What does EntityManager.flush do and why do I need to use it?

Question

I have an EJB where I am saving an object to the database. In an example I have seen, once this data is saved (EntityManager.persist) there is a call to EntityManager.flush(); Why do I need to do this? The object I am saving is not attached and not used later in the method. In fact, once saved the method returns and I would expect the resources to be released. (The example code does this on a remove call as well.)

if (somecondition) {     entityManager.persist(unAttachedEntity); } else {     attachedEntityObject.setId(unAttachedEntity.getId()); } entityManager.flush(); 

Answer 1

A call to EntityManager.flush(); will force the data to be persist in the database immediately as EntityManager.persist() will not (depending on how the EntityManager is configured : FlushModeType (AUTO or COMMIT) by default it's set to AUTO and a flush will be done automatically by if it's set to COMMIT the persitence of the data to the underlying database will be delayed when the transaction is commited).

Answer 2

EntityManager.persist() makes an entity persistent whereas EntityManager.flush() actually runs the query on your database.

So, when you call EntityManager.flush(), queries for inserting/updating/deleting associated entities are executed in the database. Any constraint failures (column width, data types, foreign key) will be known at this time.

The concrete behaviour depends on whether flush-mode is AUTO or COMMIT.