What s the use of session flush in Hibernate

Question

When we are updating a record  we can use session flush   with Hibernate  What s the need for flush

User · Answer

The flush    method causes Hibernate to flush the session  You can configure Hibernate to use flushing mode for the session by using setFlushMode   method  To get the flush mode for the current session  you can use getFlushMode   method  To check  whether session is dirty  you can use isDirty   method  By default  Hibernate manages flushing of the sessions   As stated in the documentation   https   docs jboss org hibernate orm 5 2 userguide html single chapters flushing Flushing html     Flushing      Flushing is the process of synchronizing the state of the persistence   context with the underlying database  The EntityManager and the   Hibernate Session expose a set of methods  through which the   application developer can change the persistent state of an entity       The persistence context acts as a transactional write-behind cache    queuing any entity state change  Like any write-behind cache  changes   are first applied in-memory and synchronized with the database during   flush time  The flush operation takes every entity state change and   translates it to an INSERT  UPDATE or DELETE statement       The flushing strategy is given by the flushMode of the current   running Hibernate Session  Although JPA defines only two flushing   strategies  AUTO and COMMIT   Hibernate has a much   broader spectrum of flush types          ALWAYS  Flushes the Session before every query    AUTO  This is the default mode and it flushes the Session only if necessary    COMMIT  The Session tries to delay the flush until the current Transaction is committed  although it might flush prematurely too    MANUAL  The Session flushing is delegated to the application  which must call Session flush   explicitly in order to apply the   persistence context changes          By default  Hibernate uses the AUTO flush mode which triggers a   flush in the following circumstances          prior to committing a Transaction    prior to executing a JPQL HQL query that overlaps with the queued entity actions    before executing any native SQL query that has no registered synchronization

User · Answer

As rightly said in above answers  by calling flush   we force hibernate to execute the SQL commands on Database  But do understand that changes are not  committed  yet   So after doing flush and before doing commit  if you access DB directly  say from SQL prompt  and check the modified rows  you will NOT see the changes    This is same as opening 2 SQL command sessions  And changes done in 1 session are not visible to others until committed

User · Answer

I only know that when we call session flush   our statements are execute in database but not committed   Suppose we don t call flush   method on session object and if we call commit method it will internally do the work of executing statements on the database and then committing   commit flush commit  in case of functionality   Thus  I conclude that when we call method flush   on Session object  then it doesn t get commit but hits the database and executes the query and gets rollback too   In order to commit we use commit   on Transaction object

User · Answer

You might use flush to force validation constraints to be realised and detected in a known place rather than when the transaction is committed  It may be that commit gets called implicitly by some framework logic  through declarative logic  the container  or by a template  In this case  any exception thrown may be difficult to catch and handle  it could be too high in the code    For example  if you save   a new EmailAddress object  which has a unique constraint on the address  you won t get an error until you commit   Calling flush   forces the row to be inserted  throwing an Exception if there is a duplicate   However  you will have to roll back the session after the exception

User · Answer

I would just like to club all the answers given above and also relate Flush   method with Session save   so as to give more importance   Hibernate save   can be used to save entity to database  We can invoke this method outside a transaction  that   s why I don   t like this method to save data  If we use this without transaction and we have cascading between entities  then only the primary entity gets saved unless we flush the session   flush    Forces the session to flush  It is used to synchronize session data with database   When you call session flush    the statements are executed in database but it will not committed  If you don   t call session flush   and if you call session commit     internally commit   method executes the statement and commits   So commit    flush commit  So session flush   just executes the statements in database  but not commits  and statements are NOT IN MEMORY anymore  It just forces the session to flush   Few important points   We should avoid save outside transaction boundary  otherwise mapped entities will not be saved causing data inconsistency  It   s very normal to forget flushing the session because it doesn   t throw any exception or warnings  By default  Hibernate will flush changes automatically for you  before some query executions when a transaction is committed Allowing to explicitly flush the Session gives finer control that may be required in some circumstances  to get an ID assigned  to control the size of the Session

User · Answer

Flushing the session forces Hibernate to synchronize the in-memory state of the Session with the database  i e  to write changes to the database   By default  Hibernate will flush changes automatically for you    before some query executions  when a transaction is committed   Allowing to explicitly flush the Session gives finer control that may be required in some circumstances  to get an ID assigned  to control the size of the Session

User · Answer

Calling EntityManager flush does have side-effects  It is conveniently used for entity types with generated ID values  sequence values   such an ID is available only upon synchronization with underlying persistence layer  If this ID is required before the current transaction ends  for logging purposes for instance   flushing the session is required

User · Answer

Flushing the Session gets the data that is currently in the session synchronized with what is in the database   More on the Hibernate website    http   docs jboss org hibernate core 3 3 reference en html objectstate html objectstate-flushing   flush   is useful  because there are absolutely no guarantees about when the Session executes the JDBC calls  only the order in which they are executed - except you use flush

User · Answer

With this method you evoke the flush process  This process synchronizes  the state of your database with state of your session by detecting state changes and executing the respective SQL statements

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