Dependency inversion principle

In conventional application architecture, lower-level components are designed to be consumed by higher-level components which enable increasingly complex systems to be built. In this composition, higher-level components depend directly upon lower-level components to achieve some task. This dependency upon lower-level components limits the reuse opportunities of the higher-level components.

The goal of the dependency inversion principle is to decouple high-level components from low-level components such that reuse with different low-level component implementations becomes possible. This is facilitated by the separation of high-level components and low-level components into separate packages/libraries, where interfaces defining the behavior/services required by the high-level component are owned by, and exist within the high-level component's package. The implementation of the high-level component's interface by the low level component requires that the low-level component package depend upon the high-level component for compilation, thus inverting the conventional dependency relationship. Various patterns such as Plugin, Service Locator, or Dependency Injection are then employed to facilitate the run-time provisioning of the chosen low-level component implementation to the high-level component.

Applying the dependency inversion principle can also be seen as applying the Adapter pattern, i.e. the high-level class defines its own adapter interface which is the abstraction that the high-level class depends on. The adaptee implementation also depends on the adapter interface abstraction (of course, since it implements its interface) while it can be implemented by using code from within its own low-level module. The high-level has no dependency to the low-level module since it only uses the low-level indirectly through the adapter interface by invoking polymorphic methods to the interface which are implemented by the adaptee and its low-level module.