Spring Basics - Inversion of Control

Spring is an open source Java application development framework that supports all types of Java applications such as: enterprise applications, web applications, cloud based applications, and more. Spring applications are simple, easily testable, reusable, and maintainable.

Spring modules are loosely coupled. Developers can pick and choose modules as needed to build applications.

Demos for this post:

• demo without IoC
• demo with IoC

Introduction to the Spring Framework

To start, I'll give a brief introduction to the features and modules of the Spring framework.

Spring 5.x has the following key module groups:

• Core Container: core modules that provide key features of the Spring framework.
• Data Access/Integration: These modules support JDBC and ORM data access
• Web: these modules provide support for web applications.
• Others: Spring also provides other modules such as Test for testing Spring applications.

Spring Inversion of Control

Usually, it's the developer's responsibility to create the dependent application objects using the new operator. Hence any change in the application dependencies requires code changes. This results in tight coupling and more complexity as the application grows bigger.

Inversion of Control (IoC) creates a more loosely coupled application. IoC inverts the responsibility of the application objects’ creation, initialization, and destruction from the application to a third-party aka the framework. The third party will take care of application object management and dependencies. This makes an application easy to maintain, test, and reuse. The Spring framework provides IoC implementation using dependency injection (DI).

Objects managed by Spring are called beans. Thanks to Spring, we don’t need to create objects ourselves. Dependency injection allows us to describe how objects should be created through configuration.

DI is a software design pattern that:

• Helps to create loosely coupled application architecture facilitating re-usability and easy testing.
• Separates responsibility by keeping code and configuration apart. Dependencies can be easily modified using configuration without changing the code.
• Allows us to replace actual objects with mock objects for testing. This improves testability by writing simple JUnit tests that use mock objects.

As mentioned before, the core container module of the Spring framework provides IoC using dependency injection. The Spring container knows which objects to create and when to create them through the additional details that we provide in our application called configuration metadata.

There are two types of containers Spring provides: BeanFactory and ApplicationContext (interfaces). The ApplicationContext is the preferred container for development and inherits from BeanFactory. ApplicationContext provides added features to support enterprise services such as internationalization, validation, etc.

// org.springframework.context.support.ClassPathXmlApplicationContext
// is the most common implementation of ApplicationContext
ApplicationContext context = new ClassPathXmlApplicationContext("config.xml");
Object obj = context.getBean("exampleService");

Spring allows multiple methods to configure the metadata into our POJOs:

• XML configuration
• Annotation based configuration
• Java based configuration

Conclusion

To conclude, I will introduce a small IoC example with XML based configuration.

package com.maxdemaio.service;

public class BlogPostService {
 public void display() {
        System.out.println("Hi, Welcome to the Blog Post Generation application");
    }
}

<!-- Example object in a config.xml file -->
<!--
1. <beans> is the root element & also includes namespace declarations

2. Bean definition

3. id attribute represents a unique bean identifier

4. class attribute represents a fully qualified class name
-->
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="blogPostService" class="com.maxdemaio.service.BlogPostService" />

</beans>

package com.maxdemaio.client;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.maxdemaio.service.BlogPostService;

public class Client {

 public static void main(String[] args) {

 // ApplicationContext container is instantiated by loading the configuration
 // from config.xml available in application classpath
 ApplicationContext context = new ClassPathXmlApplicationContext("config.xml");

 // Access bean with id “blogPostService"
 // Typecast from Object type to blogPostService type
 BlogPostService blogPostService = (BlogPostService) context.getBean("blogPostService");
 // Invoke display method of blogPostService to display greeting on console
        blogPostService.display();
    }

}

Rather than using the new keyword, we configured a managed Spring bean to take its place. This allows us to have loosely coupled code!

Works Cited

• https://docs.spring.io/spring-framework/docs/5.0.0.RC2/spring-framework-reference/overview.html