1. Introduction

Spring Boot provides the @Primary annotation to give higher preference to a bean when multiple candidates are qualified to autowire a single-valued dependency. This helps in determining which bean should be given priority when multiple beans are present of the same type.

Key Points:

1. @Primary indicates that a bean should be given preference when multiple candidates are present for a single autowiring scenario.

2. It is used to avoid ambiguity by marking one bean as the primary autowire candidate.

3. @Primary can be used alongside @Component, @Service, @Repository, or @Controller annotations.

2. Implementation Steps

1. Define an interface with a method that will be implemented by multiple beans.

2. Create at least two implementations of this interface.

3. Annotate one of the implementations with @Primary to indicate it should be the default bean for autowiring.

4. Autowire the interface in a component without using @Qualifier to rely on the @Primary bean.

5. Implement the CommandLineRunner in the main application class to test the autowiring.

3. Implementation Example

// Step 1: Define a service interface public interface CommunicationService { String communicate(); } // Step 2: Create the first bean implementation @Service public class EmailService implements CommunicationService { public String communicate() { return "Communicating via Email"; } } // Step 2: Create the second bean implementation and annotate it with @Primary @Service @Primary public class SmsService implements CommunicationService { public String communicate() { return "Communicating via SMS"; } } // Step 4: Autowire the CommunicationService interface in a component @Component public class NotificationManager { private final CommunicationService communicationService; @Autowired public NotificationManager(CommunicationService communicationService) { this.communicationService = communicationService; } public String sendNotification() { return communicationService.communicate(); } } // Step 5: Create the main application class @SpringBootApplication public class PrimaryAnnotationExampleApplication implements CommandLineRunner { @Autowired private NotificationManager notificationManager; public static void main(String[] args) { SpringApplication.run(PrimaryAnnotationExampleApplication.class, args); } @Override public void run(String... args) { System.out.println(notificationManager.sendNotification()); } } 

Output:

Communicating via SMS 

Explanation:

1. CommunicationService is a simple interface with a method communicate.