Dependency Injection (DI) is a fundamental aspect of the Spring framework, through which the Spring container "injects" objects into other objects or "dependencies". In other words, DI is a design pattern that removes the dependency from the programming code so that it can be easy to manage and test the application. The Spring-Core module is responsible for injecting dependencies through either Constructor or Setter methods. The design principle of Inversion of Control emphasizes keeping the Java classes independent of each other and the container frees them from object creation and maintenance. These classes, managed by Spring, must adhere to the standard definition of Java-Bean.
Dependency Injection makes programming code loosely coupled and easier to test. Loose coupling between classes can be possible by defining interfaces for common functionality, and the injector will instantiate the objects of required implementation. The task of instantiating objects is done by the container.
Dependency Injection can be configured with annotations or XML file-based configuration. The container performs bean dependency resolution by creating and initializing the ApplicationContext with configuration metadata that describes all the beans. Configuration metadata can be specified by XML, Java code, or annotations. If no circular dependencies exist, when one or more collaborating beans are being injected into a dependent bean, each collaborating bean is totally configured prior to being injected into the dependent bean.
Overall, Dependency Injection in Spring Boot is a powerful feature that allows for the creation of loosely coupled, easily testable code.
