I did some badness today.
I wanted an integration test to check to see if my @Transactional method actually did rollback on an optimistic lock failure. It’s a pretty business critical method so I wanted to be sure that the whole @Transactional and @Version annotation voodoo actually works.
The approach I took was to inject, what I called a “slowRepository” into the object with the @Transactional method. The slowRepository is just a mockito mock of the object which “answers” (using “thenAnswer”) to the “findBy…” with some custom code.
The idea is that at some point during the @Transactional method the slowRepository is called and the “answer” code is invoked. In the test, the answer code follows along the lines of:
1. Grab the data it requires from the (not slow) repository (the repository the slowRepository is mocking)
2. Switch back the slowRepository for the not slow repository
3. Invoke the @Transactional method again on a separate thread
4. Sleep for a bit
5. Wake up and return the data from step 1
The @Transactional call on the separate thread completes and commits before slowRepository wakes up. When it does, slowRepository returns stale data and the @Transactional method then fails accordingly.
The problem I encountered was using the setters for my @Autowired dependencies on the bean with the @Transactional method. I needed to use the setters so that I could switch in and out the slowRepository, but couldn’t because Spring AOP proxies are implementations of an interface, not subclasses (by default – yes I realise you can get Spring to proxy the target class using CGLIB). I found this blog post which explains how to get at the target object behind a Spring proxy. I converted it to Scala:
…which is pretty ninja.
How do I name my interfaces and the classes that implement my interfaces? Through much research on the internet and my own gut feelings, this is what I reckon:
I never prefix my interfaces with “I”. The value of coding to interfaces is that you can swap out the implementation with minimal hassle. It means that instead of using implementation specific class names in your code, you use interface names instead – right? So, if I’m going to be using interface names everywhere, I don’t want to have to prefix everything with “I”. Coding to interfaces is a best practice, so I don’t need to explicitly state that I am doing it…It should just happen – which makes the “I” prefix redundant.
Furthermore, the “Impl” suffix is also redundant. Yes, indeed, your dilemma isn’t as simple as picking between the two. Firstly, “Impl” doesn’t translate. “Impl” in other languages has no meaning, and yes I’m talking about Polish and Scala. In Polish (according to Google translate), “implementation” is “realizacja” and in Scala, there are no interfaces. The closest thing Scala has to interfaces is traits, which can contain implementation specific code. So really, everything is Scala is an implementation, so why you use “Impl” suffix?
Secondly “Impl” is restrictive. What happens when you want to add another implementation of Foo? You gonna call it “FooImplImpl”? “AnotherFooImpl”? “FooImpl2″? I didn’t think so. Perhaps you were going to just call it “FooImpl” and put it in a different package…
Thirdly “Impl” doesn’t convey any meaning about what the implementation is. You wouldn’t create an “AnimalImpl” would you? You’d create a “Lion” or an “Elephant”. There is almost always something that distinguishes your implementation that you can use in the class name. Even if it is a long name, it doesn’t matter – you’re coding to interfaces (remember?), so you’re rarely going to see that class name.
Lastly, and probably most importantly – you still can’t think of a name? You don’t need an interface. If you really can’t think of a name, your class must be generic enough to not need an interface and you’re probably not going to have any other implementations. Your class methods become the interface contract, and that is that.