Introduction

• The @Primary annotation in Spring helps to choose one bean as the default when there are multiple beans of the same type in the Spring container.
• It makes a bean the first choice for autowiring if no specific bean is selected using @Qualifier.

• Placement of @Primary :

  The @Primary annotation can be applied at Class Level or Bean Definition in Configuration Class, as explained below:
  
  
  1. Class Level:
     • Mark a class as the primary bean to be used for autowiring when multiple beans of the same type exist.
     • Example:

       @Primary
       @Component
       public class PetrolEngine implements Engine
       {
           // Implementation
       }

       
       
  2. Bean Definition in Configuration Class:
     • Use @Primary on a @Bean method in a configuration class to specify the primary bean among multiple beans of the same type.
     • Example:

       @Configuration
       public class AppConfig
       {
           @Bean
           @Primary
           public Engine petrolEngine()
           {
               return new PetrolEngine();
           }
       
           @Bean
           public Engine dieselEngine()
           {
               return new DieselEngine();
           }
       }

       
       

• Program for @Primary

  We have created the below program for @Primary annotation which is used at class level.

  Engine.java

  package in.sp.beans;
  
  public interface Engine
  {
      void start();
  }

  DieselEngine.java

  // Diesel Engine Implementation
  
  package in.sp.beans;
  
  import org.springframework.stereotype.Component;
  
  @Component
  public class DieselEngine implements Engine
  {
      @Override
      public void start() 
      {
          System.out.println("Diesel Engine started...");
      }
  }

  PetrolEngine.java

  // Petrol Engine Implementation with @Primary
  
  package in.sp.beans;
  
  import org.springframework.context.annotation.Primary;
  import org.springframework.stereotype.Component;
  
  @Component
  @Primary
  public class PetrolEngine implements Engine
  {
      @Override
      public void start()
      {
          System.out.println("Petrol 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;
  
      @Autowired
      public Car(Engine engine)
      {
          this.engine = engine;
      }
  
      public void drive()
      {
          engine.start();
          System.out.println("Car is running...");
      }
  }

  AppConfig.java

  // Configuration Class
  package in.sp.config;
  
  import org.springframework.context.annotation.ComponentScan;
  import org.springframework.context.annotation.Configuration;
  
  @Configuration
  @ComponentScan(basePackages = "in.sp.beans")
  public class AppConfig
  {
  	//no bean configuration required
  }

  MainApp.java

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

  Below is the output

  Output:

  Petrol Engine started...
  Car is running...