是否需要使用Scala蛋糕模式进行重新编译以进行依赖项注入？

Question

Using Spring dependency injection it is possible to swap out a dependency at run time by tweaking the XML file.

I don't think this is possible using the Scala cake pattern dependency injection since the dependencies are wired at compile time and to change the dependencies would require re-compilation ? If so is this an advantage that Spring dependency injection has over using the Scala cake pattern for dependency injection ?

Answer 1

Yes using the cake pattern you will need to recompile. The whole point of the cake pattern is to do compile time dependency injection :) With spring you will return to "classic" (in java world) runtime dependency injection, which is done through reflection.

Using runtime dependency injection, you might be able to swap out a dependency at runtime either by restarting the whole container or by using specific modules like jrebel to update only part of the context. In exchange you pay the price at application startup as your container has to parse the XML, instanciate all the objects and wire them up. In the case of spring the wiring can be fairly complex requiring multiple passes to complete. Usually you don't want to swap out dependencies at runtime in production code (there are valid use cases for this but I have rarely encountered them).

On the other hand, the cake pattern is validated at compile time, the wiring is necessarily predetermined (forget about @PostConstruct). Avoiding complex wiring cycles is actually a good thing :) You will also avoid loading and parsing XML files (though in my understanding the current preferred configuration method is using code not XML). Errors in the wiring will appear sooner in cake giving you a quicker feedback.

Last but not least, there are other DI mechanisms available in functionnal programming languages (such as scala) such as using an IO or a Reader Monad. For more about such mechanisms you can start at : http://blog.originate.com/blog/2013/10/21/reader-monad-for-dependency-injection/ http://fr.slideshare.net/debasishg/dependency-injection-in-scala-beyond-the-cake-pattern or Using Reader Monad for Dependency Injection