What is JavaConfig in Spring? [closed]

Question

I just try to understand the meaning and the usage of annotation @Bean, and I encounter the word called JavaConfig in this documentation (2.2.1. chapter). The context was the following:

To declare a bean, simply annotate a method with the @Bean annotation. When JavaConfig encounters such a method, it will execute that method and register(...)

I don't understand what is JavaConfig is Spring... What does it work exactly? When it run? Why is run?

I saw this documentation as well, but did not bring me closer to understanding it.

Answer 1

It refers to the annotation based configuration, as opposed to the older, original XML based configuration.

The actual JavaConfig component is the one that goes through the class files and annotations to build the configuration (as opposed to going through an XML file to build the configuration).

Answer 2

Annotating a class with the @Configuration indicates that the class can be used by the Spring IoC container as a source of bean definitions. The @Bean annotation (that you ask about) tells Spring that a method annotated with @Bean will return an object that should be registered as a bean in the Spring application context. The simplest possible @Configuration class would be as follows −

package com.test;
import org.springframework.context.annotation.*;

@Configuration
public class HelloWorldConfig {
   @Bean 
   public HelloWorld helloWorld(){
      return new HelloWorld();
   }
}

The above code will be equivalent to the following XML configuration −

<beans>
   <bean id = "helloWorld" class = "com.test.HelloWorld" />
</beans>

Here, the method name is annotated with @Bean works as bean ID and it creates and returns the actual bean. Your configuration class can have a declaration for more than one @Bean. Once your configuration classes are defined, you can load and provide them to Spring container using AnnotationConfigApplicationContext as follows −

public static void main(String[] args) {
   ApplicationContext ctx = new AnnotationConfigApplicationContext(HelloWorldConfig.class);

   HelloWorld helloWorld = ctx.getBean(HelloWorld.class);
   helloWorld.setMessage("Hello World!");
   helloWorld.getMessage();
}

You can load various configuration classes as follows −

public static void main(String[] args) {
   AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();

   ctx.register(AppConfig.class, OtherConfig.class);
   ctx.register(AdditionalConfig.class);
   ctx.refresh();

   MyService myService = ctx.getBean(MyService.class);
   myService.doStuff();
}

Example of usage:

Here is the content of HelloWorldConfig.java file

package com.test;
import org.springframework.context.annotation.*;

@Configuration
public class HelloWorldConfig {
   @Bean 
   public HelloWorld helloWorld(){
      return new HelloWorld();
   }
}

Here is the content of HelloWorld.java file

package com.test;

public class HelloWorld {
   private String message;

   public void setMessage(String message){
      this.message  = message;
   }
   public void getMessage(){
      System.out.println("Your Message : " + message);
   }
}

Following is the content of the MainApp.java file

package com.test;

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.*;

public class MainApp {
   public static void main(String[] args) {
      ApplicationContext ctx = 
         new AnnotationConfigApplicationContext(HelloWorldConfig.class);

      HelloWorld helloWorld = ctx.getBean(HelloWorld.class);
      helloWorld.setMessage("Hello World!");
      helloWorld.getMessage();
   }
}

Once you are done creating all the source files and adding the required additional libraries, let us run the application. You should note that there is no configuration file required. If everything is fine with your application, it will print the following message −

Your Message : Hello World!