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

The JPA specification says the following about persist().

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.

So using persist() would be suitable when the object ought not to be a detached object. You might prefer to have the code throw the PersistenceException so it fails fast.

Although the specification is unclear, persist() might set the @GeneratedValue@Id for an object. merge() however must have an object with the @Id already generated.