What is the difference between persist and merge in JPA and Hibernate

Question

What is the difference between persist   and merge   in Hibernate   persist   can create a UPDATE  amp  INSERT query  eg   SessionFactory sef   cfg buildSessionFactory    Session session   sef openSession    A a new A    session persist a   a setName  Mario    session flush      in this case query will be generated like this   Hibernate  insert into A  NAME  ID  values        Hibernate  update A set NAME   where ID     so persist   method can generate an Insert and an Update   Now with merge     SessionFactory sef   cfg buildSessionFactory    Session session   sef openSession    Singer singer   new Singer    singer setName  Luciano Pavarotti    session merge singer   session flush      This is what I see in the database   SINGER ID   SINGER NAME 1           Ricky Martin 2           Madonna 3           Elvis Presley 4           Luciano Pavarotti   Now update a record using merge    SessionFactory sef   cfg buildSessionFactory    Session session   sef openSession    Singer singer   new Singer    singer setId 2   singer setName  Luciano Pavarotti    session merge singer   session flush      This is what I see in the database   SINGER ID   SINGER NAME 1           Ricky Martin 2           Luciano Pavarotti 3           Elvis Presley

User · Answer

Persist should be called only on new entities, while merge is meant to reattach detached entities.

If you're using the assigned generator, using merge instead of persist can cause a redundant SQL statement.

Also, calling merge for managed entities is also a mistake since managed entities are automatically managed by Hibernate, and their state is synchronized with the database record by the dirty checking mechanism upon flushing the Persistence Context.

User · Answer

This is coming from JPA  In a very simple way    persist entity  should be used with totally new entities  to add them to DB  if entity already exists in DB there will be EntityExistsException throw    merge entity  should be used  to put entity back to persistence context if the entity was detached and was changed

User · Answer

The most important difference is this    In case of persist method  if the entity that is to be managed in the persistence context  already exists in persistence context  the new one is ignored   NOTHING happened  But in case of merge method  the entity that is already managed in persistence context will be replaced by the new entity  updated  and a copy of this updated entity will return back   from now on any changes should be made on this returned entity if you want to reflect your changes in persistence context

User · Answer

JPA specification contains a very precise description of semantics of these operations  better than in javadoc      The semantics of the persist   operation  applied to an entity X are   as follows           If X is a new entity  it   becomes managed  The entity X will be   entered into the database at or before   transaction commit or as a result of   the flush operation    If X is a   preexisting managed entity  it is   ignored by the persist operation    However  the persist operation is   cascaded to entities referenced by X    if the relationships from X to these   other entities are annotated with the   cascade PERSIST or cascade ALL   annotation element value or specified   with the equivalent XML descriptor   element    If X is a removed entity    it becomes managed    If X is a   detached object  the   EntityExistsException may be thrown   when the persist operation is invoked    or the EntityExistsException or   another PersistenceException may be   thrown at flush or commit time    For   all entities Y referenced by a   relationship from X  if the   relationship to Y has been annotated   with the cascade element value   cascade PERSIST or cascade ALL  the   persist operation is applied to Y            The semantics of the merge operation   applied to an entity X are as follows          If X is a detached entity  the state   of X is copied onto a pre-existing   managed entity instance X  of the same   identity or a new managed copy X  of X   is created    If X is a new entity   instance  a new managed entity   instance X  is created and the state   of X is copied into the new managed   entity instance X     If X is a   removed entity instance  an   IllegalArgumentException will be   thrown by the merge operation  or the   transaction commit will fail     If X   is a managed entity  it is ignored by   the merge operation  however  the   merge operation is cascaded to   entities referenced by relationships   from X if these relationships have   been annotated with the cascade   element value cascade MERGE or   cascade ALL annotation    For all   entities Y referenced by relationships   from X having the cascade element   value cascade MERGE or cascade ALL  Y   is merged recursively as Y   For all   such Y referenced by X  X  is set to   reference Y    Note that if X is   managed then X is the same object as   X      If X is an entity merged to X     with a reference to another entity Y    where cascade MERGE or cascade ALL is   not specified  then navigation of the   same association from X  yields a   reference to a managed object Y  with   the same persistent identity as Y

[java] What is the difference between persist() and merge() in JPA and Hibernate?

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