inversion of control

After years of near-ubiquitous usage of Dependency Injection, I see more and more posts and talks questioning its value. Some even go to the point where they argue against it. Most of it however is based on a whole lot of misconceptions, half-truths and blatant lies. In this post, I’d like to go back to the roots of DI, describe some related features and lists available frameworks.

Inversion of Control is not only possible at the class level, but at the module level. OSGi has been doing it for a long time. However, there are IoC approaches directly available in Java, as well as in Spring. Out-of-the-box, the Java API offers a specific form of Inversion of Control. It’s implemented by the Service Loader class.

Spring core comes out-of-the-box with two scopes: singletons and prototypes. Singletons implement the Singleton pattern, meaning there’s only a single instance at runtime (in a JVM). Spring instantiate them during context creation, caches them in the context, and serves them from the cache when needed (or something like that). Prototypes are instantiated each time you access the context to get the bean. Problems arise when you need to inject a prototype-scoped bean in a singleton-scoped b