Java: What is the difference between findById() and getReferenceById()?

findById() — loads entity immediately. getReferenceById() — returns proxy (lazy).

findById():

• Executes SELECT query.
• Returns Optional<T>.
• Entity loaded immediately.
• Safe for all use cases.

1Optional<User> user = userRepository.findById(1L);
2// SQL executed: SELECT * FROM users WHERE id = 1

getReferenceById():

• Returns proxy object.
• No SQL executed.
• Entity loaded on access.
• Throws EntityNotFoundException if not found.
• More efficient for setting foreign key references.

1User user = entityManager.getReferenceById(User.class, 1L);
2// No SQL yet
3String name = user.getName();
4// SQL executed: SELECT * FROM users WHERE id = 1

How it works:

1. findById() executes a SELECT query and hydrates the entity.
2. getReferenceById() returns a proxy that loads data on access.
3. findById() is safe for all scenarios.
4. getReferenceById() is optimized for setting foreign key references.

When to use:

• findById() — need entity immediately.
• getReferenceById() — just setting foreign key reference.

Common pitfalls:

• Using getReferenceById() when entity doesn't exist (throws exception).
• Accessing proxy outside transaction (LazyInitializationException).
• Confusing proxy with actual entity.