Introduction

• The @Qualifier annotation in Spring is used to resolve ambiguity when multiple beans of the same type are available for dependency injection.
• It works in conjunction with @Autowired to specify which bean to inject.

• How @Qualifier Works ?

  • Bean Identification: It uses the bean name specified in the @Qualifier annotation to identify the correct bean.
  • Combines with @Autowired: It works only when used alongside @Autowired or similar annotations like @Inject.

• Placement of @Qualifier :

  @Qualifier annotation annotation can be applied to Constructor Parameters, Setter Methods & Fields which are explained as below:
  
  
  1. Constructor Parameters:
     • Example:

       @Autowired
       public Car(@Qualifier("petrolEngine") Engine engine)
       {
           this.engine = engine;
       }

       
       
  2. Setter Injection:
     • Example:

       @Autowired
       @Qualifier("electricEngine")
       public void setEngine(Engine engine)
       {
           this.engine = engine;
       }

       
       
  3. Field Injection:
     • Example:

       @Autowired
       @Qualifier("dieselEngine")
       private Engine engine;

• Program for @Qualifier

  We have created the below program for @Qualifier annotation which is used with setter method, try to create program in which @Qualifier is used in constructor parameter or with field.

  Engine.java

  package in.sp.beans;
  
  public class Engine 
  {
      private String type;
  
      public Engine(String type) 
      {
          this.type = type;
      }
  
      public void start() 
      {
          System.out.println(type + " engine started...");
      }
  }

  Car.java

  package in.sp.beans;
  
  import org.springframework.beans.factory.annotation.Autowired;
  import org.springframework.beans.factory.annotation.Qualifier;
  import org.springframework.stereotype.Component;
  
  @Component
  public class Car 
  {
      private Engine engine;
  
      // Setter method for dependency injection with @Qualifier
      @Autowired
      @Qualifier("dieselEngine")  // Specify the exact bean to inject
      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")
  public class AppConfig
  {
      // Define multiple beans of the same type
  	
      @Bean(name = "petrolEngine")
      public Engine petrolEngine() 
      {
          return new Engine("Petrol");
      }
  
      @Bean(name = "dieselEngine")
      public Engine dieselEngine() 
      {
          return new Engine("Diesel");
      }
  }

  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:

  Diesel engine started...
  Car is running...