A natural identifier nevertheless identifies a database record or an object in the real world. So for that cases is necessary make use of a naturalId.
Hibernate allows us model fields as a natural identifier of an entity and retrieve them from database.
If you want more information you can visit: https://refactorizando.com/naturalid-hibernate-spring-boot
Hibernate use the annotation NaturalId to indicate in an entity the use of this functionality. So to use NaturalId you have to add in your entity @NaturalId.
@NaturalId private String isbn;