Spring Advice: autowiring interface types rather than class types

gistfile1.txt

There's another good reason to autowire interface types rather than class types, which is that if Spring needs
to generate a proxy object around the bean before injecting it, then if the bean's class defines any
interfaces, then the proxy will implement those interfaces, and will not be type-compatible with the bean class
itself. If you then try and autowire that bean by class type, it will fail. The easiest way to avoid this
annoying scenario is to always autowire by interface type, that way it will lways work as you expect.

http://stackoverflow.com/questions/5288153/using-spring-to-wire-directly-a-concrete-class