Hibernate flush and commit

Question

Is it good practice to call org hibernate Session flush   separately   As said in org hibernate Session docs       Must be called at the end of a unit of work  before commiting the transaction and closing the session  depending on flush-mode  Transaction commit   calls this method     Could you explain the purpose of calling flush   explicitely if org hibernate Transaction commit   will do it already

User · Answer

It is usually not recommended to call flush explicitly unless it is necessary. Hibernate usually auto calls Flush at the end of the transaction and we should let it do it's work. Now, there are some cases where you might need to explicitly call flush where a second task depends upon the result of the first Persistence task, both being inside the same transaction.

For example, you might need to persist a new Entity and then use the Id of that Entity to do some other task inside the same transaction, on that case it's required to explicitly flush the entity first.

@Transactional
void someServiceMethod(Entity entity){
    em.persist(entity); 
    em.flush() //need to explicitly flush in order to use id in next statement
    doSomeThingElse(entity.getId());    
}

Also Note that, explicitly flushing does not cause a database commit, a database commit is done only at the end of a transaction, so if any Runtime error occurs after calling flush the changes would still Rollback.

User · Answer

flush    Flushing is the process of synchronizing the underlying persistent store with persistable state held in memory  It will update or insert into your tables in the running transaction  but it may not commit those changes       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  So  you should commit but it also ends the unit of work  transaction commit

User · Answer

By default flush mode is AUTO which means that   The Session is sometimes flushed before query execution in order to ensure that queries never return stale state   but most of the time session is flushed when you commit your changes  Manual calling of the flush method is usefull when you use FlushMode MANUAL or you want to do some kind of optimization  But I have never done this so I can t give you practical advice

User · Answer

flush   will synchronize your database with the current state of object objects held in the memory but it does not commit the transaction  So  if you get any exception after flush   is called  then the transaction will be rolled back  You can synchronize your database with small chunks of data using flush   instead of committing a large data at once using commit   and face the risk of getting an OutOfMemoryException   commit   will make data stored in the database permanent  There is no way you can rollback your transaction once the commit   succeeds

User · Answer

In the Hibernate Manual you can see this example  Session session   sessionFactory openSession    Transaction tx   session beginTransaction     for  int i   0  i  lt  100000  i          Customer customer   new Customer           session save customer       if  i   20    0       20  same as the JDBC batch size            flush a batch of inserts and release memory          session flush            session clear             tx commit    session close      Without the call to the flush method  your first-level cache would throw an OutOfMemoryException   Also you can look at this post about flushing

User · Answer

session flush   is synchronise method means to insert data in to database sequentially if we use this method data will not store in database but it will store in cache if any exception will rise in middle we can handle it  But commit   it will store data in database if we are storing more amount of data then  there may be chance to get out Of Memory Exception As like in JDBC program in Save point topic

User · Answer

One common case for explicitly flushing is when you create a new persistent entity and you want it to have an artificial primary key generated and assigned to it  so that you can use it later on in the same transaction  In that case calling flush would result in your entity being given an id    Another case is if there are a lot of things in the 1st-level cache and you d like to clear it out periodically  in order to reduce the amount of memory used by the cache  but you still want to commit the whole thing together  This is the case that Aleksei s answer covers

[java] Hibernate: flush() and commit()

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