What are the benefits of dependency injection containers?

Question

I understand benefits of dependency injection itself. Let\'s take Spring for instance. I also understand benefits of other Spring featureslike AOP, helpers of different kind

Answer 1

From a Spring perspecitve I can give you two answers.

First the XML configuration isn't the only way to define the configuration. Most things can be configured using annotations and the things that must be done with XML are configuration for code that you aren't writing anyways, like a connection pool that you are using from a library. Spring 3 includes a method for defining the DI configuration using Java similar to the hand rolled DI configuration in your example. So using Spring does not mean that you have to use an XML based configuration file.

Secondly Spring is a lot more than just a DI framework. It has lots of other features including transaction management and AOP. The Spring XML configuration mixes all these concepts together. Often in the same configuration file I'm specifying bean dependencies, transaction settings and adding session scoped beans that actually handled using AOP in the background. I find the XML configuration provides a better place to manage all these features. I also feel that the annotation based configuration and XML configuration scale up better than doing Java based configuration.

But I do see your point and there isn't anything wrong with defining the dependency injection configuration in Java. I normally do that myself in unit tests and when I'm working on a project small enough that I haven't added a DI framework. I don't normally specify configuration in Java because to me that's the kind plumbing code that I'm trying to get away from writing when I chose to use Spring. That's a preference though, it doesn't mean that XML configuration is superior to Java based configuration.