Spring Data JPA

1. How does Spring data JPA determine whether an object is a new object?

Spring data provides three strategies:

Able 2.2. Options for detection whether a entity is new in Spring Data JPA

Id-property Inspection (default)	By default Spring Data JPA inspects the id-property of the given Entity. If null The Id-property is and then the entity would be assumed as new, otherwise as not new.
ImplementingPersistable	If an entity implements Persistable the interface, Spring Data JPA would delegate the New-detection isNew to the -Method of the Entity. See Thejavadoc for details.
ImplementingEntityInformation	One can customize The entityinformation  abstraction used in The  simplejparepository   implementation by creating a subclass Ofjparepositoryfactory  and overriding The  getentityinformation -method accordingly. One then have to register the custom implementation Ofjparepositoryfactory  as a Spring bean. Note that this should is rarely necessary. See The javadoc for details.

You can see that the default is to determine if there is an ID. You can then take other strategies, such as implementing an Persistable interface, and then overriding the IsNew method.

2. Spring Data annotations

Spring Data JPA