Understanding the need for a DI framework

Question

This might be a naive question. I\'m currently learning the Spring framework and dependency injection. While the basic principle of DI is rather easy to grasp, it\'s

Answer 1

Spring has three features that are equally important:

1. dependency injection
2. aspect oriented programming
3. library of framework classes to help with persistence, remoting, web mvc, etc.

I'll agree that it's hard to see an advantage to dependency injection when you compare that to a single call to new. In that case, the latter will certainly look simpler, because it's a single line of code. Spring's configuration will always increase the lines of code, so it's not a winning argument.

It starts looking a lot better when you can take a cross-cutting concern like transactions out of your classes and use aspects to set them up in a declarative way. The comparison to a single "new" call isn't what Spring was created for.

Perhaps the best outcome from using Spring is the way its recommended idiom uses interfaces, layering, and good principles like DRY. It's really just the distillation of object-oriented best practices that Rod Johnson used in his consulting gigs. He found that the code he built up over time helped him make a buck delivering better software for his clients. He summarized his experience in "Expert 1:1 J2EE" and ended up open sourcing the code as Spring.

I'd say buy into the framework to the degree that you think his experience can help you write better code, too.

I don't think you can get the full value of Spring until you combine all three of those features.