Java: What is the difference between FetchType.LAZY and FetchType.EAGER?

LAZY — load on access. EAGER — load immediately.

LAZY:

• Loads related entities when accessed.
• Better performance.
• Default for @OneToMany, @ManyToMany.
• Requires open session/view pattern.
• Uses proxies or collections.

1@OneToMany(mappedBy = "user", fetch = FetchType.LAZY)
2private List<Order> orders;
3// Orders loaded when getOrders() called

EAGER:

• Loads related entities immediately.
• Can cause N+1 problem.
• Default for @ManyToOne, @OneToOne.
• May load unnecessary data.

1@ManyToOne(fetch = FetchType.EAGER)
2private User user;
3// User loaded with Order

How it works:

1. LAZY loading uses proxies to defer loading until access.
2. EAGER loading fetches related entities in the same query.
3. LAZY loading reduces initial query complexity.
4. EAGER loading can cause performance issues with large datasets.

N+1 problem:

• 1 query for parent + N queries for children.
• Solution: JOIN FETCH or @EntityGraph.

1// JOIN FETCH to solve N+1
2@Query("SELECT u FROM User u JOIN FETCH u.orders WHERE u.id = :id")
3User findByIdWithOrders(@Param("id") Long id);

Best practice:

• Default to LAZY.
• Use JOIN FETCH when needed.
• Avoid EAGER for collections.
• Use @EntityGraph for complex queries.