Difference between findById() and getOne().

Question

Accepted Answer

findById() and getOne() (now referenceById() in newer Spring Data versions) both retrieve an entity by its primary key, but they differ in when the database is actually hit -- findById() fetches eagerly and returns an Optional, while getOne() returns a lazy proxy that only queries the database when its fields are first accessed.

Key Points:
• findById() returns an Optional, immediately querying the database and returning empty if no match is found.
• getOne() returns a proxy object without hitting the database yet, deferring the actual SELECT until a field on the entity is accessed.
• If the entity referenced by getOne() doesn't actually exist, the EntityNotFoundException is thrown lazily, at the point the proxy is accessed, not when getOne() is called.
• getOne() is useful when you only need the entity's reference (its ID) to set up a relationship, avoiding an unnecessary SELECT.
• getOne() is deprecated in newer Spring Data JPA in favor of getReferenceById(), which behaves the same way but with clearer naming.

Example:
Setting a foreign key relationship, like assigning an order to a customer, only needs the customer's reference, so getOne(customerId) (or getReferenceById()) avoids a wasted SELECT, whereas displaying the customer's actual details requires findById() to eagerly load the real data.

Code Example:
```
// Eager fetch, safe existence check via Optional
Optional customer = customerRepository.findById(id);

// Lazy proxy, defers the query until fields are accessed
Customer customerRef = customerRepository.getReferenceById(id);
order.setCustomer(customerRef);
orderRepository.save(order);
```

Interview Tip:
A concise interview answer is:

"findById() eagerly queries the database and returns an Optional, so I can safely check whether the entity exists. getOne(), now getReferenceById(), returns a lazy proxy that defers the actual query until a field is touched, and throws EntityNotFoundException at that point if the row doesn't exist -- I use it when I just need a reference to set up a relationship without an unnecessary SELECT."