Dependency Injection

In software engineering, a design pattern is a general repeatable solution to a commonly occurring problem in software design.
It is a description or template for how to solve a problem that can be used in many different situations.

Design patterns can speed up the development process by providing tested, proven development paradigms.
Design patterns provide general solutions, documented in a format that doesn't require specifics tied to a particular problem.

Given a class A which needs to use an instance of class B. How do you implement this idea without creating a direct dependency between 2 classes?

Dependency injection is a programming technique that makes a class independent of its dependencies.
The goal of the dependency injection technique is to remove this dependency by separating the usage from the creation of the object. This reduces the amount of required boilerplate code and improves flexibility.

The service (B) you want to use.
The client (A) that uses the service.
An interface that’s used by the client (A) and implemented by the service (B).
The injector which creates a service instance (B) and injects it into the client (A).

Traditional Approach --> need to instantiate an implementation of the Item interface within the Store class itself.

public class Store {
    private Item item;

    public Store() {
        item = new ItemImpl1();    
    }
}

DI Approach --> no need to specify the implementation of the Item that we want:

public class Store {
    private Item item;
    public Store(Item item) {
        this.item = item;
    }
}

In the Spring framework, the interface ApplicationContext is responsible for instantiating, configuring and assembling objects known as beans, as well as managing their life cycles
The Spring framework provides several implementations of the ApplicationContext interface: --> ClassPathXmlApplicationContext && FileSystemXmlApplicationContext for standalone applications --> WebApplicationContext for web applications.

To instantiate a container

ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");

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