I have question about final classes in Hibernate. reading Hibernate documentation I have read:



Then I have developed a simple example, I have created two final classes Foo and Bar. Foo has a one to many relationship to Bar. Foo and Bar are annotated with @Entity and relationship with @OneToMany(mappedBy="foo", cascade={CascadeType.ALL}) and @ManyToOne, at field level access.

Then I create a test, which inserts a Foo and many Bar. Then I find by id a Foo object and then I call getBars() and it acts as a lazy, Hibernate does a query and returns all Bars.

What I am missing?, it is supposed to not create this query because classes are final, but query is executed.

Thank you very much for your help.

Hi asotobu,

you seem to have misunderstood something.
At first thing we must distinct between lazy loading on collections (=ToMany relations) and
lazy loading on entity references (= ToOne relations).
Only the latter one uses proxies, so the difference turns out only in following scenario:



In case that you declare class Foo non final,
hibernate can fetch the bar instance from database without need to read also its parent foo record,
because bar.foo will be filled with a proxy of Foo containing just the primary key information of that foo.
That proxy is a runtime subclass of class Foo.

In case that you declare class Foo final,
nobody is allowed to extend Foo, that means nobody can create subclasses of Foo, so hibernate cannot create a proxy for Foo and thus hibernate must load the parent foo instance eagerly. This results in having a second query being executed for fetching that foo record from database.

Hope now its clear.

Thank you very much for your answer, now I understand exactly what it means that line.

Thanks again,
Alex.

Hi, I am sorry to disturb you again, but I can't still watch any difference. I am using EntityManager find method to load an instance of Bar.



Foo is final (ManyToOne part) but not Bar.

I execute the code and a select from Bar with a left outer join to Foo is also executed. Seems normal because Foo is final an a proxy cannot be created. In fact if I debug, Foo is not a proxy.

Then I change Foo as not final, and the same behavior is seen.

Note that I am using JPA annotations, where by default ManyToOne is Eager.

Thank you very much again for your help.

Ok, you have already found the key to the explanation!

If you want to use lazy association fetching then you must declare

on your relation.
Only with lazy option set, hibernate will try to instantiate the associated entity as proxy instead to resolve it from db, OK?