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Dependency injection

Programming technique of receiving dependencies

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  • How can a class be independent from the creation of the objects it depends on?
  • How can an application and the objects it uses support different configurations?

Dependency injection is often used to keep code in-line with the dependency inversion principle.

In statically typed languages using dependency injection means that a client only needs to declare the interfaces of the services it uses, rather than their concrete implementations, making it easier to change which services are used at runtime without recompiling.

Application frameworks often combine dependency injection with inversion of control. Under inversion of control, the framework first constructs an object (such as a controller), and then passes control flow to it. With dependency injection, the framework also instantiates the dependencies declared by the application object (often in the constructor method's parameters), and passes the dependencies into the object.

Dependency injection implements the idea of "inverting control over the implementations of dependencies", which is why certain Java frameworks generically name the concept "inversion of control" (not to be confused with inversion of control flow).

Roles

Dependency injection involves four roles: services, clients, interfaces, and injectors.

Services and clients

A service is any class which contains useful functionality. In turn, a client is any class which uses services. The services that a client requires are the client's dependencies.

Any object can be a service or a client; the names relate only to the role the objects play in an injection. The same object may even be both a client (it uses injected services) and a service (it is injected into other objects). Upon injection, the service is made part of the client's state, available for use.

Interfaces

Clients should not know how their dependencies are implemented, only their names and API. A service which retrieves emails, for instance, may use the IMAP or POP3 protocols behind the scenes, but this detail is likely irrelevant to calling code that merely wants an email retrieved. By ignoring implementation details, clients do not need to change when their dependencies do.

Injectors

The injector, sometimes also called an assembler, container, provider or factory, introduces services to the client.

The role of injectors is to construct and connect complex object graphs, where objects may be both clients and services. The injector itself may be many objects working together, but must not be the client, as this would create a circular dependency.

Because dependency injection separates how objects are constructed from how they are used, it often diminishes the importance of the new keyword found in most object-oriented languages. Because the framework handles creating services, the programmer tends to only directly construct value objects which represent entities in the program's domain (such as an Employee object in a business app or an Order object in a shopping app).

Analogy

As an analogy, cars can be thought of as services which perform the useful work of transporting people from one place to another. Car engines can require gas, diesel or electricity, but this detail is unimportant to the client—a passenger—who only cares if it can get them to their destination.

Cars present a uniform interface through their pedals, steering wheels and other controls. As such, which engine they were 'injected' with on the factory line ceases to matter and drivers can switch between any kind of car as needed.

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