Dependency Injection - Motivation

Motivation

The primary purpose of the dependency injection pattern is to allow selection among multiple implementations of a given dependency interface at runtime, or via configuration files, instead of at compile time. The pattern is particularly useful for providing "mock" test implementations of complex components when testing, but is often used for locating plugin components, or locating and initializing software services.

Unit testing of components in large software systems is difficult, because components under test often require the presence of a substantial amount of infrastructure and set up in order to operate at all. Dependency injection simplifies the process of bringing up a working instance of an isolated component for testing. Because components declare their dependencies, a test can automatically bring up only those dependencies required to perform testing.

More importantly, injectors can be configured to swap in simplified "mock" implementations of sub-components when testing, the idea being that the component under test can be tested in isolation as long as the substituted sub-components implement the contract of the dependent interface sufficiently to perform the unit test in question.

As an example, consider an automatic stock trading program that communicates with a live online trading service and stores historical analytic data in a distributed database. To test the component which recommends trades, one would ordinarily need to have a connection to the online service and an actual distributed database suitably populated with test data. Using dependency injection, the components that provide access to the online service and back-end databases could be replaced altogether with a test implementation of the dependency interface contracts that provide just enough behavior to perform tests on the component under test.