What is Inversion of Control (IoC)?

Inversion of Control (IoC) is a software design pattern that involves delegating control over the creation and management of objects from application code to the framework itself. This shift means that the framework, rather than the application, handles the lifecycle of objects, which simplifies code complexity and enhances scalability and maintainability.

Within the IoC paradigm, the framework is tasked with instantiating and managing objects, while the application only specifies the types of objects it requires. The framework subsequently provides these objects for the application's use.

What is Dependency Injection (DI)?

Dependency Injection (DI) is a practical implementation of the IoC principle. The essence of DI is that it allows the injection of dependencies into objects rather than having the application code create them directly.

With DI, the application remains oblivious to the dependencies of the objects it utilizes. The framework oversees the creation of these dependencies, injects them into the required objects, and then makes them accessible to the application.

The first video offers a thorough tutorial on Dependency Injection and Inversion of Control in Spring, discussing their importance and functionality.

IoC and DI in the Spring Framework

Spring serves as an IoC container that leverages the DI pattern for the management of objects and their dependencies. In Spring, the container is responsible for the creation and management of all objects. The application merely indicates which objects to instantiate and how to inject their dependencies.

Spring utilizes either XML configuration files or annotations for object and dependency management. The XML configuration outlines the creation and injection process, whereas annotations within the code designate the objects and their dependencies.

For example, consider the following implementation of the DI pattern in Spring:

public class MyApp {

private MyService myService;

public MyApp(MyService myService) {

this.myService = myService;

}

public void run() {

myService.doSomething();

}

}

In this illustration, the constructor of the MyApp class accepts a MyService object as a parameter and assigns it to the myService member variable. When the run() method is executed, it calls the doSomething() method on the injected myService object. Here, the Spring container creates and injects the MyService instance into the MyApp object.

Differentiating IoC and DI

While IoC represents a design paradigm that emphasizes the transfer of control from application code to a framework, DI serves as a method to enact IoC through the injection of dependencies. The key distinction lies in IoC’s focus on control transfer and DI’s emphasis on dependency injection.

How Spring Implements IoC and DI

Spring acts as an IoC container that implements DI for managing objects and their dependencies. In this framework, all objects are instantiated and handled by the container. The application code only needs to specify which objects to create and how to inject their respective dependencies.

Spring manages these objects and dependencies using XML configuration files or annotations.

What are Beans in Spring?

In the context of Spring, a Bean refers to any object that is instantiated and managed by the Spring container. Beans can encompass a variety of Java objects, including components, services, and controllers. When a Bean is created, the Spring container refers to the definitions in the configuration files or annotations and assembles the Bean accordingly, injecting it into any dependent objects.

Injection Methods in Spring

Spring supports three methods for dependency injection: constructor injection, setter method injection, and field injection. Constructor injection delivers dependencies through the Bean's constructor. Setter method injection utilizes the Bean's setter methods, while field injection directly assigns dependencies to the Bean's member fields. Constructor and setter method injections are the most prevalent techniques.

Handling Circular Dependencies

Circular dependency arises when two or more Beans depend on each other, creating a loop. The Spring Framework tackles this challenge using the BeanPostProcessor interface, which processes circular dependencies after the Beans are created but before their initialization methods are invoked. Spring handles circular dependencies by generating a proxy object that temporarily substitutes the original object until it is fully instantiated.