Spring IOC Container Java Config Example

In a previous article, we have discussed  What is Spring IOC Container and how it works, Now in this article, we will discuss a simple example to demonstrate Spring IOC Container with Java-based configuration metadata?
We will use the latest Spring release - Spring 5.1.0.RELEASE
The Spring IOC container is responsible for instantiating, configuring, and assembling the Spring beans. The container gets its instructions on what objects to instantiate, configure, and assemble by reading configuration metadata. The configuration metadata is represented in XML, Java annotations, or Java code. It lets you express the objects that compose your application and the rich interdependencies between those objects.
Three ways we can supply Configuration Metadata to Spring IoC container
  1. XML-based configuration
  2. Annotation-based configuration
  3. Java-based configuration
In this example, we will supply Java-based configuration metadata to Spring IoC container.

Spring IOC Container Java Config Example

Spring Application Development Steps

Follow these five steps to develop a spring application:
  1. Create a simple Maven Project
  2. Add Maven Dependencies
  3. Configure HellowWorld Spring Beans
  4. Create a Spring Container
  5. Retrieve Beans from Spring Container

Tools and technologies used

  • Spring Framework - 5.1.0.RELEASE
  • JDK - 8 or later
  • Maven - 3.2+
  • IDE - Eclipse Mars/STS

1. Create a simple Maven Project

Create a simple maven project using your favorite IDE and refer below diagram for packaging structure. If you are new to maven then read this article How to Create a Simple Maven Project.

Project Structure

Below diagram shows a project structure for your reference - 

2. Add Maven Dependencies

<project xmlns="http://maven.apache.org/POM/4.0.0"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>net.javaguides.spring</groupId>
    <artifactId>spring-ioc-example</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <url>http://maven.apache.org</url>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <dependencies>
        <!-- https://mvnrepository.com/artifact/org.springframework/spring-context -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>5.1.0.RELEASE</version>
        </dependency>
    </dependencies>
    <build>
        <sourceDirectory>src/main/java</sourceDirectory>
        <plugins>
            <plugin>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.5.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

3. Configure HelloWorld Spring Beans

What is a Spring Bean?

This is a very simple question that is often overcomplicated. Usually, Spring beans are Java objects that are managed by the Spring container.
Here is a HelloWorld Spring bean:
package net.javaguides.spring.ioc;

public class HelloWorld {
    private String message;

    public void setMessage(String message) {
        this.message = message;
    }

    public void getMessage() {
        System.out.println("My Message : " + message);
    }
}

Configuration Metadata - Configure HelloWorld Spring Beans

package net.javaguides.spring.ioc;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AppConfig {

    @Bean
    public HelloWorld helloWorld() {
        HelloWorld helloWorld = new HelloWorld();
        helloWorld.setMessage("Hello World!");
        return helloWorld;
    }
}
Spring @Configuration annotation is part of the spring core framework. Spring Configuration annotation indicates that the class has @Bean definition methods. So Spring container can process the class and generate Spring Beans to be used in the application.

4. Create a Spring Container

If we have spring bean configuration XML file in a standalone application, then we can use ClassPathXmlApplicationContext class to load the file and get the container object.
package net.javaguides.spring.ioc;

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

public class Application {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext  context = new AnnotationConfigApplicationContext(AppConfig.class);
        context.close();
    }
}

5. Retrieve Beans from Spring Container

ApplicationContext interface provides getBean() method to retrieve bean from spring container.
package net.javaguides.spring.ioc;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class Application {
    public static void main(String[] args) {
        // ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
        HelloWorld obj = (HelloWorld) context.getBean("helloWorld");
        obj.getMessage();
        context.close();
    }
}

Output

My Message : Hello World!
The source code of this example is available on my GitHub repository https://github.com/RameshMF/spring-core-tutorial

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