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

User · Answer

The EntityManager flush   operation can be used the write all changes to the database before the transaction is committed  By default JPA does not normally write changes to the database until the transaction is committed  This is normally desirable as it avoids database access  resources and locks until required  It also allows database writes to be ordered  and batched for optimal database access  and to maintain integrity constraints and avoid deadlocks  This means that when you call persist  merge  or remove the database DML INSERT  UPDATE  DELETE is not executed  until commit  or until a flush is triggered

User · Answer

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

User · Answer

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

User · Answer

So when you call EntityManager persist    it only makes the entity get managed by the EntityManager and adds it  entity instance  to the Persistence Context  An Explicit flush   will make the entity now residing in the Persistence Context to be moved to the database  using a SQL     Without flush    this  moving of entity from Persistence Context to the database  will happen when the Transaction to which this Persistence Context is associated is committed

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

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