Difference between lazy and eager loading in Hibernate

Lazy and Eager are two data loading strategies used in ORM frameworks such as Hibernate and EclipseLink. These strategies determine when related entities are fetched from the database − for example, when an Employee entity has a reference to a collection of Phone entities.

Lazy Loading

Lazy Loading means associated data is loaded only when you explicitly access it (by calling a getter or size method). Until that point, the related data is not fetched from the database.

Use Lazy Loading when −

• You are working with one-to-many or many-to-many collections.
• You are not always going to use the related entities.

Example

@OneToMany(fetch = FetchType.LAZY)
private List<Phone> phones;

// phones are NOT loaded when Employee is fetched
// phones are loaded only when you call:
employee.getPhones();  // triggers SQL query here

Eager Loading

Eager Loading means associated data is loaded immediately at the time the parent entity is fetched. A single query (or a join) fetches both the parent and all related entities together.

Use Eager Loading when −

• The number of related entities is small and predictable.
• You are sure you will need the related entities everywhere the main entity is used.

Example

@OneToMany(fetch = FetchType.EAGER)
private List<Phone> phones;

// phones are loaded immediately when Employee is fetched
// no additional query needed when calling:
employee.getPhones();  // already in memory

How They Work

Key Differences

Conclusion

Use Lazy Loading when related data is large or not always needed, and Eager Loading when related data is small and always required. Choosing the right strategy helps balance between fewer database queries and faster initial load times.