Hibernate / EclipseLink Interview questions

Explain the internal working of EclipseLink's Unit of Work commit process?

When a transaction backed by a UnitOfWork commits, EclipseLink doesn't simply replay every change immediately; it goes through a structured sequence designed to compute the minimal, correctly-ordered set of SQL statements needed.

flowchart TD A[Transaction commit triggered] --> B[Compute change set: compare managed objects vs originals] B --> C[Order operations to satisfy foreign key / dependency constraints] C --> D[Generate SQL: INSERT / UPDATE / DELETE per change] D --> E[Execute SQL against the database connection] E --> F{All statements succeed?} F -->|Yes| G[Merge changes into shared cache] F -->|No| H[Roll back transaction, propagate exception] G --> I[Release UnitOfWork resources]

The change-set computation step relies on whichever change-tracking policy is active (attribute change tracking or deferred detection) to determine exactly which entities and fields actually changed. Dependency ordering matters because, for example, a new child row referencing a new parent row via a foreign key can't be inserted before the parent row exists; EclipseLink topologically sorts the pending operations so inserts happen in an order that respects those relationships, and deletes happen in the reverse order to avoid violating constraints.

Only after every generated SQL statement succeeds does EclipseLink merge the now-committed changes into the shared cache, making them visible to other EntityManagers from the same factory; if any statement fails, the whole unit of work rolls back and none of the changes reach either the database or the shared cache.

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Why does EclipseLink order INSERT statements based on foreign key dependencies? So a child row isn't inserted before the parent row it references exists

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To alphabetize the generated SQL for readability

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It's a purely cosmetic step with no functional purpose

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Changes are merged into the shared cache: Before any SQL statements are executed

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Only after every generated SQL statement succeeds

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Regardless of whether the transaction ultimately commits or rolls back

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What is EclipseLink? What is JPA (Jakarta Persistence API)? What is the relationship between EclipseLink and Jakarta Persistence? What is a Persistence Unit in EclipseLink? What is persistence.xml used for? What is an EntityManager in EclipseLink? What is an EntityManagerFactory? What is a JPA entity? What are EclipseLink's core components? What is EclipseLink MOXy? What is class weaving in EclipseLink? What is the EclipseLink shared cache (L2 cache)? What is a fetch group in EclipseLink? What is optimistic locking in EclipseLink and JPA? What is the @Convert annotation used for? What is a Customizer in EclipseLink? What is JPQL? What are EclipseLink query hints? What is DDL generation in EclipseLink? What is a UnitOfWork in EclipseLink's native API? What is EclipseLink's native Session API? What is static weaving in EclipseLink? What are EclipseLink descriptors? What is EclipseLink DBWS? What is EclipseLink SDO? Explain the lifecycle of a JPA entity managed by EclipseLink? Why is class weaving important for lazy loading in EclipseLink? How does EclipseLink's shared cache differ from Hibernate's second-level cache? What is the difference between static weaving and dynamic weaving? How do you configure batch fetching using EclipseLink query hints? When should you use pessimistic locking instead of optimistic locking? How do you troubleshoot lazy loading failures caused by missing weaving in EclipseLink? What is the difference between EAGER and LAZY fetch strategies, and how does EclipseLink implement lazy loading without a live session? How does EclipseLink implement change tracking for detecting entity modifications? Explain the internal working of EclipseLink's cache coordination? What is the difference between EclipseLink and Hibernate? How do you implement multitenancy in EclipseLink? Why use fetch groups instead of always fetching the full entity? What is the difference between attribute change tracking and deferred change detection? How does EclipseLink cache query results, and what invalidation strategies are available? When would you choose isolated cache over shared cache for a multitenant entity? How do you configure a Customizer to add native EclipseLink mappings not expressible via annotations? What is the difference between JPQL and the EclipseLink native Expression/Criteria API? Explain the internal working of EclipseLink's Unit of Work commit process? How do you optimize EclipseLink performance for a high-throughput application? What is the difference between EclipseLink's @Convert and JPA's standard AttributeConverter? How does EclipseLink decide when to use join fetch versus batch fetch? Why should you use @Version for optimistic locking instead of manual version checks? What is the difference between EntityManager persistence context types: transaction-scoped vs extended? How do you troubleshoot stale data issues caused by EclipseLink's shared cache?

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Hibernate / EclipseLink Interview questions

Explain the internal working of EclipseLink's Unit of Work commit process?

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