How @Autowired works with JavaConfig

[Micheal John John]

I am learning Spring Annotation and I want to know how each annotation works similair to the xml configuration.
Anyway, I want to use Spring without any xml file. So I started to use the JavaConfig.

In order to tell Spring, that we are using JavaConfig, we are using @Configuration.

Now I am just studying how the @Bean and @Autowired are working.

@Bean is for defining the bean which is similiar to the xml coniguration <bean></bean>

Below is my sample code, which is working fine for testing @Configuration and @Bean:

package com.example.service;

public class Greeting {
   public Greeting(){
	   System.out.println("In Greeting Constructor");
   }
   
   public String getMessage(){
	   return "Hello Greeting Message";
   }
}


package com.example.config;

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

import com.example.service.Greeting;

@Configuration
public class AnotherConfig {
	@Bean
	public Greeting getGreet(){
		return new Greeting();
	}
}	


package com.example;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import com.example.config.AnotherConfig;
import com.example.service.Greeting;

public class HelloApp {
	public static void main(String[] args) {
		AnnotationConfigApplicationContext context1 = new AnnotationConfigApplicationContext(AnotherConfig.class);
		Greeting obj3 = (Greeting)context1.getBean("getGreet");
		System.out.println("obj3 ("+obj3+")");
		System.out.println(obj3.getMessage());
	}
}

Output:
In Greeting Constructor
obj3 (com.example.service.Greeting@3043fe0e)
Hello Greeting Message


Now I want to use @Autowired in the above example:

package com.example.service;

public class Greeting {
   public Greeting(){
	   System.out.println("In Greeting Constructor");
   }
   
   public String getMessage(){
	   return "Hello Greeting Message";
   }
}


package com.example.config;

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

import com.example.service.Greeting;

@Configuration
public class AnotherConfig {
	
	@Autowired
	Greeting greeting;
	
	@Bean
	public Greeting getGreet(){
		return this.greeting;
	}
}		


package com.example;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import com.example.config.AnotherConfig;
import com.example.service.Greeting;

public class HelloApp {
	public static void main(String[] args) {
		AnnotationConfigApplicationContext context1 = new AnnotationConfigApplicationContext(AnotherConfig.class);
		Greeting obj3 = (Greeting)context1.getBean("getGreet");
		System.out.println("obj3 ("+obj3+")");
		System.out.println(obj3.getMessage());
	}
}

Error: Not working!!!

Oct 15, 2016 2:49:04 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@439f5b3d: startup date [Sat Oct 15 02:49:04 SGT 2016]; root of context hierarchy
Oct 15, 2016 2:49:05 AM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@7d417077: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor, org.springframework.context.annotation.internalCommonAnnotationProcessor,anotherConfig,org.springframework.context.annotation.ConfigurationClassPostProcessor.importAwareProcessor,getGreet]; root of factory hierarchy
Exception in thread "main" obj3 (null)
java.lang.NullPointerException
at com.example.HelloApp.main(HelloApp.java:12)


I have googled and in one the example they have mentioned @AnnotationDrivenConfig is required for @Autowired to work, (http://www.basilv.com/psd/blog/2009/java-based-configuration-of-spring-dependency-injection)
I am using Spring 3.2, but @AnnotationDrivenConfig is not resolving and it seems not available?!!

So, I have tried other option of ComponentScan:

package com.example.service;

import org.springframework.stereotype.Component;

@Component
public class Greeting {
   public Greeting(){
	   System.out.println("In Greeting Constructor");
   }
   
   public String getMessage(){
	   return "Hello Greeting Message";
   }
}


package com.example.config;

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

import com.example.service.Greeting;

@Configuration
@ComponentScan("com.example.service")
public class AnotherConfig {
	
	@Autowired	
	@Bean
	public Greeting getGreet(Greeting greeting){
		return greeting;
	}
}	


package com.example;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import com.example.config.AnotherConfig;
import com.example.service.Greeting;

public class HelloApp {
	public static void main(String[] args) {
		AnnotationConfigApplicationContext context1 = new AnnotationConfigApplicationContext(AnotherConfig.class);
		Greeting obj3 = (Greeting)context1.getBean("getGreet");
		System.out.println("obj3 ("+obj3+")");
		System.out.println(obj3.getMessage());
	}
}

Output: Working with construtor injection

In Greeting Constructor
obj3 (com.example.service.Greeting@7a9273a8)
Hello Greeting Message


But why same thing not working with property setter?

package com.example.config;

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

import com.example.service.Greeting;

@Configuration
@ComponentScan("com.example.service")
public class AnotherConfig {
	
	@Autowired	
	Greeting greeting;
	
	@Bean
	public Greeting getGreet(){
		return this.greeting;
	}
}

Error:
Oct 15, 2016 2:54:41 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@439f5b3d: startup date [Sat Oct 15 02:54:41 SGT 2016]; root of context hierarchy
Oct 15, 2016 2:54:42 AM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@1134affc: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor, org.springframework.context.annotation.internalCommonAnnotationProcessor,anotherConfig,org.springframework.context.annotation.ConfigurationClassPostProcessor.importAwareProcessor,greeting,getGreet]; root of factory hierarchy
In Greeting Constructor
obj3 (null)
Exception in thread "main" java.lang.NullPointerException
at com.example.HelloApp.main(HelloApp.java:12)

[Ankit Garg]

@Configuration
@ComponentScan("com.example.service")
public class AnotherConfig {
     
    @Autowired 
    Greeting greeting;
     
    @Bean
    public Greeting getGreet(){
        return this.greeting;
    }
}

Autowiring is used to inject bean which you create. In this case you are not creating any instance of the Greeting class (there is no new Greeting() getting called). Spring will not automatically create an object of greeting class and inject it for you. You should be using it something like this:

class GreetingClient {
     // injecting Greeting object
    @Autowired 
    Greeting greeting;

    public void sendGreetings() {
        System.out.println(greeting.getMessage());
    }
}

@Configuration
@ComponentScan("com.example.service")
public class AnotherConfig {
     
    @Bean
    public GreetingClient getGreetingClient(){
        return new GreetingClient();
    }
     
    @Bean
    public Greeting getGreet(){
        return new Greeting();
    }
}

public class HelloApp {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext context1 = new AnnotationConfigApplicationContext(AnotherConfig.class);
        GreetingClient greetingClient = context1.getBean(GreetingClient.class);
        System.out.println("greetingClient ("+greetingClient+")");
        greetingClient.sendGreetings();
    }
}

[Mohammed Sardar.]

Thanks Ankit Garg. Your answer above also helps me. But what if we want more objects to be created and given to Spring container ? So that they can Autowired in other related pojos ? Can we specify a package of more java classes to be created as well instead creating each one ?