In a previous post to his site Andrew Embler provided a "concrete guide to dependency injection", a method to help make code easier to test and more flexible/maintainable. In this latest post he continues the series and moves to the next topic: dependency injection containers.
I recently posted about dependency injection, a guide that I hope was easy to read and understand. It's a topic that's confusing, due in no small part to its name. As I mention there, the same term defines the simple process of externally providing instance variables to a class as it does to the Container object's magical auto-instantiation of object (which also "injects dependencies.")
I'd like to talk a bit more about that. Now that we understand the term "Dependency Injection" (which is simply providing classes to classes through a constructor or a setter), let's talk about how we might make this a little less onerous.
He starts with an example from the concrete5 project where a class would require three object instances each time it would be created. Fortunately the software includes a
make command that does some of this work for you. It makes use of a dependency injection container behind the scenes to get the instances it needs. He then covers cascading dependencies, some special logic for class creation and how to "get fancy with it" by defining custom logic when an instance is created.