Various Dependencies

Optional Dependency

• In situations where a Bean may or may not exist, optional injection is necessary

@Autowired(required = false)

@Autowired(required = false)
private NotificationService notificationService;

• prevents an exception from being thrown during the injection process even if the specified Bean does not exist
• constructor injection cannot be used here since that implies that the bean is absolutely required for the object’s creation
  • see [[DI (Dependency Injection)#Methods#Constructor Injection|constructor injection]]

Optional<T>

@Autowired
private Optional<NotificationService> notificationService;

@Nullable

@Autowired
@Nullable
private NotificationService notificationService;

Collection/List Dependencies

• When multiple Beans of the same type exist, Spring can automatically inject them into a List, Map, or array
  • List<T>: All Beans corresponding to the type T are injected (automatic sorting is by Bean registration order).
  • Map<String, T>: Beans are injected with the Bean name as the key and the Bean object as the value.
  • T[]: Beans of the same type are injected as an array.
• But we will probably never use this lol

List

@Service("pokemonServiceCollection")
public class PokemonService {
	
	/* 1. List 타입으로 주입 */
	private List<Pokemon> pokemonList;
 
	@Autowired
	public PokemonService(List<Pokemon> pokemonList) {
		this.pokemonList = pokemonList;
	}
 
	public void pokemonAttack() {
		pokemonList.forEach(Pokemon::attack);
	}
}

Map

@Service("pokemonServiceCollection")
public class PokemonService {
	
	/* 2. Map 타입으로 주입 */
	private Map<String, Pokemon> pokemonMap;
	
	@Autowired
	public PokemonService(Map<String, Pokemon> pokemonMap) {
		this.pokemonMap = pokemonMap;
	}
	
	public void pokemonAttack() {
      pokemonMap.forEach((k, v) -> {
          System.out.println("key : " + k);
          System.out.print("공격 : ");
          v.attack();
      });
  }
}