jpa-patterns

Fix N+1 queries in JPA by using JOIN FETCH in a JPQL query, applying @EntityGraph on repository methods, or enabling batch fetching with @BatchSize. Each approach loads related entities in one or few queries instead of one query per parent row.

LazyInitializationException occurs when a lazy association is accessed outside an active transaction. Resolve it by fetching the association with JOIN FETCH in the query, keeping the service method @Transactional, or mapping to a DTO inside the transaction.

Use LAZY fetching for all associations, including @ManyToOne and @OneToOne which default to EAGER. EAGER loading fetches data whether needed or not, causing unnecessary joins and N+1-style overhead; fetch explicitly with JOIN FETCH when required.

JOIN FETCH is written directly in a JPQL @Query for explicit control over the SQL, while @EntityGraph is a declarative annotation on repository methods specifying attribute paths to fetch. Both produce a single query that loads associations eagerly for that call.

Detect N+1 queries by enabling SQL logging: set spring.jpa.show-sql to true and logging levels for org.hibernate.SQL to DEBUG. Count the SELECT statements per request; more than a few for a single operation indicates an N+1 problem.

Use @BatchSize when fetching multiple collections or when JOIN FETCH would cause a cartesian product across several associations. It loads lazy collections in batches of the configured size, reducing query count without changing query structure.