Preventing infinite loop data retrieval with Hibernate

Question

I was wondering: Imagine a scenario where e.g.

//POJO

public class User {

   private String userName;
   private String name;
   private String surname;
   private List<Blog> blogList;

   //All getters and setters are found here and assume they're generated.
}

public class Blog {
    private String title;
    private String content;
    private User author;
    private Date datePublished;
    private Date dateLastModified;

    //All getters and setters have been generated (by Eclipse or NetBeans)
}

Imagine that these objects have been correctly mapped into their respective Hibernate configuration files.

My question:

How would I retrieve my user with the list of all the user blogs on code level? (i.e., not allow hibernate to populate blogList automatically for me. I want to add paging (i.e. from list 5, retrieve 20 list) and also, if you think carefully, this might be an infinite loop as a Blog has a User entity which has a List<Blog> entity.

How do I prevent this?

PS Just out of curiousity, how would I let Hibernate populate my blogList on the configuration side?

Thanks in advance.

score 4 · Accepted Answer · answered Jul 15 '10 at 16:54

4

Hibernate detects such loops and doesn't let them happen
You can mark your collection with fetch type=lazy (fetchType=FetchType.LAZY) so that the collection elements are not fetched when the owning object is
you can used a Query with setFirstResult(..) and setMaxResults(..) in order to achieve paging. (and get rid of the collection then)

answered Jul 15 '10 at 16:54

Bozho

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Beat me by 10 seconds. Nice. – aperkins Jul 15 '10 at 16:55
I get what you mean, so I guess it's bye, bye DAO's :) – Buhake Sindi Jul 15 '10 at 17:03
What does `FetchType.LAZY` do? – Buhake Sindi Jul 15 '10 at 17:04
1

it is by no means "bye" to DAOs. The DAOs use the session/entity manager, Query objects, etc. The service layer should not know of these. FetchType.LAZY means that collection is fetched only if it is accessed. – Bozho Jul 15 '10 at 17:07
OK...so would Hibernate know if the collection is accessed? by the `getBlogList()` method? – Buhake Sindi Jul 15 '10 at 17:09
it creates a proxy around your list. So it becomes `PersistentList`, instead of `ArrayList` for example. – Bozho Jul 15 '10 at 17:10
I'm beginning to like Hibernate now. – Buhake Sindi Jul 15 '10 at 17:23

Preventing infinite loop data retrieval with Hibernate

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