Correct use of flush in JPA Hibernate

Question

I was gathering information about the flush   method  but I m not quite clear when to use it and how to use it correctly  From what I read  my understanding is that the contents of the persistence context will be synchronized with the database  i  e  issuing outstanding statements or refreshing entity data   Now I got following scenario with two entities A and B  in a one-to-one relationship  but not enforced or modelled by JPA   A has a composite PK  which is manually set  and also has an auto-generated IDENTITY field recordId  This recordId should be written to entity B as a foreign-key to A  I m saving A and B in a single transaction  The problem is that the auto-generated value A recordId is not available within the transaction  unless I make an explicit call of em flush   after calling em persist   on A   If I have an auto-generated IDENTITY PK then the value is directly updated in the entity  but that s not the case here    Can em flush   cause any harm when using it within a transaction

User · Answer

Actually  em flush    do more than just sends the cached SQL commands  It tries to synchronize the persistence context to the underlying database  It can cause a lot of time consumption on your processes if your cache contains collections to be synchronized   Caution on using it

User · Answer

Probably the exact details of em flush   are implementation-dependent  In general anyway  JPA providers like Hibernate can cache the SQL instructions they are supposed to send to the database  often until you actually commit the transaction  For example  you call em persist    Hibernate remembers it has to make a database INSERT  but does not actually execute the instruction until you commit the transaction  Afaik  this is mainly done for performance reasons   In some cases anyway you want the SQL instructions to be executed immediately  generally when you need the result of some side effects  like an autogenerated key  or a database trigger   What em flush   does is to empty the internal SQL instructions cache  and execute it immediately to the database   Bottom line  no harm is done  only you could have a  minor  performance hit since you are overriding the JPA provider decisions as regards the best timing to send SQL instructions to the database

User · Answer

Can em flush   cause any harm when using it within a transaction    Yes  it may hold locks in the database for a longer duration than necessary   Generally  When using JPA you delegates the transaction management to the container  a k a CMT - using  Transactional annotation on business methods  which means that a transaction is automatically started when entering the method and commited   rolled back at the end  If you let the EntityManager handle the database synchronization  sql statements execution will be only triggered just before the commit  leading to short lived locks in database  Otherwise your manually flushed write operations may retain locks between the manual flush and the automatic commit which can be long according to remaining method execution time   Notes that some operation automatically triggers a flush   executing a native query against the same session  EM state must be flushed to be reachable by the SQL query   inserting entities using native generated id  generated by the database  so the insert statement must be triggered thus the EM is able to retrieve the generated id and properly manage relationships

[java] Correct use of flush() in JPA/Hibernate

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