What is dependency injection? And how can be applied?

Answer 1

It is a very long subject to describe here. Read this link:

https://www.sitepoint.com/introduction-contexts-dependency-injection-cdi/

I try to write it short. In your code you declare a class member and you don't mind how and when it will be set by a concrete value. The CDI framework will handle it for you to find the proper implementation, create an instance of it and set your variable. You can create loose coupling between interfaces and concrete implementations with this. You don't have to declare it in your code but it in configuration level.

Answer 2

Dependency Injection design pattern allows us to remove the hard-coded dependencies and make our application loosely coupled, extendable and maintainable. We can implement dependency injection in java to move the dependency resolution from compile-time to runtime.

Spring is the framework from where dependecy injection comes into limelight.

There are various ways through which you can apply dependency injection. Please refer below article and try to understand different examples of dependency injection.

https://en.m.wikipedia.org/wiki/Dependency_injection