Jeffrey Aaron Farr has managed to “accurately sum up(IoC > Dependency Management)”:http://www.jadetower.org/muses/archives/000020.html some of the points I’ve been wanting to make concerning the feedback (mine included) to the “recent article(Inversion of Control Containers and the Dependency Injection pattern)”:http://www.martinfowler.com/articles/injection.html by Martin Fowler, and has done so far more eloquently that I would of. He correctly points out that IoC is about more than resolution of dependencies, a point Martin himself makes in the article:
The conclusion I’m coming to is that, as Fowler points out, there are lots of forms of inversion with Dependency Injection merely one of them. For example, in all three of the container frameworks mentioned above you will find at least three types of IoC:
- Dependency IoC
- Lifecycle IoC
- Configuration IoC
It’s important to recognize these other aspects as part of the IoC design pattern. IoC is not just about dependency management.
I did initially take issue with Jeffrey classing Configuration IoC as being different to Dependency IoC, although realised he may have a point. Clearly a dependency is something which a component requires to do its job – this may include information traditionally thought of as configuration data, such as the location of a database. What about a path for logging? There may be a default directory that can be overridden – in which case is this path a dependency, or simple configuration? In this case the path is not required for the component to work.
Clearly dependency is required before the object is used (something which Type 3 IoC containers such as PicoContainer can enforce), however many configuration-type values may be legitimately changed after creation, something which Type 2 IoC containers such as Spring would be very good at, but which Type 3 IoC containers could not do without recreating the component with the new configuration data. The question remains as to how useful configuration IoC would really be, although the “JMX”:http://java.sun.com/products/JavaManagement/ specification (which can be considered a Type1/2 IoC hybrid container) makes great use of it.