Hibernate MultipleBagFetchException: How to Resolve Eager Fetching Issues During SessionFactory Creation?

Hibernate: Fixing MultipleBagFetchException During SessionFactory Creation

When using Hibernate, you may encounter an exception during SessionFactory creation:

org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags

This is typically caused when fetching multiple collections of entities eagerly (using FetchType.EAGER) within an entity. This can lead to performance issues, data inconsistency, and memory overhead.

Test Case:

Consider the following example:

@Entity
public class Parent {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @OneToMany(mappedBy = "parent", fetch = FetchType.EAGER)
    private List<Child> children;

}

@Entity
public class Child {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    private Parent parent;

}

In this case, fetching Parent.children eagerly tries to simultaneously fetch multiple child collections, leading to the exception.

Potential Solutions:

1. Remove FetchType.EAGER:

Remove FetchType.EAGER from the Parent.children mapping. This will cause Hibernate to lazily fetch the children when needed, resolving the exception. However, it may affect performance.

2. Use LazyCollection:

Annotate the children field with @LazyCollection(LazyCollectionOption.FALSE) instead of using FetchType.EAGER. This instructs Hibernate to load the collection eagerly, but via a lazy initialization process, avoiding the exception.

3. Use a Set:

In most cases, it's preferable to use Set instead of List in the mapping. Sets are unordered and don't allow duplicates, eliminating the potential for Cartesian product issues.

Caution:

Using sets does not eliminate the underlying Cartesian product issue, as discussed by Vlad Mihalcea. However, it is generally a better approach and can improve performance.

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