Collection Associations Should Not Use Eager Fetching

ID

java.hibernate_collections_must_be_lazy

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Efficiency

Language

Java

Tags

best-practice, efficiency, hibernate, orm

Description

Reports collection-valued associations (@OneToMany, @ManyToMany, @ElementCollection) that explicitly set fetch = FetchType.EAGER. Eagerly fetching collections loads all associated entities immediately with the owning entity, often leading to N+1 query problems, excessive memory usage, and unpredictable performance degradation as data grows.

Rationale

When a collection association is marked EAGER, Hibernate loads the entire collection every time the parent entity is fetched, regardless of whether the collection is actually used. For @OneToMany and @ManyToMany relationships, this can cascade into loading thousands of rows and their transitive associations.

// Bad: eager-fetching a collection
@Entity
public class Department {
    @Id
    private Long id;

    @OneToMany(fetch = FetchType.EAGER)
    private List<Employee> employees;
}

Loading a single Department also loads every Employee, which may in turn eagerly load their own associations, creating a cascade of queries.

Remediation

Remove the explicit fetch = FetchType.EAGER attribute or set it to FetchType.LAZY. Use JOIN FETCH in JPQL queries or entity graphs when the collection is needed for a specific use case.

// Good: lazy-fetching (default for @OneToMany)
@Entity
public class Department {
    @Id
    private Long id;

    @OneToMany
    private List<Employee> employees;
}

References