JPA Key Notes

Prateek

2 min readJan 13, 2022

Let’s assume that we have two entities, Author and Book. There is no materialized association between them, but, both entities shares an attribute named, genre. We want to use this attribute to join the tables corresponding to Author and Book, and fetch the result in a DTO. The result should contain the Author entity and only the title attribute from Book. Well, when you are in a scenario as here, it is strongly advisable to avoid fetching the DTO via constructor expression. This approach cannot fetch the data in a single SELECT, and is prone to N+1. Way better than this consists of using Spring projections, JPA Tuple or even Hibernate ResultTransformer. These approaches will fetch the data in a single SELECT. This application is a DON'T DO THIS example. Check the number of queries needed for fetching the data
Spring deleteAllInBatch() and deleteInBatch() don't use delete batching and don't take advantage of cascading removal, orphanRemoval and automatic optimistic locking mechanism to prevent lost updates (e.g., @Version is ignored), but both of them take advantage on ON DELETE CASCADE and are very efficient. They trigger bulk operations via Query.executeUpdate(), therefore, the Persistence Context is not synchronized accordingly (it's up to you to flush (before delete) and close/clear (after delete) the Persistence Context accordingly to avoid issues created by unflushed (if any) or outdated (if any) entities). The first one simply triggers a delete from entity_name statement, while the second one triggers a delete from entity_name where id=? or id=? or id=? ... statement. For delete in batches rely on deleteAll(), deleteAll(Iterable<? extends T> entities) or delete() method. Behind the scene, the two flavors of deleteAll() relies on delete(). Mixing batching with database automatic actions (ON DELETE CASCADE) will result in a partially synchronized Persistent Context.

JPA Key Notes

Written by Prateek