Introduction

• In Java & Annotation based configuration, autowiring is achieved using the @Autowired annotation either on constructor or setter method or fields.
• The Spring container automatically resolves and injects dependencies where the @Autowired annotation is applied.
  
  
• By using @Autowired annotation, we dont need to use autowire attribute in the <bean> tag.

• Now we will create some programs of Autowiring using Java & Annotation based Configuration.
  
  

• Program 1

  First we will create program using Java Configurations with @Autowired Annotation which is applied on constructor.

  Engine.java

  package in.sp.beans;
  
  public class Engine
  {
      public void start() 
      {
          System.out.println("Engine started...");
      }
  }

  Car.java

  package in.sp.beans;
  
  import org.springframework.beans.factory.annotation.Autowired;
  import org.springframework.stereotype.Component;
  
  @Component
  public class Car
  {
      private Engine engine;
  
      // Constructor for dependency injection
      @Autowired
      public Car(Engine engine)
      {
          this.engine = engine;
      }
  
      public void drive()
      {
          engine.start();
          System.out.println("Car is running...");
      }
  }

  AppConfig.java

  package in.sp.config;
  
  import org.springframework.context.annotation.Bean;
  import org.springframework.context.annotation.ComponentScan;
  import org.springframework.context.annotation.Configuration;
  
  import in.sp.beans.Engine;
  
  @Configuration
  @ComponentScan(basePackages = "in.sp.beans")
  public class AppConfig
  {
      @Bean
      public Engine engine()
      {
          return new Engine();
      }
  }

  MainApp.java

  package in.sp.main;
  
  import org.springframework.context.ApplicationContext;
  import org.springframework.context.annotation.AnnotationConfigApplicationContext;
  
  import in.sp.beans.Car;
  import in.sp.config.AppConfig;
  
  public class MainApp
  {
      public static void main(String[] args)
      {
          // Load Spring context using Java configuration
          ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
  
          // Retrieve the Car bean
          Car car = context.getBean(Car.class);
  
          // Call the drive method
          car.drive();
      }
  }

  Below is the output

  Output:

  Engine started...
  Car is running...

  
  
  

• Program 2

  Now we are going to create program using Java Configurations with @Autowired Annotation which is applied on setter methods.
  

  Engine.java

  package in.sp.beans;
  
  public class Engine 
  {
      public void start() 
      {
          System.out.println("Engine started...");
      }
  }

  Car.java

  package in.sp.beans;
  
  import org.springframework.beans.factory.annotation.Autowired;
  import org.springframework.stereotype.Component;
  
  @Component
  public class Car
  {
      private Engine engine;
  
      // Setter method for dependency injection
      @Autowired
      public void setEngine(Engine engine)
      {
          this.engine = engine;
      }
  
      public void drive()
      {
          engine.start();
          System.out.println("Car is running...");
      }
  }

  AppConfig.java

  package in.sp.config;
  
  import org.springframework.context.annotation.Bean;
  import org.springframework.context.annotation.ComponentScan;
  import org.springframework.context.annotation.Configuration;
  
  import in.sp.beans.Engine;
  
  @Configuration
  @ComponentScan(basePackages = "in.sp.beans")  // Scans for @Component-annotated classes
  public class AppConfig 
  {
  	@Bean
      public Engine engine()
      {
          return new Engine();
      }
  }

  MainApp.java

  package in.sp.main;
  
  import org.springframework.context.ApplicationContext;
  import org.springframework.context.support.ClassPathXmlApplicationContext;
  
  import in.sp.beans.Car;
  
  public class MainApp
  {
  	public static void main(String[] args) 
  	{
  		// Loading Spring context from annotations
  		ApplicationContext context = new ClassPathXmlApplicationContext("in/sp/resources/applicationContext.xml");
          
  		Car car = context.getBean(Car.class);
          
  		car.drive();
  	}
  }

  Below is the output

  Output:

  Engine started...
  Car is running...

  
  
  

• Program 3

  Now we will create third program using Java Configurations with @Autowired Annotation which is applied on fields.
  

  Engine.java

  package in.sp.beans;
  
  public class Engine
  {
      public void start() 
      {
          System.out.println("Engine started...");
      }
  }

  Car.java

  package in.sp.beans;
  
  import org.springframework.beans.factory.annotation.Autowired;
  import org.springframework.stereotype.Component;
  
  @Component
  public class Car 
  {
      @Autowired
      private Engine engine; // Field injection with @Autowired
  
      public void drive()
      {
          engine.start();
          System.out.println("Car is running...");
      }
  }

  AppConfig.java

  package in.sp.config;
  
  import org.springframework.context.annotation.Bean;
  import org.springframework.context.annotation.ComponentScan;
  import org.springframework.context.annotation.Configuration;
  
  import in.sp.beans.Engine;
  
  @Configuration
  @ComponentScan(basePackages = "in.sp.beans")
  public class AppConfig
  {
      @Bean
      public Engine engine()
      {
          return new Engine();
      }
  }

  MainApp.java

  package in.sp.main;
  
  import org.springframework.context.ApplicationContext;
  import org.springframework.context.annotation.AnnotationConfigApplicationContext;
  
  import in.sp.beans.Car;
  import in.sp.config.AppConfig;
  
  public class MainApp 
  {
      public static void main(String[] args) 
      {
          // Load Spring context using Java configuration
          ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
  
          // Retrieve the Car bean
          Car car = context.getBean(Car.class);
  
          // Call the drive method
          car.drive();
      }
  }

  Below is the output

  Output:

  Engine started...
  Car is running...