Spring JPA CrudRepository save(Entity) is returning 0 in id field - spring

Question

I am adding spring JPA/Hibernate 5 to an old project. I am running CrudRepository .save(Entity) method against a Mainframe/DB2 table. The row is inserted just fine, but returnedEntity.getIdColumn() has 0 in it. I really need the ID for further processing. Could somebody please help? Thank you very much!
#Entity
Table(name="MY_TABLE")
public class myClass {
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
#Column(name="ID_COLUMN")
private Long idColumn;
...
}
Identity is the only strategy type that worked.
Here is the Service class:
#Transactional
public Entiry insertEntity(Entity originalEntity) {
return MyRepository.save(originalEntity);
}
Runner class:
Entity originalEntity = createEntity();
Entity returnedEntity = ServiceClass.insertEntity(originalEntity);
System.out.println(originalEntity.getIdColumn());
System.out.println(returnedEntity.getIdColumn());

Answer 1

My guess is that you're trying to get an ID before your transaction was flushed to DB. Thus JPA doesn't know what id will be assigned and returns 0.
Edited:
I'd recommend to have something like this:
#Transactional
public Entity save( .....) {
//some code
repository.save(entity);
//some code
return entity;
}
Transaction will be flushed at the end of this method and entity which will be returned from it should have a real id.

Answer 2

Turned out my table had an id sequence generator already defined in the DB. So, after I changed it to generationType.SEQUENCE it worked nicely.
#GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "MY_TABLE_SEQ")

Answer 3

Recently, I've faced with similar issue.
In my case, the new entity has been passed from UI and ID was 0 instead of NULL (since the type of ID was primitive on Java side). Therefore Hibernate didn't use right save strategy.
Eventually, I changed the type from long to Long and it helped:
private long id;
private Long id;
Also update method was changed from:
#PutMapping("/{id}")
public T update(#PathVariable ID id, #Valid #RequestBody T entity) {
this.getService().update(entity);
return entity;
}
to:
#PutMapping("/{id}")
public T update(#PathVariable ID id, #Valid #RequestBody T entity) {
return this.getService().update(entity);
}
P.S.: When entity ID was 0, Hibernate was using merge strategy, however, it should persist strategy for new Entities instead (link for implementation of save method).