spring and interfaces

Question

I read all over the place about how Spring encourages you to use interfaces in your code. I don\'t see it. There is no notion of interface in your spring xml configuration.

Answer 1

Writing separate interfaces adds complexity and boilerplate code that's normally unnecessary. It also makes debugging harder because when you click a method call in your IDE, it shows the interface instead of the implementation. Unless you're swapping implementations at runtime, there's no need to go down that path.

Tools like Mockito make it very easy to test code using dependency injection without piling on interfaces.

Answer 2

No one has mention yet that in many occasions won't be necessary to create an interface so that the implementing class can be switched quickly because simply there won't be more than one implementing class.

When interfaces are created without need, classes will be created by pairs (interface plus implementation), adding unnecessary boilerplate interfaces and creating potential dependency confusions because, on XML configuration files, components will be sometimes referenced by its interface and sometimes by its implementation, with no consequences at runtime but being incoherent regarding code conventions.

Answer 3

Most of the answers here are some form of "You can easily swap out implementations", but what I think they fail to answer is the why? part. To that I think the answer is almost definitively testability. Regardless of whether or not you use Spring or any other IOC framework, using Dependency Injection makes your code easier to test. In the case of say a writer rather than a PrinterWriter, you can Mock the Writer interface in a Unit test, and ensure that your code is calling it the way you expect it to. If you depend directly on the class implementation, your only option is to walk to the printer and check it, which isn't very automated. Furthermore, if you depend upon the result of a call to a class, not being able to Mock it may prevent you from being able to reach all code paths in your test, thus reducing their quality (potentially) Simply put, you should decouple Object graph creation from application logic. Doing so makes your code easier to test.