Lots of Java Frameworks (definitely Spring) will generate wrapper classes for your code to handle things like method level authorization, pre/post method loggin

Lots of Java Frameworks (definitely Spring) will generate wrapper classes for your code to handle things like method level authorization, pre/post method logging, Exception transformation. This is generally termed "Aspect Oriented Programming". This is exactly whats going on with Spring's @Service annotation. In order for you to still be able to call your methods even though the class will be some sort of proxy at runtime, an Interface is required. It may seem silly but it all makes perfect sense when you understand whats going on. Edit: Surprised I haven't seen this mentioned in any other comments. I had thought this was pretty common knowledge, at least amongst people with Java experience.