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Spring - Annotation Autowiring using Java

We have seen Autowiring by Annotations using application.xml. But this time we will not be using application.xml at all. Rather replace it using java.

Also we have seen Autowiring can be achieved with annotations using the @Autowired annotation.

We will be taking the same classes 'Car' and 'Wheel', where 'Car' class has a reference to an instance of 'Wheel' class. Let us define the classes with annotations. Also let us assume all the classes are in 'com.learnerslesson' package.

Wheel class

package com.learnerslesson;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class Wheel {

@Value("Solid Tire")
String tire;

public String getTire() {
return tire;
}

public void setTire(String tire) {
this.tire = tire;
}
}

Since, we have marked the 'Wheel' class with '@Component', Spring creates a 'Wheel' bean and with '@Value("Solid Tire")' it assignes a value 'Solid Tire' to the 'tire' property of 'Wheel'.

Car class

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component("car")
public class Car {

@Autowired
Wheel wheel;

public Wheel getWheel() {
return wheel;
}

// The Setter method is not needed.
}

With '@Component("car")', Spring creates a 'Car' bean with name 'car'.

Now, with '@Autowired' annotation Spring searches for a bean of 'Wheel' type. And since the 'Wheel' bean is already there, Spring injects the 'Wheel' bean to the 'Car' class.

Thats all you need to do to get the Autowiring done.

Note :Since, we have put the @Autowired annotation above 'Wheel wheel', we don't have to mention the setter method.

Finally, we can write the application-config.xml

And the replacement class for 'application-context.xml' is defined below :

package com.learnerslesson;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan({"com.learnerslesson"})
public class JavaAppConfig {

}

So, we have used @Configuration above the 'JavaAppConfig'. Which informs spring that it should be treated as a Configuration class(Or a replacement for application-context.xml).

And the body of the class is completely blank. It is all because of

@ComponentScan({"com.learnerslesson"})

It scans for all the beans annotated with @Component, creates beans and keeps in its container.And in 'Car' bean it finds @Autowired on top of 'Wheel wheel',

@Autowired
Wheel wheel;

It searches for 'Wheel' bean and injects in the reference of 'Wheel' that 'Car' is holding.

Finally, in the main() method we define our application code to interact with spring.

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

public class MySpringApp {
public static void main(String[] args) {

ApplicationContext applicationContext =
new AnnotationConfigApplicationContext(JavaAppConfig.class);

Car car = (Car)applicationContext.getBean("car");

System.out.println("The Car brand is : "+car.getWheel().getTireType());

}
}

Output :

The Car tire is : Solid Tire

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