How do JPA and Hibernate define the AUTO flush mode
Although both the Hibernate
Session and the JPA
EntityManager define a
flush() method to manually trigger this process, it’s much more convenient to let Hibernate manage the Persistence Context flushing. Unfortunately, there’s is a major difference between how JPA and Hibernate define the automatic flushing mechanism.
When using the default
AUTO flush mode, the Persistence Context should be flushed as follows:
- before the transaction is committed,
- before running a JPQL or HQL query
- before executing a native SQL query
As previously explained, Hibernate triggers the
AUTO flush only for the first two events, and the native SQL queries must either override the
ALWAYS flush mode using the
Query#setFlushMode(FlushMode flushMode) method or add a table space synchronization (e.g.
This is only required for the native Hibernate API when using a
Since Hibernate 5.2, if you bootstrap Hibernate using JPA (e.g.
persistence.xml), then even the Hibernate
FlushType.AUTOwill behave just like its JPA counterpart.
Only if you bootstrap Hibernate using the native mechanism, will the Hibernate
Sessionuse the legacy
JPA AUTO flushing
JPA is more strict, and the
AUTO flush mode must trigger a flush before any query. More the section
3.10.8 of the Java Persistence API specification says that the
AUTO flush mode should ensure that all pending changes are visible by any executing query.
This can be demonstrated by executing the following method:
assertTrue(((Number) entityManager .createNativeQuery("select count(*) from Post") .getSingleResult()).intValue() == 0); Post post = new Post("Hibernate"); post.setId(1L); entityManager.persist(post); assertTrue(((Number) entityManager .createNativeQuery("select count(*) from Post") .getSingleResult()).intValue() == 1);
When running this test, Hibernate generates the following SQL statements:
SELECT COUNT(*) FROM Post INSERT INTO post (title, version, id) VALUES ('Hibernate', 0, 1) SELECT COUNT(*) FROM Post
So, the flush was triggered and the
INSERT statement was executed prior to running the
This doesn’t happens when using a
assertTrue(((Number) entityManager .createQuery("select count(*) from Post") .getSingleResult()).intValue() == 0); Post post = new Post("Hibernate"); post.setId(1L); entityManager.persist(post); Session session = entityManager.unwrap(Session.class); assertTrue(((Number) session .createSQLQuery("select count(*) from Post") .uniqueResult()).intValue() == 0);
This time, Hibernate generates the following statements:
SELECT COUNT(*) FROM Post SELECT COUNT(*) as col_0_0_ FROM post blogentity0_ INSERT INTO post (title, version, id) VALUES ('Hibernate', 0, 1)
Flushing is a very important concept for any ORM too, and Hibernate is no different.
As a rule of thumb, it’s better to make sure that native SQL queries don’t return inconsistent results when using the Hibernate
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