Where should I put @Transactional annotation: at an interface definition or at an implementing class?

Question

The question from the title in code:

@Transactional (readonly = true)
public interface FooService {
   void doSmth ();
}


public class FooServiceImpl implements FooService {
   ...
}

vs

public interface FooService {
   void doSmth ();
}

@Transactional (readonly = true)
public class FooServiceImpl implements FooService {
   ...
}

Answer 1

From http://static.springsource.org/spring/docs/2.0.x/reference/transaction.html

The Spring team's recommendation is that you only annotate concrete classes with the @Transactional annotation, as opposed to annotating interfaces. You certainly can place the @Transactional annotation on an interface (or an interface method), but this will only work as you would expect it to if you are using interface-based proxies. The fact that annotations are not inherited means that if you are using class-based proxies then the transaction settings will not be recognised by the class-based proxying infrastructure and the object will not be wrapped in a transactional proxy (which would be decidedly bad). So please do take the Spring team's advice and only annotate concrete classes (and the methods of concrete classes) with the @Transactional annotation.

Note: Since this mechanism is based on proxies, only 'external' method calls coming in through the proxy will be intercepted. This means that 'self-invocation', i.e. a method within the target object calling some other method of the target object, won't lead to an actual transaction at runtime even if the invoked method is marked with @Transactional!

(Emphasis added to the first sentence, other emphasis from the original.)