Understanding Spring transactions - What happens when a transactional method calls another transactional method?

Just to understand the workings of Spring transactions I want to know what happens in the following case where one method marked as @Transactional calls another method marked as @Transactional.

Assume a the configuration uses all default settings.

@Service("myService")
@Transactional
public MyService{
   public void myServiceMethod(){
      myDAO.getSomeDBObjects();
   }
}

@Repository("myDAO")
@Transactional
public MyDAOWithUsesBeyondMyService{
   public void getSomeDBObjects(){...}
}

Now if I were to enter MyService.myServiceMethod() it would clearly start a transaction. Then, upon drilling into myDAO.getSomeDBObjects() what would happen? Would the fact that a transaction already exist cause no new transaction to be born, or am I creating two transactions here?

The documentation (quoted below) on Propagation seems to cover this, but I'd like to verify my understanding, it was a little much for my virgin brain to comprehend all at once.

Propagation: Typically, all code executed within a transaction scope will run in that transaction. However, you have the option of specifying the behavior in the event that a transactional method is executed when a transaction context already exists. For example, code can continue running in the existing transaction (the common case); or the existing transaction can be suspended and a new transaction created. Spring offers all of the transaction propagation options familiar from EJB CMT. To read about the semantics of transaction propagation in Spring, see Section 10.5.7, “Transaction propagation”.

Solution 1:

Two answers:

a) don't do it. Use @Transactional in the service layer or the dao layer, but not both (the service layer is the usual choice, as you probably want one transaction per service method)

b) if you do it, what happens depends on the propagation attribute of the @Transactional annotation and is described in this section: 10.5.7 Transaction propagation. Basically: PROPAGATION_REQUIRED means the same transaction will be used for both methods, while PROPAGATION_REQUIRES_NEW starts a new transaction.

About your comments:

Of course I kept reading and realized that, as I'm using proxies, this second method won't be managed by the transactional proxy, thus it's like any other method call.

That's not true in your situation (only if both methods were within the same class).

If a bean has methods a and b, and a calls b, then b is called on the actual method, not the proxy, because it is called from within the proxy (a bean doesn't know that it is proxied to the outside world).

proxy      bean  
a() -->    a()
            |
            V  
b() -->    b()

In your situation, however, a service would have an injected dao object, which would be a proxy itself, so you'd have a situation like this:

           proxy      bean
service    a() -->    a()
                       |
             /---------/
             |                 
             V
dao        b() -->    b()