Spring @Configuration Annotation

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.

Spring @Configuration

Spring @Configuration annotation allows us to use annotations for dependency injection. Let’s understand how to create Spring Configuration classes. Let’s create a simple java bean class.

package com.journaldev.spring;

public class MyBean {

	public MyBean() {
		System.out.println("MyBean instance created");
	}
	
}

Before we use any of the Spring framework classes, we will have to add it’s dependencies to the maven project.

<dependency>
		<groupId>org.springframework</groupId>
		<artifactId>spring-context</artifactId>
		<version>5.0.6.RELEASE</version>
</dependency>

Now let’s create the Spring Configuration class.

package com.journaldev.spring;

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

@Configuration
public class MyConfiguration {

    @Bean
    public MyBean myBean() {
		return new MyBean();
	}
	
}

Let’s write a simple class and configure our simple Spring configuration class.

package com.journaldev.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class MySpringApp {

	public static void main(String[] args) {
		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
		ctx.register(MyConfiguration.class);
		ctx.refresh();

		// MyBean mb1 = ctx.getBean(MyBean.class);

		// MyBean mb2 = ctx.getBean(MyBean.class);

		ctx.close();
	}

}

If you run above application, it will produce output like this:

May 23, 2018 12:34:54 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@ff5b51f: startup date [Wed May 23 12:34:54 IST 2018]; root of context hierarchy
MyBean instance created
May 23, 2018 12:34:54 PM org.springframework.context.support.AbstractApplicationContext doClose
INFO: Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@ff5b51f: startup date [Wed May 23 12:34:54 IST 2018]; root of context hierarchy

Notice that Spring loads beans into it’s context before we have even requested it. This is to make sure all the beans are properly configured and application fail-fast if something goes wrong. Also ctx.refresh() must be called, otherwise we will get following error when we will try to get any bean from the context.

Exception in thread "main" java.lang.IllegalStateException: org.springframework.context.annotation.AnnotationConfigApplicationContext@f0f2775 has not been refreshed yet
	at org.springframework.context.support.AbstractApplicationContext.assertBeanFactoryActive(AbstractApplicationContext.java:1076)
	at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1106)
	at com.journaldev.spring.MySpringApp.main(MySpringApp.java:11)

If you uncomment the statements where I am getting MyBean instances, you will notice that it’s not calling the constructor of MyBean. It’s because the default scope of spring beans is Singleton. We can change it using @Scope annotation.

What if we remove @Configuration annotation?

What will happen if we remove the @Configuration annotation from MyConfiguration class. You will notice that it still works as expected and spring beans are registered and retrieved as singleton classes. But in this case, if we make a call to myBean() method then it will be a plain java method call and we will get a new instance of MyBean and it won’t remain singleton. To prove this point, let’s define another bean that will be using MyBean instance.

package com.journaldev.spring;

public class MyBeanConsumer {

	public MyBeanConsumer(MyBean myBean) {
		System.out.println("MyBeanConsumer created");
		System.out.println("myBean hashcode = "+myBean.hashCode());
	}

}

Our updated Spring Configuration class is:

package com.journaldev.spring;

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

//@Configuration
public class MyConfiguration {

	@Bean
    public MyBean myBean() {
		return new MyBean();
	}
	
	@Bean
    public MyBeanConsumer myBeanConsumer() {
		return new MyBeanConsumer(myBean());
	}
}

Now when we run the MySpringApp class, it generates following output.

MyBean instance created
MyBean instance created
MyBeanConsumer created
myBean hashcode = 1647766367

So MyBean is not singleton anymore, now let’s annotate MyConfiguration with @Configuration annotation again and run the MySpringApp class. This time output will be like below.

MyBean instance created
MyBeanConsumer created
myBean hashcode = 1095088856

So it’s better to use @Configuration annotation with configuration classes to make sure our spring container is behaving like the way we want it to. If you don’t want to use @Configuration annotation for some weird reasons, we can still create our configuration class by not calling the myBean() method and rather using an instance variable of MyBean configured through @Autowired annotation. Something like below code will work as well.

package com.journaldev.spring;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

//@Configuration
public class MyConfiguration {

	@Autowired
	MyBean myBean;
	
	@Bean
    public MyBean myBean() {
		return new MyBean();
	}
	
	@Bean
    public MyBeanConsumer myBeanConsumer() {
		return new MyBeanConsumer(myBean);
	}
}

That’s all for Spring Configuration annotation, we will look into other spring annotations in future posts.

You can download the example code from our GitHub Repository.