手动设置自动生成的id属性

Question

In Hibernate is there a way to skip generating the value of a @GeneratedValue property by manually setting it before calling save on the session?

I am building an import/export facility and I would like to preserve the IDs from a previously made export.

Answer 1

Is important to say, if you use this way:

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

And you don't have the id you set in the database it will ignore your id set and the database will create a new one. I mean, if you have an empty database and set user.setId(100L) and save, when you find it in database the service.findById(100L) will not find any object.

Answer 2

By default, Hibernate won't override value if it was already set, so everything should work as you expect. Just make sure you're not using AUTO.

If you need a more complex logic, take a look at @GenericGenerator. https://docs.jboss.org/hibernate/stable/annotations/reference/en/html_single/#entity-hibspec-identifier

Answer 3

@GeneratedValue will only apply it's strategy if the id on which you're applying this annotation is null / 0 .

This means that if you've manually assigned an id field, then @GeneratedValue annotation is USELESS.

Answer 4

The problem is that I was using the default ( AUTO ) strategy for the @GeneratedValue . If I change it to explicitly specify IDENTITY then it works!

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

I don't really understand why though... :-)