lazy fetching defers loading an association until code accesses it, usually through a proxy or Hibernate collection. Eager fetching requires the association to be available with the entity, but it does not guarantee one SQL join; the provider may use additional queries. JPA defaults to eager for to-one associations and lazy for to-many associations, though I usually make important choices explicit. The right data shape depends on the use case, not on one global setting.
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Marking everything eager can load large graphs and produce repeated queries, while marking everything lazy can move database access into mapping or serialization and fail after the context closes. I keep mappings conservative and define a fetch plan in the repository query for each read operation. List endpoints normally project only needed columns, while a command loads the aggregate state required to enforce its invariant. SQL count and row multiplication both matter.

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Saying eager always means one join.
Making every relationship eager to avoid exceptions.
Letting JSON serialization choose the fetch plan.

How do lazy and eager fetching work in JPA?

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