What is dependency injection and how is it implemented in Java?

Answer 1

Such a pattern involves at least three elements: adependent, its dependencies and an injector(sometimes referred to as a provider or container). The dependent is a consumer that needs to accomplish a task in a computer program. In order to do so, it needs the help of various services (the dependencies) that execute certain sub-tasks. The provider is the component that is able to compose the dependent and its dependencies so that they are ready to be used, while also managing these objects' life-cycles. This injector may be implemented, for example, as a service locator, an abstract factory, a factory method or a more complex abstraction such as a framework.

The following is an example. A car (the consumer) depends upon an engine (the dependency) in order to move. The car's engine is made by an automaker (the dependency provider). The car does not know how to install an engine into itself, but it needs an engine in order to move. The automaker installs an engine into the car and the car utilizes the engine to move.

When the concept of dependency injection is used, it decouples high-level modules from low-level services. The result is called the dependency inversion principle. public interfaceEngine { public float getEngineRPM(); publicvoid setFuelConsumptionRate(floatflowInGallonsPerMinute); } public interface Car { public float getSpeedInMPH(); publicvoid setPedalPressure(float pedalPressureInPounds); }