Hibernate ManyToOne with FetchType.LAZY not fetching lazy

问题

I am using Hibernate with spring.

I have a model-class like this.


@Entity
@Table(name = "forumtopic")
public final class Forumtopic extends AbstractUserTracking implements
    java.io.Serializable {

/**SNIP **/

    private Forumcategory forumcategory;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "FkForumcategoryId", nullable = false)
    public Forumcategory getForumcategory() {
        return this.forumcategory;
    }

    public void setForumcategory(final Forumcategory forumcategory) {
        this.forumcategory = forumcategory;
    }
}

It works in general, but the Category is not loaded lazy, but eagerly after the ForumEntry has been loaded.

Hibernate: 
    select
        forumtopic0_.PkId as PkId19_0_,
        forumtopic0_.CreateDate as CreateDate19_0_,
        forumtopic0_.FkCreateUserId as FkCreate3_19_0_,
        forumtopic0_.FkLastUserId as FkLastUs4_19_0_,
        forumtopic0_.LastChange as LastChange19_0_,
        forumtopic0_.FkForumcategoryId as FkForum10_19_0_,
        forumtopic0_.PublishCategory as PublishC6_19_0_,
        forumtopic0_.State as State19_0_,
        forumtopic0_.Text as Text19_0_,
        forumtopic0_.Topic as Topic19_0_,
        forumtopic0_.FkTpUserId as FkTpUserId19_0_ 
    from
        forumtopic forumtopic0_ 
    where
        forumtopic0_.PkId=?
Hibernate: 
    select
        forumcateg0_.PkId as PkId17_0_,
        forumcateg0_.CreateDate as CreateDate17_0_,
        forumcateg0_.Name as Name17_0_,
        forumcateg0_.FkRequestId as FkReques4_17_0_,
        forumcateg0_.FkTpUserId as FkTpUserId17_0_ 
    from
        forumcategory forumcateg0_ 
    where
        forumcateg0_.PkId=?

Altough the getter was not called the ForumCategory is loaded right after ForumTopic.

This problems appears in all my @ManyToOne-associations. However @OneToMany associating are loaded lazily.

I am using maven2 for the build. These are my dependencies.

  <dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring</artifactId>
    <version>2.5.6</version>
  </dependency>


  <dependency>
    <groupId>org.hibernate</groupId> 
    <artifactId>hibernate-core</artifactId> 
    <version>3.3.1.GA</version> 
  </dependency>
  <dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>ejb3-persistence</artifactId>
    <version>1.0.2.GA</version>
  </dependency>
  <dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-annotations</artifactId>
    <type>jar</type>
    <version>3.4.0.GA</version>
  </dependency>
  <dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-entitymanager</artifactId>
    <type>jar</type>
    <version>3.4.0.GA</version>
  </dependency>
  <dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-search</artifactId>
    <version>3.1.0.GA</version>
</dependency>

Can someone please help me understand what is happening?

回答1:

I guess it's caused by the fact that your classes are declared as final, therefore Hibernate cannot generate lazy proxies for them. Try to remove final from class declarations.

回答2:

Looks like the transaction/session is closed after returning ForumTopic. Hence it becomes detached entity. When you try to do a getForumCategory there is no session to do the operation. You can prefetch the ForumCategory in the same session like,

In your getForumTopic, before returning a list (assuming you have a getAllForumTopic)

public List<ForumTopic> getAllForumTopic() {
  <snip> 
  List<ForumTopic> topics = <SNIP: get the list of ForumTopic>

  for(ForumTopic topic: topics)
      topic.getForumCategory()

  return topics;
}

This will fetch the ForumCategory in the same session. Otherwise you have to create a transaction in the calling function of getAllForumTopic and use the same transaction where ever you need to call getForumCategory.

来源：https://stackoverflow.com/questions/5594297/hibernate-manytoone-with-fetchtype-lazy-not-fetching-lazy

标签

hibernate

Spring

maven-2

lazy-loading

many-to-one