Spring Transaction: What will happen if I don't give @Transaction annotation on method

Question

I am using Spring-Boot, Spring Rest Controller and Spring Data JPA. If I don't specify @Transaction then also record get's created but I will like to understand how it happens.My understanding is that Spring by default adds a transaction with default parameters but not sure where it adds is it add Service layer or at Repository.

 public interface CustomerRepository extends CrudRepository<Customer, Long> {

    List<Customer> findByLastName(String lastName);
 }

 @Service
 public class CustomerServiceImpl implements CustomerService> {

   List<Customer> findByLastName(String lastName){
     //read operation
    }

 // What will happen if @Transaction is missing. How record get's created without the annotation
  public Customer insert(Customer customer){
  // insert operations
   }
 }

Answer 1

Spring Data JPA adds the @Transactional annotation at the Repository layer specifically in the class SimpleJpaRepository.This is the base Repository class which is extended for all Spring Data JPA repositories

e.g

/*
     * (non-Javadoc)
     * @see org.springframework.data.repository.CrudRepository#save(java.lang.Object)
     */
    @Transactional
    public <S extends T> S save(S entity) {

        if (entityInformation.isNew(entity)) {
            em.persist(entity);
            return entity;
        } else {
            return em.merge(entity);
        }
    }

Answer 2

Although Spring add automatically the @Transactional annotation at DAO layer, it is not the correct place nor correct behavior. The annotation must be located at service layer, there is an answered question here.