hibernate - Why use @Transactional with @Service instead of with @Controller

Question

I have seen many comments in stack-overflow articles I found certain things about either @Transactional use with @Service or with @Controller

"Usually, one should put a transaction at the service layer."

"The normal case would be to annotate on a service layer level"

"Think transactions belong on the Service layer. It's the one that knows about units of work and use cases. It's the right answer if you have several DAOs injected into a Service that need to work together in a single transaction." [Source]

Drawback to use @transactional with @service layer

If I had 2 methods for example saveUser() and saveEmail() (because I store the emails in a database to send them later - like a queue) I would create in my service a method saveUserAndSendEmail(User user) which would be transactional. [Source]

It means I create many methods in service layer instead of one Save Generic Method as follow

public <T> long save(T entity) throws DataAccessException {
    Session session = sessionFactory.getCurrentSession();
    long getGenVal=(Long) session.save(entity);
    return getGenVal;
}

According to the above solution , it means we have many methods like following LOL..

public <T> long saveAccount(T entity)....

public <T> long saveWithAuditLog(T entity, K entity1)....

public <T> long saveWithAuditLogAndEntries(T entity, K entity, M entity)....

OVERCOME this situation

I USE THE @Transactional in @Controller and Just make a Generic Save Method and save all the entities/ model using this simple save method. and if any method fail to save then all the transactions in controller rollback successfully.

Other situation that ensure that @Transactional should be use with @Controller

In @Controller:

pt.save(entity1);
pt.save(entity2);
int a = 2/0;
pt.save(entity3);

In case , @Transactional on Service, first 2 entity successfully saved but third not it not rollback all the transaction

In case , @Transactional on @Controller all the transaction rollback as exception occur

why stack-overflow asked , "Don't do transactions in your controller. Put them in your service layer classes."? [source]

Answer 1

You are asking about best practice, and best practice is to mark @Transactional in the service layer because a @Controller should not be aware of data persistence in a MVC logic.
@Service is constructed on use-case generated from analysis and knows about unit of works and is also realized thinking in terms of reuse: if you switch from a web context to a desktop one (for example, or some other visual frontend) where @Controller layer doesn't exist you don't have problems because all is encapsulated in service layer.
A @Service is a contract and a modification in presentation layer should not require a rewrite of @Service code.
But Spring don't care about where you put your your transaction boundaries, you can put on @Controller but your application may will be harder to be maintained.

I hope this is clear enough. Sorry if not; English is not my native language.