18.11. Dependency Injection

When building a large system out of components, with different implementations for each component, one needs to assemble the component choices. For example, there may be a mock database and a real database, or console logging and file logging. A particular implementation may want the real database and console logging for running an experiment, or the mock database and file logging for running an automated test script.

Usually, there is some dependency among the components. For example, the data access component may require logging.