Answer by Josep Panadero for JPA EntityManager: Why use persist() over merge()?

Persist and merge are for two different purposes (they aren't alternatives at all).

(edited to expand differences information)

persist:

• Insert a new register to the database
• Attach the object to the entity manager.

merge:

• Find an attached object with the same id and update it.
• If exists update and return the already attached object.
• If doesn't exist insert the new register to the database.

persist() efficiency:

• It could be more efficient for inserting a new register to a database than merge().
• It doesn't duplicates the original object.

persist() semantics:

• It makes sure that you are inserting and not updating by mistake.

Example:

{    AnyEntity newEntity;    AnyEntity nonAttachedEntity;    AnyEntity attachedEntity;    // Create a new entity and persist it            newEntity = new AnyEntity();    em.persist(newEntity);    // Save 1 to the database at next flush    newEntity.setValue(1);    // Create a new entity with the same Id than the persisted one.    AnyEntity nonAttachedEntity = new AnyEntity();    nonAttachedEntity.setId(newEntity.getId());    // Save 2 to the database at next flush instead of 1!!!    nonAttachedEntity.setValue(2);    attachedEntity = em.merge(nonAttachedEntity);    // This condition returns true    // merge has found the already attached object (newEntity) and returns it.    if(attachedEntity==newEntity) {            System.out.print("They are the same object!");    }    // Set 3 to value    attachedEntity.setValue(3);    // Really, now both are the same object. Prints 3    System.out.println(newEntity.getValue());    // Modify the un attached object has no effect to the entity manager    // nor to the other objects    nonAttachedEntity.setValue(42);}

This way only exists 1 attached object for any register in the entity manager.

merge() for an entity with an id is something like:

AnyEntity myMerge(AnyEntity entityToSave) {    AnyEntity attached = em.find(AnyEntity.class, entityToSave.getId());    if(attached==null) {            attached = new AnyEntity();            em.persist(attached);    }    BeanUtils.copyProperties(attached, entityToSave);    return attached;}

Although if connected to MySQL merge() could be as efficient as persist() using a call to INSERT with ON DUPLICATE KEY UPDATE option, JPA is a very high level programming and you can't assume this is going to be the case everywhere.