Posted on Jun 21, 2024 • Originally published at springmasteryhub.com

Understanding @Primary in Spring

If you read my post about the @Qualifier annotation, you have noticed that defining two beans of the same type can be a challenge. By distinguishing it with a qualifier name, @Qualifier helps Spring determine which bean to inject.

The @Primary annotation will help Spring decide which of those same types of beans it should pick primarily.

For the bean annotated with the @Primary annotation, the @Qualifier will not be necessary. It is only required for the other bean.

Example 1: Using @primary and @Qualifier in Spring

Let’s see the example in practice:

public interface GreetingService { String greet(); } @Service @Primary public class EnglishGreetingService implements GreetingService { @Override public String greet() { return "Hello!"; } } @Service("spanishGreetingService") public class SpanishGreetingService implements GreetingService { @Override public String greet() { return "Hola!"; } } @RestController @RequestMapping("/greet") public class GreetingController { private static final org.slf4j.Logger log = org.slf4j.LoggerFactory.getLogger(GreetingController.class); @Autowired private GreetingService greetingService; @Autowired @Qualifier("spanishGreetingService") private GreetingService spanishGreetingService; @GetMapping public ResponseEntity<String> printGreetings() { log.info(greetingService.greet()); log.info(spanishGreetingService.greet()); return ResponseEntity.ok("greetings"); } }

You won’t need to use the @Qualifier on the bean you defined as @Primary

When you call this controller with this curl:

curl http://localhost:8080/greet

You are going to see in the logs that the primary one was automatically set:

2024-06-20T21:08:53.884-03:00 INFO 1868 --- [nio-8080-exec-1] c.s.mastery.primary.GreetingController : Hello! 2024-06-20T21:08:53.884-03:00 INFO 1868 --- [nio-8080-exec-1] c.s.mastery.primary.GreetingController : Hola!

Example 2: Using @primary at the Configuration Level

Another example is when you need to use the bean at the configuration level, so it will not be possible to use the @Qualifier to identify which bean Spring should inject.

@Configuration public class AppPrimaryConfig { @Bean @Primary public ConfigGreetingService portugueseGreetings() { return new PortugueseGreetings(); } @Bean("italianGreetings") public ConfigGreetingService italianGreetings() { return new ItalianGreetings(); } } @RestController @RequestMapping("/new-greetings") public class NewGreetingController { private static final org.slf4j.Logger log = org.slf4j.LoggerFactory.getLogger(GreetingController.class); @Autowired private ConfigGreetingService greetingService; @Autowired @Qualifier("italianGreetings") private ConfigGreetingService italianGreetings; @GetMapping public ResponseEntity<String> printGreetings() { log.info(greetingService.greet()); log.info(italianGreetings.greet()); return ResponseEntity.ok("greetings"); } }

When you call the controller:

curl http://localhost:8080/new-greetings

You should see:

2024-06-20T21:10:17.517-03:00 INFO 8768 --- [nio-8080-exec-1] c.s.mastery.primary.GreetingController : Ola 2024-06-20T21:10:17.517-03:00 INFO 8768 --- [nio-8080-exec-1] c.s.mastery.primary.GreetingController : Ciao