Dependency injection is a design pattern in software development that is used to reduce the coupling between components of a software application. It involves passing dependent objects to a class or method from the outside, rather than having the class or method create the dependent objects itself. This allows for greater flexibility, modularity, and testability in the code.
For example, a class that processes payments may depend on a class that handles communication with a payment gateway. In a traditional approach, the payment processing class would create an instance of the payment gateway communication class within its own code. This creates a tight coupling between the two classes, making it difficult to change or replace one without affecting the other.
With dependency injection, the payment processing class would not create an instance of the payment gateway communication class itself. Instead, it would expect to receive an instance of that class from somewhere else - typically, from an object called a "container" that manages dependencies between components. The container creates and manages the instances of the dependent classes, and injects them into the appropriate components when they are needed.
Dependency injection can be implemented in several ways, including constructor injection, setter injection, and interface injection. In constructor injection, the dependent object is passed as an argument to the constructor of the class that uses it. In setter injection, the dependent object is passed to a setter method of the class that uses it. In interface injection, the dependent object is passed to a method that implements an interface that the using class also implements.
Dependency injection benefits include: reduced coupling between components; improved flexibility and modularity; improved testability in isolation; better code organization and readability.