Do I really need to create interfaces in Spring?

Question

In my Spring project I have many simple services to fetching data (just a simple CRUD). The design of the developers that started this project was to create the implementation for each of the service like

public interface UserService

and then implementation like

public class UserServiceImpl implements UserService

Since there is no chance that UserService will have more implementation I'm really sick of these Impl suffix and the more I read (e.g. this article) I'm realising that I have reasons to being sick

I had a discussion with a friend from a team last week and I shared my thoughts with him but what he answered was 'basically you're right but Spring likes interfaces and works with them better than with classes'.

Unfortunately I'm not an expert in Spring and, however I was trying to look for some arguments, I was not able to find an answer was he right.

Are there some strong arguments to use such approach in Spring to have interface for every little service class?

Answer 1

I can tell from real world projects, that it works well without interfaces only having the implementing class. Following the principle "You aren't gonna need it" (YAGNI), you simplify your code if you follow that rule. Dependency Injection works also well with classes, interfaces are not a requirement for it.

Sure you can write and reuse test implementations, but you can do the same with mocks e.g. with mockito and overwrite the behavior of your implementation class for test cases.

Answer 2

I've gone through all the Answers here, But would like to add more on proxy

AOP can use JDK proxy OR CGlib proxy

If Class has implemented interface it will use JDK proxy(preferred whenever you have a choice). If Class has not implemented interface it will use CGlib proxy.

Answer 3

Wherever you want to reap benefits of dependecy injection (DI) pattern you need to program against abstractions, usually an interface.

There are more benefits to DI, but the most persuasive seems to be it allows unit testing. There your interfaces will get at least one more implementation (the mock implementantion), when you will want to test your class in isolation from its dependencies (those production implementations of the interfaces).

That said, that doesn't mean every class must implement some interface. Some parts of code can be tightly coupled together without problem.

Note that using Spring or not doesn't play role in the use DI/not use DI decision.