Hibernate Tips: How to avoid Hibernate’s MultipleBagFetchException

Avatar photoByThorben Janssen

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Hibernate Tips is a series of posts in which I describe a quick and easy solution for common Hibernate questions. If you have a question for a future Hibernate Tip, please leave a comment below.

Question:

You explained that I should use a JOIN FETCH clause to initialize all associations of my entity that I will use in my use case. But as soon as I do that for more than 1 association, Hibernate throws a MultipleBagFetchException.

How can I JOIN FETCH multiple associations?

Solution:

Hibernate throws a MultipleBagFetchException whenever you try to fetch multiple Bags in a query.

org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple Bags: [org.thoughts.on.java.model.Book.authors, org.thoughts.on.java.model.Book.reviews]

As I explained in my post about the most efficient data type for to-many associations, a Bag is an unordered collection. Hibernate uses it if you model your association as a List.

@Entity
public class Book {
	
	// Don't do this!

	@ManyToMany
	@JoinTable(
		      name="BookAuthor",
		      joinColumns={@JoinColumn(name="bookId", referencedColumnName="id")},
		      inverseJoinColumns={@JoinColumn(name="authorId", referencedColumnName="id")})
	private List authors = new ArrayList();
	
	@OneToMany(mappedBy = "book")
	private List reviews = new ArrayList();
	
	...
	
}

You can avoid the MultipleBagFetchException by using a Set instead of a List.

@Entity
public class Book {
	
	@ManyToMany
	@JoinTable(
		      name="BookAuthor",
		      joinColumns={@JoinColumn(name="bookId", referencedColumnName="id")},
		      inverseJoinColumns={@JoinColumn(name="authorId", referencedColumnName="id")})
	private Set authors = new HashSet();
	
	@OneToMany(mappedBy = "book")
	private Set reviews = new HashSet();
	
	...
	
}

In contrast to the previously used Bags, Hibernate fetches multiple Sets without throwing an exception.

Book b = em.createQuery("SELECT b "
				+ "FROM Book b "
					+ "JOIN FETCH b.authors a "
					+ "JOIN FETCH b.reviews r "
				+ "WHERE b.id = 1",
				Book.class).getSingleResult();

Learn more:

I explained the differences between a Set and a Bag in more details in How to Choose the Most Efficient Data Type for To-Many Associations – Bag vs. List vs. Set.

And if you want to learn more about Hibernate associations and how you can handle them efficiently, you should read the following posts:

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4 Comments

  1. Avatar photo Hans-Martin Fetzer says:

    Hi,
    this tip avoids the exception, but findBy may then return wrong results!

    I had a simular situation with two eager associations. Translated to book example: with a book b, having
    two authors a1 and a2 and two reviews r1 and r2.

    findBy returned b having two reviews r1 and r2 but four authors a1, a2, and again a1, a2!
    The problem seams to be an, in this case, incorrect generated sql like:
    select . from book left outer join author . left outer join review ..

    Looks to be an error in Hibernate, to use a list and a set together should also be forbidden?!

    1. Avatar photo Thorben Janssen says:

      Hi Hans-Martin,

      I wasn’t able to reproduce that issue. But what happens is, that you will get the Book entity 4 times, if it’s associated with 2 Authors and 2 Reviews. You can avoid that by adding the DISTINCT keyword to your query and by setting the QueryHints.PASS_DISTINCT_THROUGH to false, e.g.:

      TypedQuery q = em.createQuery(“SELECT DISTINCT b ”
      + “FROM Book b ”
      + “JOIN FETCH b.authors a ”
      + “JOIN FETCH b.reviews r ”
      + “WHERE b.id = 1”,
      Book.class);
      q.setHint(QueryHints.PASS_DISTINCT_THROUGH, false);
      List       b = q.getResultList();

      Regards,
      Thorben

  2. If you change the collection to a Set how will you be able to add multiple new entities to the collection? Even if you add them one by one and have a save call to the child entity, you will run into a problem if the entity manager does not flush between the calls to add the new entities to the parent entity.

    1. Avatar photo Thorben Janssen says:

      Hi Om Tara,

      That depends on the equals() implementation of your entity. In most cases, I don’t recommend implementing equals. But if you do, you need to make sure that an entity object with a null primary key value is never equal to another entity object.

      Regards,
      Thorben

Comments are closed.