difference between entityManager.persist() and entityManager.merge()

EntityManager_persist_vs_merge.md

QUESTION:

EntityManager.merge() can insert new objects and update existing ones.

Why would one want to use persist() (which can only create new objects)?

ANSWER:

Either way will add an entity to a PersistenceContext, the difference is in what you do with the entity afterwards.

Persist takes an entity instance, adds it to the context and makes that instance managed (ie future updates to the entity will be tracked)

Merge creates a new instance of your entity, copies the state from the supplied entity, and makes the new copy managed. The instance you pass in will not be managed (any changes you make will not be part of the transaction - unless you call merge again).

more informations,
http://stackoverflow.com/questions/1069992/jpa-entitymanager-why-use-persist-over-merge
http://blog.xebia.com/2009/03/23/jpa-implementation-patterns-saving-detached-entities/
http://www.stevideter.com/2008/12/07/saveorupdate-versus-merge-in-hibernate/

The problem is that everyone, including me, uses the return of the merge that is after all, managed. :P

Good explanation!

When I found that post I was looking for the difference between session.saveOrUpdate() e entityManager.merge(), anyways, I learned something interesting in the end.

More informations about them and CASCADE,
http://stackoverflow.com/questions/4509086/what-is-the-difference-between-persist-and-merge-in-hibernate
http://stackoverflow.com/questions/18373383/jpa-onetoone-difference-between-cascade-merge-and-persist (EXCELLENT)

http://webdev.apl.jhu.edu/~jcs/ejava-javaee/coursedocs/605-784-site/docs/content/html/jpa-relationex-one2many-uni.html#jpa-relationex-o2m-uni-orphanRemoval

http://stackoverflow.com/questions/306144/jpa-cascadetype-all-does-not-delete-orphans

Ok.. Thanks for the xplanation.... But how does someone make use of the merge. I am new to spring jpa and hibernate.