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in JPA, if we call EntityTransaction.commit(), does it automatically call EntityManager.flush()? or should we call them both? what is the difference? because i have problem with JPA, when i insert an entity to database, i call persist(). in the database, the data has been inserted (can be fetched), but that data doesn't show up in my app (i fetch it using findAll()). and on another entity, it showed up. is there something i don't know? i'm using standard Spring CRUD, JPA resource_local, and postgresql. sorry for my english, thanks in advance
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JPA also defines a COMMIT flush mode, which is described as follows: If FlushModeType. COMMIT is set, the effect of updates made to entities in the persistence context upon queries is unspecified. When executing a JPQL query, the persistence context is only flushed when the current running transaction is committed.
Flushing is the process of synchronizing the underlying persistent store with persistable state held in memory. In other words, on a flush() all the insert, update, delete or whatever statements are actually called on the database, before a flush() nothing happens on your database.
You need to flush in batch processing otherwise it may give OutOfMemoryException. Commit(); Commit will make the database commit. When you have a persisted object and you change a value on it, it becomes dirty and hibernate needs to flush these changes to your persistence layer.
Commit a transaction by calling the commit() method on the Connection interface. This tells your database to perform all required consistency checks and persist the changes permanently. Rollback all operations performed during the transaction by calling the rollback() method on the Connection interface.
em.flush() - It saves the entity immediately to the database with in a transaction to be used further and it can be rolled back.
em.getTransaction().commit - It marks the end of transaction and saves all the chnages with in the transaction into the database and it can't be rolled back.
Refer https://prismoskills.appspot.com/lessons/Hibernate/Chapter_14_-_Flush_vs_Commit.jsp
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If you have a @Version annotated column in your entity and call entityManager.flush(), then you will either (immediately!) get an OptimisticLockException, or the database will lock this row (or table). In the later case you can still call setRollbackOnly(), and the lock will later be released without a DB change.
Or from a different perspective, with flush() you can create a (pessimistic) lock on that database row. The others will still see the old entry, but if they try to update they will be blocked, until the lock is released.
All this is also true for CMT (container managed transactions). Instead of waiting for the moment, where the service method is finished and the CMT commit is performed, you can call flush() (even several times) in your service method and handle the OptimisticLockException(s) immediately.
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