How to Resolve Hibernate's MultipleBagFetchException During SessionFactory Initialization?

MultipleBagFetchException in Hibernate: Simultaneous Fetching of Multiple Bags

Hibernate encounters the org.hibernate.loader.MultipleBagFetchException when attempting to simultaneously fetch multiple bags during SessionFactory initialization. This exception manifests in the following scenario:

A Parent entity maintains a relationship with multiple Child entities using a OneToMany association. In the Parent class, the children property represents the collection of Child objects.

When using FetchType.EAGER on the children property, Hibernate attempts to eager load all Child entities associated with the Parent during SessionFactory creation. However, this leads to the MultipleBagFetchException exception because Hibernate cannot simultaneously fetch multiple bags (i.e., the children collection) from the database.

To resolve this issue, it is recommended to remove the FetchType.EAGER attribute from the @OneToMany annotation. Alternatively, you can annotate the collection fields with @LazyCollection(LazyCollectionOption.FALSE) to achieve the same Eager fetch strategy.

Additional Considerations

In the case where the Parent entity contains another entity with a FetchType.EAGER collection, such as AnotherParent with a collection of AnotherChild objects, Hibernate will also encounter the MultipleBagFetchException. This occurs because Hibernate cannot handle simultaneous Eager fetching of multiple collections.

To address this issue, change the FetchType.EAGER attribute to @LazyCollection(LazyCollectionOption.FALSE) for both the Parent and AnotherParent collections. Remember, using Set instead of List for collections can reduce the likelihood of encountering this exception.

Caution:

Using Set does not eliminate the underlying Cartesian Product issue, as described by Vlad Mihalcea in another answer. This is why it is crucial to use Eager fetching with caution, carefully considering the performance implications and potential side effects.

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