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 {
...
}
Spring's recommendation is that you annotate the concrete implementations instead of an interface. It's not incorrect to use the annotation on an interface, it's just possible to misuse that feature and inadvertently bypass your @Transaction declaration.
If you've marked something transactional in an interface and then refer to one of its implementing classes elsewhere in spring, it's not quite obvious that the object that spring creates will not respect the @Transactional annotation.
In practice it looks something like this:
public class MyClass implements MyInterface {
private int x;
public void doSomethingNonTx() {}
@Transactional
public void toSomethingTx() {}
}