Primary

Indicates that a specific bean is a primary candidate for injection.

Why @Primary is needed?

There are situations when it's necessary to register more than one bean of the same type.

Take the following scenario, for example, where we have catAnimal and dogAnimal beans, both belonging to the Animal type:

@ClawjectApplication
class Application {
  @Bean
  catAnimal = new Animal('meow');

  @Bean
  dogAnimal = new Animal('woof');

  @PostConstruct
  postConstruct(
    animal: Animal // <-- Which bean will be injected here?
  ) {}
}

In this case, if we attempt to compile application, Clawject will report that two possible candidates found for animal parameter. Normally, we can set argument name same as bean name, but what if we just need some any bean of type Animal? To overcome this problem, Clawject provides the @Primary decorator.

The @Primary decorator can be used on Bean to indicate that a Bean should be given preference when multiple beans of the same type are available.

@ClawjectApplication
class Application {
  @Bean
  @Primary
  catAnimal = new Animal('meow');

  @Bean
  dogAnimal = new Animal('woof');

  @PostConstruct
  postConstruct(
    animal: Animal // <-- catAnimal will be injected here
  ) {
    console.log(animal.sound);
  }
}

In this case, catAnimal will be injected into the postConstruct method because it is marked as @Primary. But if you want to inject dogAnimal, even if catAnimal is marked as @Primaru you should specify parameter name of the dependency same as bean name.

@ClawjectApplication
class Application {
  @Bean
  @Primary
  catAnimal = new Animal('meow');

  @Bean
  dogAnimal = new Animal('woof');

  @PostConstruct
  postConstruct(
    dogAnimal: Animal // <-- dogAnimal will be injected here
  ) {
    console.log(dogAnimal.sound);
  }
}